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Release 2.6.2
Guido van Rossum Fred L. Drake, Jr., editor
April 09, 2009
Python Software Foundation Email: email@example.com
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Whetting Your Appetite Using the Python Interpreter 2.1 Invoking the Interpreter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 The Interpreter and Its Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . An Informal Introduction to Python 3.1 Using Python as a Calculator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 First Steps Towards Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . More Control Flow Tools 4.1 if Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 for Statements . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 The range() Function . . . . . . . . . . . . . . . . . . . . . . 4.4 break and continue Statements, and else Clauses on Loops 4.5 pass Statements . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Defining Functions . . . . . . . . . . . . . . . . . . . . . . . . . 4.7 More on Defining Functions . . . . . . . . . . . . . . . . . . . . 4.8 Intermezzo: Coding Style . . . . . . . . . . . . . . . . . . . . . Data Structures 5.1 More on Lists . . . . . . . . . . . . . . 5.2 The del statement . . . . . . . . . . . 5.3 Tuples and Sequences . . . . . . . . . 5.4 Sets . . . . . . . . . . . . . . . . . . . 5.5 Dictionaries . . . . . . . . . . . . . . . 5.6 Looping Techniques . . . . . . . . . . 5.7 More on Conditions . . . . . . . . . . 5.8 Comparing Sequences and Other Types Modules 6.1 More on Modules . . 6.2 Standard Modules . . 6.3 The dir() Function . 6.4 Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 5 5 6 9 9 18 21 21 21 22 23 23 24 25 29 31 31 35 36 37 37 38 39 40 41 42 44 45 46 51 51 54
Input and Output 7.1 Fancier Output Formatting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Reading and Writing Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . .8 Dates and Times .2 History Substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. . . . . . . . . . . . . . . . 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Tools for Working with Lists . . . . . . . . . .4 Random Remarks . . . . . . . . . . . . . . . . . 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Brief Tour of the Standard Library 10. . . . . . . . . . . . . . . . . . . .4 Multi-threading . . . .12 Batteries Included .3 Handling Exceptions . . .8 Errors and Exceptions 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. . . . . . . . . . . . . . . . . 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 File Wildcards . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Syntax Errors . . . . . . . . . . . . . . . . . . . . .10 Performance Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. . . . . . . . . . . . . . . . .1 A Word About Terminology . . . . . . .9 Iterators . . . . . .11 Quality Control . . . . . . 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13. . . . . . . . . . . . . . . . . . .6 Defining Clean-up Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. . . 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. . . . . . . . . . 10. . . 9. . . . . . . . . . . . . .1 Operating System Interface . . . . . . . . . . . . . . . . . 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 95 95 95 97 99 ii . . . . . . . . 9.1 Output Formatting . . . . . . . . . . . . . . . . . . . . 8. . . . . . . . . . 10. . . . . . . . . . . . . . .3 A First Look at Classes . . . . . . . . . . . . . . . . 9. . . . . . . . . .4 Error Output Redirection and Program Termination 10. . . . . . . . . . . . . . . . . . . . . . . .6 Private Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Decimal Floating Point Arithmetic . . . . . . . . . . . . . . . . 13. . . . . 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 Python Scopes and Name Spaces 9. . . . . . . . . . .3 Key Bindings . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Generator Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. . . . . .2 Templating . . . . . 9. . . . . . . . . . . . . . . . . . . . . . . . . . . 13. . .5 String Pattern Matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. . . . . . 14 Floating Point Arithmetic: Issues and Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Exceptions Are Classes Too . . . . . . . . . . . . . . 12 What Now? 13 Interactive Input Editing and History Substitution 13. . . . . . . . . . . . . . . . . . 8. . . . . . . . . . . .7 Internet Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 Commentary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Predefined Clean-up Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Working with Binary Data Record Layouts 11. . . . . . . . . . . . . . . . . . . .5 Logging . . . . 9. . . . . . . . . . . . . . . . 10. . . . . . . . . . . . . . . . . . . . . . . . 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Generators . 11. . . . . . . . . . . . . .9 Data Compression . .1 Line Editing . . . . . . . . . . . . . . . . . . . . . . . .5 User-defined Exceptions . . . . . . 10. . . . . . . .3 Command Line Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Odds and Ends . . . . . . . . . . . . . . . . .4 Raising Exceptions . . . 11 Brief Tour of the Standard Library – Part II 11. . . . . . . . . . . . .2 Exceptions . . . . . . . . . . . . . . . . . .6 Weak References . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Mathematics . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 59 59 60 62 62 64 65 67 67 67 69 71 72 74 74 75 75 77 77 79 79 79 80 80 80 80 81 81 82 82 82 83 85 85 86 87 87 88 89 89 90 93 9 Classes 9. . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . .1 Representation Error . . 111 C History and License C. . . . . . . . . . . . . . . D Copyright Index 113 113 114 117 125 127 iii . . . . . . . . . . . . . . . . . . . 101 A Glossary 103 B About these documents 111 B. . . . . . . . . . . . . . . . C. . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 History of the software . . . . . . . . . . . . . . . . . . .14. . . . . . C. . . . . .3 Licenses and Acknowledgements for Incorporated Software . . . . . . . . . . . . .1 Contributors to the Python Documentation . . . . . . . . . . . . . . .2 Terms and conditions for accessing or otherwise using Python . . . . .
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Python's elegant syntax and dynamic typing. http://www.org/. so the tutorial can be read off-line as well. There are also several books covering Python in depth. but all examples are self-contained. This tutorial introduces the reader informally to the basic concepts and features of the Python language and system.Python Tutorial. and you will be ready to learn more about the various Python library modules described in the Python Library Reference. make it an ideal language for scripting and rapid application development in many areas on most platforms. read Extending and Embedding the Python Interpreter and Python/C API Reference. This tutorial does not attempt to be comprehensive and cover every single feature. CONTENTS 1 . To write extensions in C or C++.2 Release 2. and will give you a good idea of the language's flavor and style.6 Date April 09. The Python interpreter is easily extended with new functions and data types implemented in C or C++ (or other languages callable from C). Instead.python. The Python Reference Manual gives a more formal definition of the language. The Glossary is also worth going through. It helps to have a Python interpreter handy for hands-on experience. together with its interpreted nature. After reading it. Release 2. programs and tools. The Python interpreter and the extensive standard library are freely available in source or binary form for all major platforms from the Python Web site. it introduces many of Python's most noteworthy features. and additional documentation. Python is also suitable as an extension language for customizable applications.6. The same site also contains distributions of and pointers to many free third party Python modules. see the Python Library Reference document. For a description of standard objects and modules. and may be freely distributed. you will be able to read and write Python modules and programs. powerful programming language. It has efficient high-level data structures and a simple but effective approach to object-oriented programming. or even every commonly used feature. 2009 Python is an easy to learn.
Python Tutorial.2 2 CONTENTS .6. Release 2.
you can 3 . or to test functions during bottom-up program development. Or maybe you've written a program that could use an extension language. you may have to work with several C/C++/Java libraries but find the usual write/compile/test/re-compile cycle is too slow. Python is extensible: if you know how to program in C it is easy to add a new built-in function or module to the interpreter. or a specialized GUI application. not well-suited for GUI applications or games. system calls. yet many things are at least as easy in Python as in those languages. or a simple game. but it is a real programming language. Perhaps you're writing a test suite for such a library and find writing the testing code a tedious task. such as flexible arrays and dictionaries. For example. and will help you get the job done more quickly. offering much more structure and support for large programs than shell scripts or batch files can offer. Python is simpler to use. but shell scripts are best at moving around files and changing text data. Python enables programs to be written compactly and readably. being a very-high-level language. You could write a Unix shell script or Windows batch files for some of these tasks. it has high-level data types built in. either to perform critical operations at maximum speed. and you don't want to design and implement a whole new language for your application.CHAPTER ONE WHETTING YOUR APPETITE If you do much work on computers. The interpreter can be used interactively. Python also offers much more error checking than C. Because of its more general data types Python is applicable to a much larger problem domain than Awk or even Perl. Python allows you to split your program into modules that can be reused in other Python programs. or rename and rearrange a bunch of photo files in a complicated way. C++. Python is an interpreted language. It comes with a large collection of standard modules that you can use as the basis of your programs — or as examples to start learning to program in Python. Once you are really hooked. On the other hand. which makes it easy to experiment with features of the language. and Unix operating systems. Python is just the language for you. and even interfaces to graphical user interface toolkits like Tk. available on Windows. eventually you find that there's some task you'd like to automate. Mac OS X. and. Python is simple to use. Some of these modules provide things like file I/O. or Java programs. • no variable or argument declarations are necessary. but it can take a lot of development time to get even a first-draft program. which can save you considerable time during program development because no compilation and linking is necessary. Perhaps you'd like to write a small custom database. If you're a professional software developer. Programs written in Python are typically much shorter than equivalent C. It is also a handy desk calculator. or to link Python programs to libraries that may only be available in binary form (such as a vendor-specific graphics library). You could write a C/C++/Java program. • statement grouping is done by indentation instead of beginning and ending brackets. for several reasons: • the high-level data types allow you to express complex operations in a single statement. sockets. to write throw-away programs. you may wish to perform a search-and-replace over a large number of text files.
through functions and modules. Making references to Monty Python skits in documentation is not only allowed. Whetting Your Appetite . the tutorial invites you to play with the Python interpreter as you read. Release 2. beginning with simple expressions. This is rather mundane information. and finally touching upon advanced concepts like exceptions and user-defined classes.Python Tutorial. it is encouraged! Now that you are all excited about Python. By the way. the language is named after the BBC show "Monty Python's Flying Circus" and has nothing to do with reptiles.6. but essential for trying out the examples shown later. Since the best way to learn a language is to use it. In the next chapter. you'll want to examine it in some more detail.2 link the Python interpreter into an application written in C and use it as an extension or command language for that application. statements and data types. 4 Chapter 1. The rest of the tutorial introduces various features of the Python language and system through examples. the mechanics of using the interpreter are explained.
sys. Note that there is a difference between python file and python <file. These can be invoked using python -m module [arg] . The interpreter's line-editing features usually aren't very sophisticated.C:\python26 Typing an end-of-file character (Control-D on Unix. (E.. you can exit the interpreter by typing the following commands: import sys. analogous to the shell's -c option. it reads and executes commands interactively.g. command line editing isn't available. the program will encounter end-of-file 5 . other places are possible..exit(). In the latter case. The interpreter operates somewhat like the Unix shell: when called with standard input connected to a tty device. check with your local Python guru or system administrator. Since this file has already been read until the end by the parser before the program starts executing. which adds more elaborate interactive editing and history features. the Python installation is usually placed in C:\Python26. Perhaps the quickest check to see whether command line editing is supported is typing Control-P to the first Python prompt you get. are satisfied from file. If that doesn't work.. or if ^P is echoed.. such as calls to input() and raw_input(). when called with a file name argument or with a file as standard input. though you can change this when you're running the installer. Control-Z on Windows) at the primary prompt causes the interpreter to exit with a zero exit status.. To add this directory to your path. /usr/local/python is a popular alternative location.) On Windows machines. Since Python statements often contain spaces or other characters that are special to the shell. you can type the following command into the command prompt in a DOS box: set path=%path%. If it beeps. Some Python modules are also useful as scripts.. you have command line editing. Since the choice of the directory where the interpreter lives is an installation option. On Unix.1 Invoking the Interpreter The Python interpreter is usually installed as /usr/local/bin/python on those machines where it is available.. you'll only be able to use backspace to remove characters from the current line. putting /usr/local/bin in your Unix shell's search path makes it possible to start it by typing the command python to the shell. A second way of starting the interpreter is python -c command [arg] . it is usually advised to quote command in its entirety with single quotes. If nothing appears to happen.CHAPTER TWO USING THE PYTHON INTERPRETER 2. input requests from the program. which executes the statement(s) in command. see Appendix Interactive Input Editing and History Substitution for an introduction to the keys. whoever installed the interpreter may have enabled support for the GNU readline library. which executes the source file for module as if you had spelled out its full name on the command line. it reads and executes a script from that file.
sys. In interactive mode. the script name and additional arguments thereafter are passed to the script in the variable sys.2. (Exceptions handled by an except clause in a try statement are not errors in this context.1 Error Handling When an error occurs.2 Interactive Mode When commands are read from a tty. >>> Continuation lines are needed when entering a multi-line construct. As an example. When a script file is used.. When -m module is used. normal output from executed commands is written to standard output. by default three dots (. In this mode it prompts for the next command with the primary prompt. which is a list of strings.) 2..argv for the command or module to handle.) Some errors are unconditionally fatal and cause an exit with a nonzero exit.). "copyright".argv[0] is an empty string.6. this applies to internal inconsistencies and some cases of running out of memory.argv[0] is set to '-'. when input came from a file. it is sometimes useful to be able to run the script and enter interactive mode afterwards. "credits" or "license" for more information.Python Tutorial. it exits with a nonzero exit status after printing the stack trace. which may be handled by a try statement. 2.argv[0] is set to '-c'.argv. Its length is at least one. All error messages are written to the standard error stream. The interpreter prints a welcome message stating its version number and a copyright notice before printing the first prompt: python Python 2. (This does not work if the script is read from standard input. When -c command is used. usually three greater-than signs (>>>). Typing the interrupt character (usually Control-C or DEL) to the primary or secondary prompt cancels the input and returns to the primary prompt.1 Argument Passing When known to the interpreter. When the script name is given as '-' (meaning standard input). sys. it then returns to the primary prompt. when no script and no arguments are given.. This can be done by passing -i before the script. 6 Chapter 2. print "Be careful not to fall off!" .2 immediately. Feb 28 2007...argv[0] is set to the full name of the located module.1.2 The Interpreter and Its Environment 2. 1 Typing an interrupt while a command is executing raises the KeyboardInterrupt exception. sys. take a look at this if statement: >>> the_world_is_flat = 1 >>> if the_world_is_flat: . the interpreter is said to be in interactive mode. for the same reason as explained in the previous paragraph. the interpreter prints an error message and a stack trace. Options found after -c command or -m module are not consumed by the Python interpreter's option processing but left in sys.1. 00:02:06) Type "help". Release 2. sys. Using the Python Interpreter .. for continuation lines it prompts with the secondary prompt. In the former case (which is usually what you want) they are satisfied from whatever file or device is connected to standard input of the Python interpreter. 1 A problem with the GNU Readline package may prevent this. Be careful not to fall off! 2.6 (#1.
py On Windows systems. this first line must end with a Unix-style line ending ('\n').2. with the Euro symbol having the ordinal value 164. and also not understood by the operating system for script files with #! lines (only used on Unix systems). or pound. or permission.pyw. This is similar to the . is used to start a comment in Python. the console window that normally appears is suppressed. To display all these characters properly. and it will be possible to directly write Unicode string literals in the selected encoding. like shell scripts.coding: iso-8859-15 -*currency = u" " print ord(currency) If your editor supports saving files as UTF-8 with a UTF-8 byte order mark (aka BOM). the ISO-8859-15 encoding can be used. to write Unicode literals including the Euro currency symbol. The best way to do it is to put one more special comment line right after the #! line to define the source file encoding: # -*. Using non-ASCII characters in identifiers is not supported. characters of most languages in the world can be used simultaneously in string literals and comments.2. The Interpreter and Its Environment 7 . Note that the hash. The #! must be the first two characters of the file.3 Source Code Encoding It is possible to use encodings different than ASCII in Python source files. IDLE supports this capability if Options/General/Default Source Encoding/UTF-8 is set.2. On some platforms. character.coding: encoding -*With that declaration. in that case.py files with python. and it must use a font that supports all the characters in the file.4 The Interactive Startup File When you use Python interactively. By using UTF-8 (either through the signature or an encoding declaration).2 Executable Python Scripts On BSD'ish Unix systems. You can do this by setting an environment variable named PYTHONSTARTUP to the name of a file containing your start-up commands.2. Release 2. in the section on codecs. 2. The list of possible encodings can be found in the Python Library Reference. all characters in the source file will be treated as having the encoding encoding. you can use that instead of an encoding declaration. Notice that this signature is not understood in older Python releases (2.profile feature of the Unix shells. there is no notion of an "executable mode". The Python installer automatically associates . The extension can also be .2 2. it is frequently handy to have some standard commands executed every time the interpreter is started. 2. For example.exe so that a double-click on a Python file will run it as a script. '#'. Python scripts can be made directly executable. This script will print the value 8364 (the Unicode codepoint corresponding to the Euro symbol) and then exit: # -*. your editor must recognize that the file is UTF-8. 2. The script can be given an executable mode. using the chmod command: $ chmod +x myscript.2 and earlier).6. not a Windows ('\r\n') line ending.Python Tutorial. by putting the line #! /usr/bin/env python (assuming that the interpreter is on the user's PATH) at the beginning of the script and giving the file an executable mode.
isfile('.Python Tutorial. not when Python reads commands from a script. If you want to use the startup file in a script. You can also change the prompts sys. Release 2. Using the Python Interpreter . It is executed in the same namespace where interactive commands are executed.path.ps2 in this file.get('PYTHONSTARTUP') if filename and os. you can program this in the global start-up file using code like if os.py'): execfile('.ps1 and sys. you must do this explicitly in the script: import os filename = os.2 This file is only read in interactive sessions.path.pythonrc.py').6.isfile(filename): execfile(filename) 8 Chapter 2. and not when /dev/tty is given as the explicit source of commands (which otherwise behaves like an interactive session). If you want to read an additional start-up file from the current directory. so that objects that it defines or imports can be used without qualification in the interactive session.pythonrc.environ.
Many of the examples in this manual. but not within a string literal. Expression syntax is straightforward: the operators +.. For example: >>> 4 >>> . input and output are distinguished by the presence or absence of prompts (>>> and . >>>. this is used to end a multi-line command. include comments. parentheses can be used for grouping. 4 >>> 4 2+2 # This is a comment 2+2 2+2 # and a comment on the same line as code 9 .1.1 Using Python as a Calculator Let's try some simple Python commands. A comment may appear at the start of a line or following whitespace or code. you must type everything after the prompt.CHAPTER THREE AN INFORMAL INTRODUCTION TO PYTHON In the following examples...1 Numbers The interpreter acts as a simple calculator: you can type an expression at it and it will write the value. Since comments are to clarify code and are not interpreted by Python. * and / work just like in most other languages (for example.) 3. #. when the prompt appears.. Note that a secondary prompt on a line by itself in an example means you must type a blank line. A hash character within a string literal is just a hash character. (It shouldn't take long. Comments in Python start with the hash character. and extend to the end of the physical line. Start the interpreter and wait for the primary prompt.): to repeat the example. and now a third! STRING = "# This is not a comment. lines that do not begin with a prompt are output from the interpreter.. Some examples: # this is the first comment SPAM = 1 # and this is the second comment # . -. even those entered at the interactive prompt." 3.. Pascal or C). they may be omitted when typing in examples.
imag) function.5 Complex numbers are also supported. n Traceback (most recent File "<stdin>".. y and z Variables must be "defined" (assigned a value) before they can be used.75 / 1. in <module> not defined There is full support for floating point. 2 >>> -3 (50-5*6)/4 # Integer division returns the floor: 7/3 7/-3 The equal sign ('=') is used to assign a value to a variable.. Complex numbers with a nonzero real component are written as (real+imagj). Afterwards. line NameError: name 'n' is undefined variable call last): 1...2 >>> 5 >>> .1) (-1+0j) >>> 3+1j*3 (3+3j) 10 Chapter 3.Python Tutorial. Release 2. An Informal Introduction to Python . operators with mixed type operands convert the integer operand to floating point: >>> 3 * 3.5 7. or can be created with the complex(real. no result is displayed before the next interactive prompt: >>> width = 20 >>> height = 5*9 >>> width * height 900 A value can be assigned to several variables simultaneously: >>> >>> 0 >>> 0 >>> 0 x = y = z = 0 x y z # Zero x. or an error will occur: >>> # try to access an .0 / 2 3. imaginary numbers are written with a suffix of j or J.6. >>> 1j * 1J (-1+0j) >>> 1j * complex(0.5 >>> 7.
5j >>> a.0625 >>> round(_. This means that when you are using Python as a desk calculator.real 1. Python can also manipulate strings.5 >>> a. use abs(z) >>> a. They can be enclosed in single quotes or double quotes: 3. Use abs(z) to get its magnitude (as a float) or z. Release 2.2 >>> (3+1j)*3 (9+3j) >>> (1+2j)/(1+1j) (1.0 >>> a.5+0.real**2 + a.1. int() and long()) don't work for complex numbers — there is no one correct way to convert a complex number to a real number. it is somewhat easier to continue calculations.0j >>> float(a) Traceback (most recent call last): File "<stdin>".1.0 >>> abs(a) # sqrt(a. line 1.0 >>> In interactive mode.5j) Complex numbers are always represented as two floating point numbers.06 >>> This variable should be treated as read-only by the user. 2) 113. for example: >>> tax = 12. use z.50 >>> price * tax 12.2 Strings Besides numbers. To extract these parts from a complex number z.0+4.real and z. which can be expressed in several ways. the last printed expression is assigned to the variable _.imag**2) 5.imag.5+0.real 3. Using Python as a Calculator 11 .Python Tutorial.imag 0.5 The conversion functions to floating point and integer (float(). Don't explicitly assign a value to it — you would create an independent local variable with the same name masking the built-in variable with its magic behavior. the real and imaginary part.6. >>> a=3.real to get its real part.5625 >>> price + _ 113. in ? TypeError: can't convert complex to float.imag 4. >>> a=1. 3.5 / 100 >>> price = 100.
This example would print the following: This is a rather long string containing several lines of text just as you would do in C.' String literals can span multiple lines in several ways.Python Tutorial." '"Yes. print """ Usage: thingy [OPTIONS] -h -H hostname """ produces the following output: Usage: thingy [OPTIONS] -h -H hostname Display this usage message Hostname to connect to Display this usage message Hostname to connect to If we make the string literal a "raw" string." she said. \n sequences are not converted to newlines. with a backslash as the last character on the line indicating that the next line is a logical continuation of the line: hello = "This is a rather long string containing\n\ several lines of text just as you would do in C. Continuation lines can be used.' '"Isn\'t.\" he said. Thus." print hello 12 Chapter 3.\n\ Note that whitespace at the beginning of the line is\ significant. are both included in the string as data.' >>> '"Isn\'t. Note that whitespace at the beginning of the line is significant.2 >>> 'spam eggs' 'spam eggs' >>> 'doesn\'t' "doesn't" >>> "doesn't" "doesn't" >>> '"Yes. the example: hello = r"This is a rather long string containing\n\ several lines of text much as you would do in C. An Informal Introduction to Python . and the newline character in the source. End of lines do not need to be escaped when using triple-quotes." she said. Or. strings can be surrounded in a pair of matching triple-quotes: """ or "'. Release 2. but they will be included in the string." he said. the newline following the trailing backslash is discarded." print hello Note that newlines still need to be embedded in the string using \n." he said.' >>> "\"Yes. but the backslash at the end of the line." he said.' '"Yes.6.
an omitted first index defaults to zero. >>> word[:2] 'He' >>> word[2:] 'lpA' # The first two characters # Everything except the first two characters Unlike a C string. Release 2. Like in Icon. Using Python as a Calculator 13 . like in C.1. an omitted second index defaults to the size of the string being sliced. described later. Python strings cannot be changed.strip() File "<stdin>". The string is enclosed in double quotes if the string contains a single quote and no double quotes. substrings can be specified with the slice notation: two indices separated by a colon.strip() # + 'ing' # <<This is ok This is ok This is invalid 'ing' # <line 1. There is no separate character type. (The print statement.strip() 'string' >>> 'str'. the first line above could also have been written word = 'Help' 'A'. not with arbitrary string expressions: >>> 'str' 'ing' 'string' >>> 'str'. and repeated with *: >>> word = 'Help' + 'A' >>> word 'HelpA' >>> '<' + word*5 + '>' '<HelpAHelpAHelpAHelpAHelpA>' Two string literals next to each other are automatically concatenated.Python Tutorial. the first character of a string has subscript (index) 0. Assigning to an indexed position in the string results in an error: 3. The interpreter prints the result of string operations in the same way as they are typed for input: inside quotes. to show the precise value. in ? 'ing' ^ SyntaxError: invalid syntax Strings can be subscripted (indexed).) Strings can be concatenated (glued together) with the + operator. this only works with two literals. a character is simply a string of size one. and with quotes and other funny characters escaped by backslashes.2 would print: This is a rather long string containing\n\ several lines of text much as you would do in C. else it's enclosed in single quotes.6. can be used to write strings without quotes or escapes. 'str'. >>> word[4] 'A' >>> word[0:2] 'He' >>> word[2:4] 'lp' Slice indices have useful defaults.
line 1. creating a new string with the combined content is easy and efficient: >>> 'x' + word[1:] 'xelpA' >>> 'Splat' + word[4] 'SplatA' Here's a useful invariant of slice operations: s[:i] + s[i:] equals s.2 >>> word[0] = 'x' Traceback (most recent call last): File "<stdin>". so it does not count from the right! >>> word[-0] 'H' # (since -0 equals 0) Out-of-range negative slice indices are truncated. An Informal Introduction to Python . line 1.6. but don't try this for single-element (non-slice) indices: 14 Chapter 3. in ? TypeError: object doesn't support item assignment >>> word[:1] = 'Splat' Traceback (most recent call last): File "<stdin>". an upper bound smaller than the lower bound returns an empty string. >>> word[1:100] 'elpA' >>> word[10:] '' >>> word[2:1] '' Indices may be negative numbers. to start counting from the right.Python Tutorial. Release 2. in ? TypeError: object doesn't support slice assignment However. For example: >>> word[-1] 'A' >>> word[-2] 'p' >>> word[-2:] 'pA' >>> word[:-2] 'Hel' # The last character # The last-but-one character # The last two characters # Everything except the last two characters But note that -0 is really the same as 0. >>> word[:2] + word[2:] 'HelpA' >>> word[:3] + word[3:] 'HelpA' Degenerate slice indices are handled gracefully: an index that is too large is replaced by the string size.
providing auto-conversions where necessary. Creating Unicode strings in Python is just as simple as creating normal strings: 3.6.1. xrange (in The Python Library Reference) Strings. line 1..1.2 >>> word[-100:] 'HelpA' >>> word[-10] # error Traceback (most recent call last): File "<stdin>".Python Tutorial. This lead to very much confusion especially with respect to internationalization (usually written as i18n — 'i' + 18 characters + 'n') of software. For non-negative indices. Unicode solves these problems by defining one code page for all scripts.3 Unicode Strings Starting with Python 2.. For example. list. Unicode has the advantage of providing one ordinal for every character in every script used in modern and ancient texts. and support the common operations supported by such types. Previously. in ? IndexError: string index out of range One way to remember how slices work is to think of the indices as pointing between characters.format() is described here. 3. and the Unicode strings described in the next section. the second row gives the corresponding negative indices. are examples of sequence types. the length of word[1:3] is 2. for example: +---+---+---+---+---+ | H | e | l | p | A | +---+---+---+---+---+ 0 1 2 3 4 5 -5 -4 -3 -2 -1 The first row of numbers gives the position of the indices 0.org/) and integrates well with the existing string objects. Texts were typically bound to a code page which mapped the ordinals to script characters. String Methods (in The Python Library Reference) Both strings and Unicode strings support a large number of methods for basic transformations and searching. Release 2. with the left edge of the first character numbered 0. respectively. if both are within bounds. the length of a slice is the difference of the indices. buffer. It can be used to store and manipulate Unicode data (see http://www. The slice from i to j consists of all characters between the edges labeled i and j. The built-in function len() returns the length of a string: >>> s = 'supercalifragilisticexpialidocious' >>> len(s) 34 See Also: Sequence Types — str. unicode.unicode. String Formatting Operations (in The Python Library Reference) The old formatting operations invoked when strings and Unicode strings are the left operand of the % operator are described in more detail here.5 in the string. Using Python as a Calculator 15 . Then the right edge of the last character of a string of n characters has index n. String Formatting (in The Python Library Reference) Information about string formatting with str. tuple.0 a new data type for storing text data is available to the programmer: the Unicode object. there were only 256 possible ordinals for script characters.
The built-in function unicode() provides access to all registered Unicode codecs (COders and DECoders). and UTF-16. UTF-8. Unicode objects provide an encode() method that takes one argument. Other characters are interpreted by using their respective ordinal values directly as Unicode ordinals. The following example shows how: >>> u'Hello\u0020World !' u'Hello World !' The escape sequence \u0020 indicates to insert the Unicode character with the ordinal value 0x0020 (the space character) at the given position. or converted with str(). which passes through characters in the range 0 to 127 and rejects any other characters with an error. Release 2. You have to prefix the opening quote with 'ur' to have Python use the Raw-Unicode-Escape encoding. It will only apply the above \uXXXX conversion if there is an uneven number of backslashes in front of the small 'u'. ASCII.6.Python Tutorial. If you have literal strings in the standard Latin-1 encoding that is used in many Western countries. Lowercase names for encodings are preferred. Some of the more well known encodings which these codecs can convert are Latin-1. An Informal Introduction to Python . If you want to include special characters in the string. there is also a raw mode just like the one for normal strings. conversion takes place using this default encoding. Python provides a whole set of other ways of creating Unicode strings on the basis of a known encoding. The default encoding is normally set to ASCII. you can do so by using the Python Unicode-Escape encoding. you will find it convenient that the lower 256 characters of Unicode are the same as the 256 characters of Latin-1. For experts. the name of the encoding. When a Unicode string is printed. >>> ur'Hello\u0020World !' u'Hello World !' >>> ur'Hello\\u0020World !' u'Hello\\\\u0020World !' The raw mode is most useful when you have to enter lots of backslashes. 16 Chapter 3. line 1. >>> u"äöü". you can use the unicode() function with the encoding name as the second argument. The latter two are variable-length encodings that store each Unicode character in one or more bytes. >>> u"abc" u'abc' >>> str(u"abc") 'abc' >>> u"äöü" u'\xe4\xf6\xfc' >>> str(u"äöü") Traceback (most recent call last): File "<stdin>". in ? UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in r To convert a Unicode string into an 8-bit string using a specific encoding.encode('utf-8') '\xc3\xa4\xc3\xb6\xc3\xbc' If you have data in a specific encoding and want to produce a corresponding Unicode string from it. written to a file. Apart from these standard encodings. as can be necessary in regular expressions.2 >>> u'Hello World !' u'Hello World !' The small 'u' in front of the quote indicates that a Unicode string is supposed to be created.
12] >>> a [1. 100. 4] >>> 3*a[:3] + ['Boo!'] ['spam'. 1234] >>> a ['spam'. 'bletch'. 1234] >>> a[2] = a[2] + 23 >>> a ['spam'. The most versatile is the list. 'eggs'. 'spam'.4 Lists Python knows a number of compound data types. 'eggs'. 100. 'utf-8') u'\xe4\xf6\xfc' 3.2 >>> unicode('\xc3\xa4\xc3\xb6\xc3\xbc'. a[1:1] = ['bletch'. a[0:2] = [1. 'eggs'.. 100. 100. >>> a = ['spam'.. 'eggs'. 123..1.. 1234] Like string indices. concatenated and so on: >>> a[0] 'spam' >>> a[3] 1234 >>> a[-2] 100 >>> a[1:-1] ['eggs'. 'spam'. 123. which can be written as a list of comma-separated values (items) between square brackets. it is possible to change individual elements of a list: >>> a ['spam'. 'xyzzy'. 100] >>> a[:2] + ['bacon'. and this can even change the size of the list or clear it entirely: >>> # Replace some items: . 1234] >>> # Insert (a copy of) itself at the beginning >>> a[:0] = a 3. and lists can be sliced. 'eggs'. 'Boo!'] Unlike strings. a[0:2] = [] >>> a [123. List items need not all have the same type.1. Using Python as a Calculator 17 .6. 12. used to group together other values. 100. 'eggs'. 1234] >>> # Remove some: . 'eggs'.. 'eggs'. which are immutable. 2*2] ['spam'. Release 2. 100..Python Tutorial. 1234] >>> # Insert some: . 'bacon'. 'xyzzy'] >>> a [123. list indices start at 0. 1234] Assignment to slices is also possible.
q = [2. 123. 1234] >>> # Clear the list: replace all items with an empty list >>> a[:] = [] >>> a [] The built-in function len() also applies to lists: >>> a = ['a'.6. 'xyzzy'. An Informal Introduction to Python . >>> [2.2 First Steps Towards Programming Of course. 3.. a+b This example introduces several new features. 3] p = [1. 'bletch'. we can use Python for more complicated tasks than adding two and two together. . 'c'. For instance. 3.Python Tutorial..... 'bletch'.. 'xtra']. 1234. 4] len(p) p[1] 3] p[1][0] p[1]. 'd'] >>> len(a) 4 It is possible to nest lists (create lists containing other lists). 'xyzzy'. 'xtra'] # See section 5. 'b'. >>> . 1 while b < 10: print b a. q.. for example: >>> >>> >>> 3 >>> [2. b = b.2 >>> a [123.append('xtra') p [2. b = 0. p[1] and q really refer to the same object! We'll come back to object semantics later.. >>> 2 >>> >>> [1. 4] q 3. . Release 2. we can write an initial sub-sequence of the Fibonacci series as follows: >>> . .. 18 Chapter 3. 1 1 2 3 5 8 # Fibonacci series: # the sum of two elements defines the next a.1 Note that in the last example..
In Python. . empty sequences are false. • The print statement writes the value of the expression(s) it is given. >= (greater than or equal to) and != (not equal to). b = b.. in fact any sequence. so you have to type a tab or space(s) for each indented line. b = 0. so you can format things nicely. zero is false. First Steps Towards Programming 19 . like this: >>> i = 256*256 >>> print 'The value of i is'. 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 Note that the interpreter inserts a newline before it prints the next prompt if the last line was not completed. > (greater than). a+b . i The value of i is 65536 A trailing comma avoids the newline after the output: >>> a.2 • The first line contains a multiple assignment: the variables a and b simultaneously get the new values 0 and 1. 1 >>> while b < 1000: . <= (less than or equal to)..Python Tutorial. In practice you will prepare more complicated input for Python with a text editor. Release 2. like in C.. • The while loop executes as long as the condition (here: b < 10) remains true. The test used in the example is a simple comparison. Note that each line within a basic block must be indented by the same amount.. • The body of the loop is indented: indentation is Python's way of grouping statements. The right-hand side expressions are evaluated from the left to the right.2.6. Strings are printed without quotes.. print b. On the last line this is used again. == (equal to). Python does not (yet!) provide an intelligent input line editing facility. demonstrating that the expressions on the right-hand side are all evaluated first before any of the assignments take place. The condition may also be a string or list value. When a compound statement is entered interactively.. The standard comparison operators are written the same as in C: < (less than). most text editors have an auto-indent facility. it must be followed by a blank line to indicate completion (since the parser cannot guess when you have typed the last line). 3. any nonzero integer value is true. It differs from just writing the expression you want to write (as we did earlier in the calculator examples) in the way it handles multiple expressions and strings. anything with a non-zero length is true. and a space is inserted between items. a.
Python Tutorial.2 20 Chapter 3. An Informal Introduction to Python . Release 2.6.
.. An if .. a = ['cat'. and the else part is optional. in the order that they appear in the sequence... or giving the user the ability to define both the iteration step and halting condition (as C).. Rather than always iterating over an arithmetic progression of numbers (like in Pascal)... print 'Negative changed to zero' . 'window'. print 'Zero' ..... Python's for statement iterates over the items of any sequence (a list or a string). print 'Single' .. elif x == 0: ... and is useful to avoid excessive indentation. Python knows the usual control flow statements known from other languages.2 for Statements The for statement in Python differs a bit from what you may be used to in C or Pascal. x = 0 . The keyword 'elif' is short for 'else if'.. sequence is a substitute for the switch or case statements found in other languages.. else: . elif . 4.. For example (no pun intended): >>> # Measure some strings: .1 if Statements Perhaps the most well-known statement type is the if statement.. elif x == 1: . elif ...CHAPTER FOUR MORE CONTROL FLOW TOOLS Besides the while statement just introduced. More There can be zero or more elif parts.. with some twists... 'defenestrate'] >>> for x in a: ... print x... 4. len(x) 21 . print 'More' . For example: >>> x = int(raw_input("Please enter an integer: ")) Please enter an integer: 42 >>> if x < 0: .
4. -30) [-10. a[i] .. 8. -100. 9] >>> range(-10.3 The range() Function If you do need to iterate over a sequence of numbers. if len(x) > 6: a. It generates lists containing arithmetic progressions: >>> range(10) [0. If you need to modify the list you are iterating over (for example. 8. 1. 'little'. 7. sometimes this is called the 'step'): >>> range(5. 6. x) . >>> a ['defenestrate'. the built-in function range() comes in handy.. you can combine range() and len() as follows: >>> a = ['Mary'. The slice notation makes this particularly convenient: >>> for x in a[:]: # make a slice copy of the entire list ..insert(0. 9] >>> range(0. 2. 3. It is possible to let the range start at another number.. 10. the legal indices for items of a sequence of length 10. it is convenient to use the enumerate() function. 10) [5. to duplicate selected items) you must iterate over a copy. cat 3 window 6 defenestrate 12 It is not safe to modify the sequence being iterated over in the loop (this can only happen for mutable sequence types. -40. range(10) generates a list of 10 values. see Looping Techniques. 22 Chapter 4.. 'defenestrate'] 4. 7.6. More Control Flow Tools . or to specify a different increment (even negative. 6.2 . -70] To iterate over the indices of a sequence. 3) [0. 3. 5.. 'a'. such as lists). 'had'. 'lamb'] >>> for i in range(len(a)): . 'window'... 0 Mary 1 had 2 a 3 little 4 lamb In most such cases.Python Tutorial. Release 2. print i. 9] The given end point is never part of the generated list. however.. 'cat'. 6..
breaks out of the smallest enclosing for or while loop. print n..2 4. pass # Busy-wait for keyboard interrupt (Ctrl+C) . n/x . For example: >>> while True: .. Another place pass can be used is as a place-holder for a function or conditional body when you are working on new code. print n. continues with the next iteration of the loop.... 'equals'. else: ...6. 4...... Release 2. and else Clauses on Loops The break statement.. 2 is a prime number 3 is a prime number 4 equals 2 * 2 5 is a prime number 6 equals 2 * 3 7 is a prime number 8 equals 2 * 4 9 equals 3 * 3 4. allowing you to keep thinking at a more abstract level..4 break and continue Statements. pass .. # loop fell through without finding a factor .. break and continue Statements.Python Tutorial.. pass # Remember to implement this! . but not when the loop is terminated by a break statement. n): .4.. which searches for prime numbers: >>> for n in range(2. it is executed when the loop terminates through exhaustion of the list (with for) or when the condition becomes false (with while).. break .. if n % x == 0: ... and else Clauses on Loops 23 . The pass is silently ignored: >>> def initlog(*args): . 10): . for x in range(2. '*'. x. The continue statement..... like in C.. This is commonly used for creating minimal classes: >>> class MyEmptyClass: . This is exemplified by the following loop. Loop statements may have an else clause. 'is a prime number' . also borrowed from C.5 pass Statements The pass statement does nothing. It can be used when a statement is required syntactically but the program requires no action..
. b = b. even functions without a return statement do return a value.. so make a habit of it. In fact. More Control Flow Tools .. since if a mutable object is passed. thus. . 1 while b < n: print b.. a. this string literal is the function's documentation string. a new local symbol table is created for that call. you might object that fib is not a function but a procedure since it doesn't return a value. b = 0.6 Defining Functions We can create a function that writes the Fibonacci series to an arbitrary boundary: >>> .. (More about docstrings can be found in the section Documentation Strings. then in the global symbol table. More precisely. call by object reference would be a better description. This serves as a general renaming mechanism: >>> fib <function fib at 10042ed0> >>> f = fib >>> f(100) 1 1 2 3 5 8 13 21 34 55 89 Coming from other languages. although they may be referenced. This value is called None (it's a built-in name). The execution of a function introduces a new symbol table used for the local variables of the function. then in the local symbol tables of enclosing functions. or docstring. instead of printing it: 1 Actually.. ..2 4. whereas variable references first look in the local symbol table.. The value of the function name has a type that is recognized by the interpreter as a user-defined function.6. .. The actual parameters (arguments) to a function call are introduced in the local symbol table of the called function when it is called. and must be indented. This value can be assigned to another name which can then also be used as a function. all variable assignments in a function store the value in the local symbol table.""" a. or to let the user interactively browse through code.. 1 When a function calls another function. and finally in the table of built-in names. it's good practice to include docstrings in code that you write. Release 2.) There are tools which use docstrings to automatically produce online or printed documentation.Python Tutorial.. Writing the value None is normally suppressed by the interpreter if it would be the only value written. a+b # Now call the function we just defined: fib(2000) 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 The keyword def introduces a function definition. . . You can see it if you really want to using print: >>> fib(0) >>> print fib(0) None It is simple to write a function that returns a list of the numbers of the Fibonacci series. 24 Chapter 4. 1 1 def fib(n): # write Fibonacci series up to n """Print a Fibonacci series up to n. not the value of the object). >>> .. global variables cannot be directly assigned a value within a function (unless named in a global statement).. arguments are passed using call by value (where the value is always an object reference. albeit a rather boring one. The first statement of the function body can optionally be a string literal. It must be followed by the function name and the parenthesized list of formal parameters. A function definition introduces the function name in the current symbol table. the caller will see any changes the callee makes to it (items inserted into a list). Thus. The statements that form the body of the function start at the next line..
please!'): while True: ok = raw_input(prompt) if ok in ('y'. In this example it is equivalent to result = result + [b].2 >>> . Release 2. For example: def ask_ok(prompt. a+b return result f100 = fib2(100) # call it f100 # write the result 1.. A method is a function that 'belongs' to an object and is named obj. 55. def fib2(n): # return Fibonacci series up to n """Return a list containing the Fibonacci series up to n. 1 while b < n: result.7 More on Defining Functions It is also possible to define functions with a variable number of arguments.7. return without an expression argument returns None. complaint='Yes or no. 'nop'.. 4. demonstrates some new Python features: • The return statement returns with a value from a function.. 'ye'. This creates a function that can be called with fewer arguments than it is defined to allow... Different types define different methods.. 21. . it adds a new element at the end of the list. 13.""" result = [] a...append(b) # see below a. Falling off the end of a function also returns None... (It is possible to define your own object types and methods. .) The method append() shown in the example is defined for list objects.. This example also introduces the in keyword..1 Default Argument Values The most useful form is to specify a default value for one or more arguments. but more efficient. 2). as usual. • The statement result. b = b. 3. retries=4. More on Defining Functions 25 .. which can be combined. 4.methodname. 34..1 if retries < 0: raise IOError. The default values are evaluated at the point of function definition in the defining scope. . Methods of different types may have the same name without causing ambiguity. 'yes'): return True if ok in ('n'. .7. >>> >>> [1. and methodname is the name of a method that is defined by the object's type. 8. 'nope'): return False retries = retries . .6. There are three forms.. 5. 2. as discussed later in this tutorial. .append(b) calls a method of the list object result. b = 0. This tests whether or not a sequence contains a certain value. 89] This example. where obj is some object (this may be an expression). using classes. 'no'..Python Tutorial. . 'refusenik user' print complaint This function can be called either like this: ask_ok('Do you really want to quit?') or like this: ask_ok('OK to overwrite the file?'. so that 4.
This makes a difference when the default is a mutable object such as a list. 'bereft of life'. voltage. Important warning: The default value is evaluated only once.2 i = 5 def f(arg=i): print arg i = 6 f() will print 5. 2] [1. 3] If you don't want the default to be shared between subsequent calls. 'jump') 26 Chapter 4. the following function: def parrot(voltage. For instance. state. type print "-. "volts through it. "!" could be called in any of the following ways: parrot(1000) parrot(action = 'VOOOOOM'. dictionary. voltage = 1000000) parrot('a thousand'. action='voom'. action.Python Tutorial." print "-. state='a stiff'.6. L=[]): L. type='Norwegian Blue'): print "-. For example. Release 2. or instances of most classes.2 Keyword Arguments Functions can also be called using keyword arguments of the form keyword = value. L=None): if L is None: L = [] L. 2. you can write the function like this instead: def f(a.This parrot wouldn't". state = 'pushing up the daisies') parrot('a million'.7.It's". the following function accumulates the arguments passed to it on subsequent calls: def f(a.append(a) return L 4. print "if you put".Lovely plumage. the". More Control Flow Tools .append(a) return L print f(1) print f(2) print f(3) This will print [1] [1.
.Do you have any Limburger ? -. ":". keywords[kw] It could be called like this: cheeseshop("Limburger". ---------------------------------------client : John Cleese shopkeeper : Michael Palin sketch : Cheese Shop Sketch 4.Do you have any".2 but the following calls would all be invalid: parrot() parrot(voltage=5. sir..) For example. in ? TypeError: function() got multiple values for keyword argument 'a' When a final formal parameter of the form **name is present. line 1. No argument may receive a value more than once — formal parameter names corresponding to positional arguments cannot be used as keywords in the same calls. It's really very. pass . Here's an example that fails due to this restriction: >>> def function(a): .6. sketch="Cheese Shop Sketch") and of course it would print: -. This may be combined with a formal parameter of the form *name (described in the next subsection) which receives a tuple containing the positional arguments beyond the formal parameter list. a=0) Traceback (most recent call last): File "<stdin>". It's not important whether a formal parameter has a default value or not.I'm sorry. **keywords): print "-.I'm sorry.".keys() keys. *arguments. it receives a dictionary (see Mapping Types — dict (in The Python Library Reference)) containing all keyword arguments except for those corresponding to a formal parameter. (*name must occur before **name. we're all out of Limburger It's very runny. "?" print "-. we're all out of". where the keywords must be chosen from the formal parameter names.". "It's really very. More on Defining Functions 27 . sir.0.sort() for kw in keys: print kw. Release 2. voltage=220) parrot(actor='John Cleese') # # # # required argument missing non-keyword argument following keyword duplicate value for argument unknown keyword In general. "It's very runny.7.Python Tutorial. 'dead') parrot(110. sir. kind for arg in arguments: print arg print "-" * 40 keys = keywords. if we define a function like this: def cheeseshop(kind. VERY runny. sir. VERY runny. kind. client="John Cleese". shopkeeper='Michael Palin'. an argument list must have any positional arguments followed by any keyword arguments... >>> function(0.
2 Note that the sort() method of the list of keyword argument names is called before printing the contents of the keywords dictionary.. if this is not done.write(separator.. . action. print "-. print "E's". >>> d = {"voltage": "four million". the least frequently used option is to specify that a function can be called with an arbitrary number of arguments. separator.. they are just syntactic sugar for a normal function definition.7. write the function call with the *-operator to unpack the arguments out of a list or tuple: >>> [3. action='voom'): .4 Unpacking Argument Lists The reverse situation occurs when the arguments are already in a list or tuple but need to be unpacked for a function call requiring separate positional arguments.. Lambda forms can be used wherever function objects are required. voltage. a few features commonly found in functional programming languages like Lisp have been added to Python. print "if you put". b: a+b. Before the variable number of arguments.7. 5] args = [3. More Control Flow Tools . range(3.join(args)) 4.7. *args): file. They are syntactically restricted to a single expression. lambda forms can reference variables from the containing scope: >>> def make_incrementor(n): . With the lambda keyword. If they are not available separately..This parrot wouldn't".". Release 2. 6) 4. state. For instance.. >>> f = make_incrementor(42) >>> f(0) 28 Chapter 4. def write_multiple_items(file. >>> >>> [3.6.. zero or more normal arguments may occur. small anonymous functions can be created. 4.. state='a stiff'. . Semantically. 5] # normal call with separate arguments # call with arguments unpacked from a list In the same fashion... E's bleedin' demised 4. dictionaries can deliver keyword arguments with the **-operator: >>> def parrot(voltage.. 6] range(*args) 4. "!" . "volts through it.Python Tutorial.5 Lambda Forms By popular demand. Like nested function definitions. "action": "VOOM"} >>> parrot(**d) -.This parrot wouldn't VOOM if you put four million volts through it.3 Arbitrary Argument Lists Finally. the order in which the arguments are printed is undefined. Here's a function that returns the sum of its two arguments: lambda a.. the built-in range() function expects separate start and stop arguments. return lambda x: x + n . "state": "bleedin' demised". These arguments will be wrapped up in a tuple (see Tuples and Sequences).
""" . since these are available by other means (except if the name happens to be a verb describing a function's operation). visually separating the summary from the rest of the description. and no tabs.. The following lines should be one or more paragraphs describing the object's calling conventions. some are more readable than others.7.. . PEP 8 has emerged as the style guide that most projects adhere to. The first line should always be a short. Making it easy for others to read your code is always a good idea. Release 2. This is done using the following convention. Every Python developer should read it at some point. it promotes a very readable and eye-pleasing coding style. . Here is an example of a multi-line docstring: >>> def my_function(): . etc. the second line should be blank. Intermezzo: Coding Style 29 . 4.. For brevity. it is a good time to talk about coding style. really.. If there are more lines in the documentation string. and adopting a nice coding style helps tremendously for that.2 42 >>> f(1) 43 4. No.8.. For Python. Equivalence of whitespace should be tested after expansion of tabs (to 8 spaces.8 Intermezzo: Coding Style Now that you are about to write longer.__doc__ Do nothing..) Whitespace "equivalent" to this indentation is then stripped from the start of all lines of the string. so tools that process documentation have to strip indentation if desired. its side effects. Lines that are indented less should not occur.6. 4 spaces are a good compromise between small indentation (allows greater nesting depth) and large indentation (easier to read). but if they occur all their leading whitespace should be stripped... pass . The Python parser does not strip indentation from multi-line string literals in Python. . really.Python Tutorial. >>> print my_function. it should not explicitly state the object's name or type. it doesn't do anything. This line should begin with a capital letter and end with a period. and are best left out.. but document it. more complex pieces of Python. here are the most important points extracted for you: • Use 4-space indentation.. """Do nothing. concise summary of the object's purpose. but document it. it doesn't do anything.6 Documentation Strings There are emerging conventions about the content and formatting of documentation strings.. No. • Wrap lines so that they don't exceed 79 characters. (We can't use the first line since it is generally adjacent to the string's opening quotes so its indentation is not apparent in the string literal.. The first non-blank line after the first line of the string determines the amount of indentation for the entire documentation string. normally). formatted) in different styles. Tabs introduce confusion. 4. Most languages can be written (or more concise.
Python Tutorial. the convention is to use CamelCase for classes and lower_case_with_underscores for functions and methods. More Control Flow Tools . Plain ASCII works best in any case. put comments on a line of their own. • Name your classes and functions consistently. • Don't use fancy encodings if your code is meant to be used in international environments. • Use blank lines to separate functions and classes.2 This helps users with small displays and makes it possible to have several code files side-by-side on larger displays. 4).6. 30 Chapter 4. but not directly inside bracketing constructs: a = f(1. • When possible. Always use self as the name for the first method argument (see A First Look at Classes for more on classes and methods). 2) + g(3. • Use docstrings. Release 2. • Use spaces around operators and after commas. and larger blocks of code inside functions.
You will see this notation frequently in the Python Library Reference.1 More on Lists The list data type has some more methods. 5. 333.insert(2.25).append(x). a. and return it.count(66. 1234. x) is equivalent to a. in place. = L. equivalent to a[len(a):] = [x]. 333. It is an error if there is no such item. pop([i]) Remove the item at the given position in the list.pop() removes and returns the last item in the list. sort() Sort the items of the list. If no index is specified.insert(0. It is an error if there is no such item. a. x) Insert an item at a given position.count('x') 0 a. so a. Here are all of the methods of list objects: append(x) Add an item to the end of the list.insert(len(a). in place. remove(x) Remove the first item from the list whose value is x.count(333). not that you should type square brackets at that position.CHAPTER FIVE DATA STRUCTURES This chapter describes some things you've learned about already in more detail. reverse() Reverse the elements of the list. and adds some new things as well. a.25. -1) 31 . and a. 1.) index(x) Return the index in the list of the first item whose value is x. The first argument is the index of the element before which to insert. count(x) Return the number of times x appears in the list. equivalent to a[len(a):] insert(i. An example that uses most of the list methods: >>> >>> 2 1 >>> a = [66.5] print a. extend(L) Extend the list by appending all the items in the given list. x) inserts at the front of the list. (The square brackets around the i in the method signature denote that the parameter is optional.
1234. where the last element added is the first element retrieved ("last-in. first-out"). >>> 6 >>> 5 >>> [3. 1234. use pop() with 0 as the index.append("Terry") # Terry arrives >>> queue.5. 6.2 >>> a.5. 6] stack. "Michael"] >>> queue. 1234.5] 5. 333. -1. 1234.Python Tutorial. use pop() without an explicit index.append(333) >>> a [66. use append(). where the first element added is the first element retrieved ("first-in.1 Using Lists as Stacks The list methods make it very easy to use a list as a stack. To retrieve an item from the top of the stack. To add an item to the top of the stack. 333] >>> a.reverse() >>> a [333.pop(0) 'Eric' >>> queue.append(7) stack 4.index(333) 1 >>> a. 333. -1. -1. first-out").2 Using Lists as Queues You can also use a list conveniently as a queue. stack = [3.sort() >>> a [-1.pop() stack 4.remove(333) >>> a [66. 333.1. To retrieve an item from the front of the queue.append(6) stack.pop() stack 4] 5. use append(). 333. 1. 5. 5] stack. 1.6.25.pop(0) 'John' >>> queue ['Michael'. 66. For example: >>> queue = ["Eric". 5. 333.25. 333. 7] stack.pop() stack. Data Structures .25. "John". Release 2. To add an item to the back of the queue. 4. 'Terry'. 333] >>> a.25] >>> a.5. >>> 7 >>> [3. 66. 1. 'Graham'] 32 Chapter 5.1. 1.append("Graham") # Graham arrives >>> queue. For example: >>> >>> >>> >>> [3.
its value is returned. 10. In this case the starting value is returned for an empty sequence. then on the result and the next item.2 5. 8. For example.. 1000] More than one sequence may be passed.. then to the result and the next item. to compute some primes: >>> def f(x): return x % 2 != 0 and x % 3 != 0 .. 11)) [1. >>> reduce(add.. and reduce().1. the function must then have as many arguments as there are sequences and is called with the corresponding item from each sequence (or None if some sequence is shorter than another). seq = range(8) def add(x. 14] reduce(function. 0) . if the sequence is empty. For example: >>> >>> .. 4. 27. and so on. 11)) 55 >>> sum([]) 0 5. 512. >>> map(cube. >>> sum(range(1. 11.y): return x+y . filter(function. If sequence is a string or tuple. range(1.. it is always a list. 12. y): return x+y map(add. For example.. sequence) returns a sequence consisting of those items from the sequence for which function(item) is true. and the function is first applied to the starting value and the first sequence item.6. 64. seq.y): return x+y . 13. 6. 216. seq) 2. sequence) calls function(item) for each of the sequence's items and returns a list of the return values.. to compute some cubes: >>> def cube(x): return x*x*x .1. 23] map(function. map(). 25)) [5. Release 2.3 Functional Programming Tools There are three built-in functions that are very useful when used with lists: filter(). 19. 11)) 55 If there's only one item in the sequence. >>> def sum(seq): . 125. def add(x. return reduce(add.. 343. seq. 7. 17. For example. sequence) returns a single value constructed by calling the binary function function on the first two items of the sequence. >>> [0.Python Tutorial. and so on.. A third argument can be passed to indicate the starting value... 729. 8. otherwise.. an exception is raised. >>> filter(f. range(1. More on Lists 33 . the result will be of the same type. to compute the sum of the numbers 1 through 10: >>> def add(x.. For example. range(2.
They are a powerful tool but – like all powerful tools – they need to be used carefully. ' loganberry '. i)) for i in range(1. list comprehensions can be nested. -54] List comprehensions are much more flexible than map() and can be applied to complex expressions and nested functions: >>> [str(round(355/113. 16).1416'. The resulting list definition tends often to be clearer than lists built using those constructs. Data Structures .6)] ['3. -54] >>> [x+y for x in vec1 for y in vec2] [6.2 Don't use this example's definition of sum(): since summing numbers is such a common need. 12. in ? [x.3. 12. '3. 16. 16].6. [6. -3] >>> [vec1[i]*vec2[i] for i in range(len(vec1))] [8. 4. '3.1.0. 10.Python Tutorial. x**2 for x in vec] ^ SyntaxError: invalid syntax >>> [(x. '3. (4. a built-in function sum(sequence) is already provided. filter() and/or lambda. 7. 4].4 List Comprehensions List comprehensions provide a concise way to create lists without resorting to use of map(). -36.5 Nested List Comprehensions If you've got the stomach for it. 9.1'.x**2] for x in vec] [[2. Each list comprehension consists of an expression followed by a for clause. 24. 6] >>> [3*x for x in vec] [6. '3. 36]] >>> [x. -9] >>> [x*y for x in vec1 for y in vec2] [8.strip() for weapon in freshfruit] ['banana'. one list per row: 34 Chapter 5. 12. 18] >>> [3*x for x in vec if x < 2] [] >>> [[x.14'. [4. -5. New in version 2.14159'] 5. Consider the following example of a 3x3 matrix held as a list containing three lists. 4. >>> freshfruit = [' banana'. (6. it must be parenthesized. The result will be a list resulting from evaluating the expression in the context of the for and if clauses which follow it. 'loganberry'. x**2) for x in vec] [(2.1. 4). -18.parens required for tuples File "<stdin>". 18. if at all. 36)] >>> vec1 = [2. 5. If the expression would evaluate to a tuple. 3. then zero or more for or if clauses. x**2 for x in vec] # error . line 1. 6] >>> vec2 = [4.142'. 18] >>> [3*x for x in vec if x > 3] [12. 6. 5. 8. and works exactly like this. -7. Release 2. 'passion fruit'] >>> vec = [2. 'passion fruit '] >>> [weapon.
5. The del statement can also be used to remove slices from a list or clear the entire list (which we did earlier by assignment of an empty list to the slice). The del statement 35 . 9]. .5] del a[:] a del can also be used to delete entire variables: >>> del a Referencing the name a hereafter is an error (at least until another value is assigned to it). 4. 1234. 333. 9]] Special care has to be taken for the nested list comprehension: To avoid apprehension when nesting list comprehensions. 66. 5.2. [7. 2. [1.2 The del statement There is a way to remove an item from a list given its index instead of its value: the del statement. 1234.25. ] Now. 333. >>> >>> [1. (2.. 5. Release 2. 6.. 4. 8]. 1. 3].. .25..5] del a[0] a 66. 6]. 333.6. if you wanted to swap rows and columns. (3. print In real world.25.5] del a[2:4] a 66. [4. . 1. [2. 8).. 1. For example: >>> >>> >>> [1. >>> >>> [] a = [-1.2 >>> mat = [ . The zip() function would do a great job for this use case: >>> zip(*mat) [(1... 5. 333.. 8. 6. 7). you could use a list comprehension: >>> print [[row[i] for row in mat] for i in [0. A more verbose version of this snippet shows the flow explicitly: for i in [0. 7].Python Tutorial. 1234. We'll find other uses for del later. read from right to left. 5. 2]] [[1. 2]: for row in mat: print row[i]. 9)] See Unpacking Argument Lists for details on the asterisk in this line. you should prefer builtin functions to complex flow statements. [3. This differs from the pop() method which returns a value.
Python Tutorial, Release 2.6.2
5.3 Tuples and Sequences
We saw that lists and strings have many common properties, such as indexing and slicing operations. They are two examples of sequence data types (see Sequence Types — str, unicode, list, tuple, buffer, xrange (in The Python Library Reference)). Since Python is an evolving language, other sequence data types may be added. There is also another standard sequence data type: the tuple. A tuple consists of a number of values separated by commas, for instance: >>> t = 12345, 54321, 'hello!' >>> t[0] 12345 >>> t (12345, 54321, 'hello!') >>> # Tuples may be nested: ... u = t, (1, 2, 3, 4, 5) >>> u ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5)) As you see, on output tuples are always enclosed in parentheses, so that nested tuples are interpreted correctly; they may be input with or without surrounding parentheses, although often parentheses are necessary anyway (if the tuple is part of a larger expression). Tuples have many uses. For example: (x, y) coordinate pairs, employee records from a database, etc. Tuples, like strings, are immutable: it is not possible to assign to the individual items of a tuple (you can simulate much of the same effect with slicing and concatenation, though). It is also possible to create tuples which contain mutable objects, such as lists. A special problem is the construction of tuples containing 0 or 1 items: the syntax has some extra quirks to accommodate these. Empty tuples are constructed by an empty pair of parentheses; a tuple with one item is constructed by following a value with a comma (it is not sufficient to enclose a single value in parentheses). Ugly, but effective. For example: >>> empty = () >>> singleton = 'hello', >>> len(empty) 0 >>> len(singleton) 1 >>> singleton ('hello',)
# <-- note trailing comma
The statement t = 12345, 54321, 'hello!' is an example of tuple packing: the values 12345, 54321 and 'hello!' are packed together in a tuple. The reverse operation is also possible: >>> x, y, z = t This is called, appropriately enough, sequence unpacking and works for any sequence on the right-hand side. Sequence unpacking requires the list of variables on the left to have the same number of elements as the length of the sequence. Note that multiple assignment is really just a combination of tuple packing and sequence unpacking.
Chapter 5. Data Structures
Python Tutorial, Release 2.6.2
5.4 Sets
Python also includes a data type for sets. A set is an unordered collection with no duplicate elements. Basic uses include membership testing and eliminating duplicate entries. Set objects also support mathematical operations like union, intersection, difference, and symmetric difference. Here is a brief demonstration: >>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana'] >>> fruit = set(basket) # create a set without duplicates >>> fruit set(['orange', 'pear', 'apple', 'banana']) >>> 'orange' in fruit # fast membership testing True >>> 'crabgrass' in fruit False >>> # Demonstrate set operations on unique letters from two words ... >>> a = set('abracadabra') >>> b = set('alacazam') >>> a # unique letters in a set(['a', 'r', 'b', 'c', 'd']) >>> a - b # letters in a but not in b set(['r', 'd', 'b']) >>> a | b # letters in either a or b set(['a', 'c', 'r', 'd', 'b', 'm', 'z', 'l']) >>> a & b # letters in both a and b set(['a', 'c']) >>> a ^ b # letters in a or b but not both set(['r', 'd', 'b', 'm', 'z', 'l'])
5.5 Dictionaries
Another useful data type built into Python is the dictionary (see Mapping Types — dict (in The Python Library Reference)). Dictionaries are sometimes found in other languages as "associative memories" or "associative arrays". Unlike sequences, which are indexed by a range of numbers, dictionaries are indexed by keys, which can be any immutable type; strings and numbers can always be keys. Tuples can be used as keys if they contain only strings, numbers, or tuples; if a tuple contains any mutable object either directly or indirectly, it cannot be used as a key. You can't use lists as keys, since lists can be modified in place using index assignments, slice assignments, or methods like append() and extend(). It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary). A pair of braces creates an empty dictionary: {}. Placing a comma-separated list of key:value pairs within the braces adds initial key:value pairs to the dictionary; this is also the way dictionaries are written on output. The main operations on a dictionary are storing a value with some key and extracting the value given the key. It is also possible to delete a key:value pair with del. If you store using a key that is already in use, the old value associated with that key is forgotten. It is an error to extract a value using a non-existent key. The keys() method of a dictionary object returns a list of all the keys used in the dictionary, in arbitrary order (if you want it sorted, just apply the sort() method to the list of keys). To check whether a single key is in the dictionary, use the in keyword. 5.4. Sets 37
Python Tutorial, Release 2.6.2
Here is a small example using a dictionary: >>> tel = {'jack': 4098, 'sape': 4139} >>> tel['guido'] = 4127 >>> tel {'sape': 4139, 'guido': 4127, 'jack': 4098} >>> tel['jack'] 4098 >>> del tel['sape'] >>> tel['irv'] = 4127 >>> tel {'guido': 4127, 'irv': 4127, 'jack': 4098} >>> tel.keys() ['guido', 'irv', 'jack'] >>> 'guido' in tel True The dict() constructor builds dictionaries directly from lists of key-value pairs stored as tuples. When the pairs form a pattern, list comprehensions can compactly specify the key-value list. >>> dict([('sape', 4139), ('guido', 4127), ('jack', 4098)]) {'sape': 4139, 'jack': 4098, 'guido': 4127} >>> dict([(x, x**2) for x in (2, 4, 6)]) # use a list comprehension {2: 4, 4: 16, 6: 36} Later in the tutorial, we will learn about Generator Expressions which are even better suited for the task of supplying key-values pairs to the dict() constructor. When the keys are simple strings, it is sometimes easier to specify pairs using keyword arguments: >>> dict(sape=4139, guido=4127, jack=4098) {'sape': 4139, 'jack': 4098, 'guido': 4127}
5.6 Looping Techniques
When looping through dictionaries, the key and corresponding value can be retrieved at the same time using the iteritems() method. >>> knights = {'gallahad': 'the pure', 'robin': 'the brave'} >>> for k, v in knights.iteritems(): ... print k, v ... gallahad the pure robin the brave When looping through a sequence, the position index and corresponding value can be retrieved at the same time using the enumerate() function. >>> for i, v in enumerate(['tic', 'tac', 'toe']): ... print i, v ... 38 Chapter 5. Data Structures
'blue'] >>> for q. >>> basket = ['apple'. To loop over a sequence in reverse. print 'What is your {0}? It is {1}. a < b == c tests whether a is less than b and moreover b equals c. not has the highest priority and or the lowest.. Comparisons can be chained. What is your quest? It is the holy grail. use the sorted() function which returns a new sorted list while leaving the source unaltered..10. first specify the sequence in a forward direction and then call the reversed() function. print i . 'quest'. this only matters for mutable objects like lists. Release 2. answers): . >>> questions = ['name'. What is your name? It is lancelot. 5. apple banana orange pear 5.'. a in zip(questions..7. As always. and the outcome of a comparison (or of any other Boolean expression) may be negated with not. 'orange'.format(q. between them. a) . The operators is and is not compare whether two objects are really the same object. parentheses can be used to express the desired composition.6...2)): .. which is lower than that of all numerical operators.. 'pear'.. 'apple'.... Comparisons may be combined using the Boolean operators and and or. 'banana'] >>> for f in sorted(set(basket)): . >>> for i in reversed(xrange(1. 'favorite color'] >>> answers = ['lancelot'. The comparison operators in and not in check whether a value occurs (does not occur) in a sequence.2 0 tic 1 tac 2 toe To loop over two or more sequences at the same time.7 More on Conditions The conditions used in while and if statements can contain any operators. These have lower priorities than comparison operators.Python Tutorial. For example. More on Conditions 39 . 'the holy grail'. so that A and not B or C is equivalent to (A and (not B)) or C.. 'orange'. print f . What is your favorite color? It is blue. 9 7 5 3 1 To loop over a sequence in sorted order. the entries can be paired with the zip() function. not just comparisons. All comparison operators have the same priority.
Release 2. ('aa'. 2. and so on. Lexicographical ordering for strings uses the ASCII ordering for individual characters.2 The Boolean operators and and or are so-called short-circuit operators: their arguments are evaluated from left to right. 'Trondheim'. 2. 2. assignment cannot occur inside expressions. For example. If two items to be compared are themselves sequences of the same type. string2. 2) < (1. a string is always smaller than a tuple. If all items of two sequences compare equal. and evaluation stops as soon as the outcome is determined. C programmers may grumble about this. It is possible to assign the result of a comparison or other Boolean expression to a variable. but it avoids a common class of problems encountered in C programs: typing = in an expression when == was intended.0. If one sequence is an initial sub-sequence of the other.0. the lexicographical comparison is carried out recursively. -1) (1. string3 = ''.8 Comparing Sequences and Other Types Sequence objects may be compared to other objects with the same sequence type. When used as a general value and not as a Boolean. 'a'). 2. ('abc'. 4] 'ABC' < 'C' < 'Pascal' < 'Python' (1. 1 Mixed numeric types are compared according to their numeric value. 4) < (1. Some examples of comparisons between sequences of the same type: (1. 'Hammer Dance' >>> non_null = string1 or string2 or string3 >>> non_null 'Trondheim' Note that in Python. 2. The comparison uses lexicographical ordering: first the first two items are compared. A and B and C does not evaluate the expression C. if they are equal. 3. and if they differ this determines the outcome of the comparison. the shorter sequence is the smaller (lesser) one. 2. the sequences are considered equal. 2.0) (1. 4) [1. 1 The rules for comparing objects of different types should not be relied upon. unlike C. >>> string1. 3. 2. 4) Note that comparing objects of different types is legal. the return value of a short-circuit operator is the last evaluated argument.Python Tutorial. so 0 equals 0.0. For example. 3] < [1. Thus. etc. 2. if A and C are true but B is false. 5. The outcome is deterministic but arbitrary: the types are ordered by their name. 2. the next two items are compared. a list is always smaller than a string. 2. 4) (1. 'ab')) < (1. 40 Chapter 5. etc. Data Structures . they may change in a future version of the language.6. 3) < (1. until either sequence is exhausted. 3) == (1.
b = 0. The file name is the module name with the suffix . 1 while b < n: result. A module is a file containing Python definitions and statements. Python has a way to put definitions in a file and use them in a script or in an interactive instance of the interpreter. b = b. a+b return result Now enter the Python interpreter and import this module with the following command: >>> import fibo This does not enter the names of the functions defined in fibo directly in the current symbol table. Using the module name you can access the functions: >>> fibo. Such a file is called a module. 1 while b < n: print b.fib(1000) 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 41 .py appended. it only enters the module name fibo there. For instance.append(b) a. Therefore. if you want to write a somewhat longer program. This is known as creating a script. definitions from a module can be imported into other modules or into the main module (the collection of variables that you have access to in a script executed at the top level and in calculator mode). the module's name (as a string) is available as the value of the global variable __name__. You may also want to use a handy function that you've written in several programs without copying its definition into each program. you may want to split it into several files for easier maintenance. As your program gets longer. use your favorite text editor to create a file called fibo. b = b. Within a module.CHAPTER SIX MODULES If you quit from the Python interpreter and enter it again. the definitions you have made (functions and variables) are lost. a. you are better off using a text editor to prepare the input for the interpreter and running it with that file as input instead.py in the current directory with the following contents: # Fibonacci numbers module def fib(n): # write Fibonacci series up to n a. To support this. b = 0. a+b def fib2(n): # return Fibonacci series up to n result = [] a.
if it's just one module you want to test interactively. These statements are intended to initialize the module. The imported module names are placed in the importing module's global symbol table.Python Tutorial. For example: >>> from fibo import fib.itemname. modname. They are executed only the first time the module is imported somewhere. There is even a variant to import all names that a module defines: >>> from fibo import * >>> fib(500) 1 1 2 3 5 8 13 21 34 55 89 144 233 377 This imports all names except those beginning with an underscore (_). fibo is not defined). fib2 >>> fib(500) 1 1 2 3 5 8 13 21 34 55 89 144 233 377 This does not introduce the module name from which the imports are taken in the local symbol table (so in the example. Modules . for that matter). On the other hand. There is a variant of the import statement that imports names from a module directly into the importing module's symbol table. 55. It is customary but not required to place all import statements at the beginning of a module (or script. 3. you must restart the interpreter – or. 42 Chapter 6. Release 2.fib2(100) [1. Therefore. 34. 6. e. 1 Each module has its own private symbol table. if you know what you are doing you can touch a module's global variables with the same notation used to refer to its functions.1. which is used as the global symbol table by all functions defined in the module. Note: For efficiency reasons.1 Executing modules as scripts When you run a Python module with 1 In fact function definitions are also 'statements' that are 'executed'. 21. 89] >>> fibo. if you change your modules.__name__ 'fibo' If you intend to use a function often you can assign it to a local name: >>> fib = fibo. 5. reload(modulename). each module is only imported once per interpreter session. the execution enters the function name in the module's global symbol table.fib >>> fib(500) 1 1 2 3 5 8 13 21 34 55 89 144 233 377 6. the author of a module can use global variables in the module without worrying about accidental clashes with a user's global variables.6. 2.1 More on Modules A module can contain executable statements as well as function definitions. use reload(). 1.2 >>> fibo. 8. Thus. 13.g. Modules can import other modules.
6.2 python fibo. this is assumed to contain an already-"byte-compiled" version of the module spam. the code is not run: >>> import fibo >>> This is often used either to provide a convenient user interface to a module. but with the __name__ set to "__main__". so a Python module directory can be shared by machines of different architectures.py used to create spam. the resulting spam.1.argv[1])) you can make the file usable as a script as well as an importable module. More on Modules 43 . that is. This will generally be an error. It is not an error if this attempt fails. That means that by adding this code at the end of your module: if __name__ == "__main__": import sys fib(int(sys.py <arguments> the code in the module will be executed. it is important that the script not have the same name as a standard module.pyc. modules are searched in the list of directories given by the variable sys.py is successfully compiled.:/usr/local/lib/python. Actually.path which is initialized from the directory containing the input script (or the current directory).pyc. Note that because the directory containing the script being run is on the search path. on Unix.3 "Compiled" Python files As an important speed-up of the start-up time for short programs that use a lot of standard modules. When PYTHONPATH is not set. you don't need to do anything to create the spam. and the . the interpreter searches for a file named spam.Python Tutorial. See section Standard Modules for more information.dependent default.pyc file is ignored if these don't match. just as if you imported it.py is found. or Python will attempt to load the script as a module when that module is imported.pyc is recorded in spam. The modification time of the version of spam. Normally. or for testing purposes (running the module as a script executes a test suite). because the code that parses the command line only runs if the module is executed as the "main" file: $ python fibo. PYTHONPATH and the installation. The contents of the spam.1. this is usually . the search continues in an installation-dependent default path. if a file called spam. an attempt is made to write the compiled version to spam. Some tips for experts: 6. a list of directory names.py 50 1 1 2 3 5 8 13 21 34 If the module is imported.pyc file are platform independent. if for any reason the file is not written completely. or when the file is not found there.pyc file will be recognized as invalid and thus ignored later. This allows Python programs that know what they're doing to modify or replace the module search path. Whenever spam.py in the current directory. Release 2.pyc file.2 The Module Search Path When a module named spam is imported.pyc exists in the directory where spam. This has the same syntax as the shell variable PATH.1.6. 6. and then in the list of directories specified by the environment variable PYTHONPATH.
When -O is used. it only removes assert statements. you should only use this option if you know what you're doing.pyo file directly on the command line. It is also possible to name a .pyc files (or . You can modify it using standard list operations: >>> import sys >>> sys. Modules .pyc files are ignored and .pyo files is the speed with which they are loaded.ps1 and sys.pyc or .py files are compiled to optimized bytecode. the only thing that's faster about .path is a list of strings that determines the interpreter's search path for modules. 6.pyc or . Thus.6. these provide access to operations that are not part of the core of the language but are nevertheless built in.pyo file. • Passing two -O flags to the Python interpreter (-OO) will cause the bytecode compiler to perform optimizations that could in some rare cases result in malfunctioning programs. the bytecode for the script is never written to a .pyc or . resulting in more compact .pyo files. One particular module deserves some attention: sys. ' >>> sys. the Python Library Reference ("Library Reference" hereafter). Currently only __doc__ strings are removed from the bytecode. the winreg module is only provided on Windows systems.. It is initialized to a default path taken from the environment variable PYTHONPATH.pyo when -O is used) without a file spam.pyo files.py for the same module. the startup time of a script may be reduced by moving most of its code to a module and having a small bootstrap script that imports that module. Since some programs may rely on having these available. Release 2.pyc or .ps1 '>>> ' >>> sys. described in a separate document. which is built into every Python interpreter. either for efficiency or to provide access to operating system primitives such as system calls.path.py file. all bytecode is optimized. . Some modules are built into the interpreter. optimized code is generated and stored in .ps2 '. • The module compileall can create .2 Standard Modules Python comes with a library of standard modules. The optimizer currently doesn't help much.. The set of such modules is a configuration option which also depends on the underlying platform For example.append('/ufs/guido/lib/python') 44 Chapter 6.Python Tutorial. • A program doesn't run any faster when it is read from a .ps1 = 'C> ' C> print 'Yuck!' Yuck! C> These two variables are only defined if the interpreter is in interactive mode.pyo files when -O is used) for all modules in a directory. The variables sys. or from a built-in default if PYTHONPATH is not set.ps2 define the strings used as primary and secondary prompts: >>> import sys >>> sys. • When a script is run by giving its name on the command line. • It is possible to have a file called spam.pyo file than when it is read from a .pyc (or spam. This can be used to distribute a library of Python code in a form that is moderately hard to reverse engineer. The variable sys.2 • When the Python interpreter is invoked with the -O flag.
The dir() Function 45 . 'AssertionError'. 'filter'. they are defined in the standard module __builtin__: >>> import __builtin__ >>> dir(__builtin__) ['ArithmeticError'. 'id'.2 6. 'SystemExit'.fib >>> dir() ['__builtins__'. 'divmod'. 'exit'. 'ValueError'. '__doc__'. 'KeyboardInterrupt'. 3. '__doc__'. 'UserWarning'. 'license'. 'NotImplemented'. '__debug__'. 'sys'] Note that it lists all types of names: variables. 'SyntaxError'. 'RuntimeWarning'. '__file__'. 6. 'UnicodeEncodeError'. 'FutureWarning'. 5] >>> import fibo >>> fib = fibo. 'OSError'. '_'.3 The dir() Function The built-in function dir() is used to find out which names a module defines. 'enumerate'. dir() does not list the names of built-in functions and variables. 'None'. 'warnoptions'] Without arguments. 'frozenset'. 'copyright'. 'hash'. 'OverflowError'. 'apply'. 'MemoryError'. 'fib2'] >>> dir(sys) ['__displayhook__'. 'setdlopenflags'. 'abs'. 'compile'. 'file'. 4. 'int'. 'ps2'. 'globals'. '__name__'. '__import__'. 'float'. 'callstats'. '__name__'. '__excepthook__'. 'argv'. 'ImportError'. 'EnvironmentError'. 'getdefaultencoding'. 'NotImplementedError'. 'hexversion'. 'False'. 'NameError'. 'Exception'. 'exc_type'. 'hasattr'. 'map'. 'stdout'. 'SyntaxWarning'. 'UnicodeError'. 'classmethod'.3. 'ZeroDivisionError'. 'coerce'.6. 'min'. 'TypeError'. sys >>> dir(fibo) ['__name__'. '__doc__'. 'locals'. 'Ellipsis'. 'list'. 'cmp'. 'dict'. 'credits'. 2. 'IOError'. 'UnicodeDecodeError'. 'StopIteration'. 'exc_clear'. 'FloatingPointError'. 'builtin_module_names'. 'getdlopenflags'. 'input'. 'LookupError'. 'issubclass'. 'version_info'. 'setrecursionlimit'. 'WindowsError'. 'prefix'. 'version'. 'intern'. 'long'. '__name__'. 'fibo'. '__stdout__'. 'True'. 'delattr'. 'settrace'. 'path'. 'DeprecationWarning'. 'maxunicode'. 'iter'. 'path_hooks'. 'hex'. 'IndexError'. It returns a sorted list of strings: >>> import fibo. 'AttributeError'. 'RuntimeError'. 'complex'. 'a'. 'setcheckinterval'. 'eval'. 'exec_prefix'. 'setprofile'. 'Warning'. 'buffer'. If you want a list of those. Release 2. 'UnicodeTranslateError'. 'IndentationError'. 'executable'. etc. '__stdin__'. 'dir'. 'getrefcount'. 'isinstance'. 'chr'. modules. 'UnboundLocalError'. 'api_version'. 'KeyError'.Python Tutorial. 'excepthook'. 'maxint'. functions. 'len'. 'displayhook'. 'TabError'. 'modules'. '__stderr__'. 'path_importer_cache'. 'ps1'. 'meta_path'. 'fib'. 'stdin'. 'stderr'. 'SystemError'. 'exit'. 'getrecursionlimit'. 'StandardError'. 'EOFError'. 'exc_info'. 'copyright'. 'PendingDeprecationWarning'. '_getframe'. 'help'. 'callable'. dir() lists the names you have defined currently: >>> a = [1. 'ReferenceError'. 'platform'. 'execfile'. 'fib'. 'bool'. 'byteorder'. 'basestring'. 'max'. 'getattr'.
.py wavread. The __init__.py karaoke.py aiffread. 'quit'.py auread.py auwrite. 'sum'. In the simplest case. 'range'. 'reload'. adding echo. __init__. so in addition you will be writing a never-ending stream of modules to perform these operations. Python searches through the directories on sys.wav. 'round'. this is done to prevent directories with a common name.py surround. filters/ __init__. 'reduce'. effects/ __init__. described later.py files are required to make Python treat the directories as containing packages. Users of the package can import individual modules from the package. 'xrange'. 'ord'. 'property'.py wavwrite.. the module name A. so you may need to create and maintain a growing collection of modules for the conversion between the various file formats. 'reversed'.py can just be an empty file. Release 2.. There are many different sound file formats (usually recognized by their extension.py . 'staticmethod'.py reverse. but it can also execute initialization code for the package or set the __all__ variable. 'open'. Top-level package Initialize the sound package Subpackage for file format conversions Subpackage for sound effects Subpackage for filters When importing the package.py vocoder.py equalizer. 'unicode'. Suppose you want to design a collection of modules (a "package") for the uniform handling of sound files and sound data. 'zip'] 6.6. the use of dotted module names saves the authors of multi-module packages like NumPy or the Python Imaging Library from having to worry about each other's module names. There are also many different operations you might want to perform on sound data (such as mixing. 'tuple'. 'raw_input'. 'str'.. .py echo. 'oct'.. applying an equalizer function. Here's a possible structure for your package (expressed in terms of a hierarchical filesystem): sound/ __init__. from unintentionally hiding valid modules that occur later on the module search path.py formats/ __init__. 'vars'. Just like the use of modules saves the authors of different modules from having to worry about each other's global variable names.py . 'slice'.B designates a submodule named B in a package named A. for example: 46 Chapter 6.path looking for the package subdirectory.aiff. 'repr'. for example: .py aiffwrite.Python Tutorial.4 Packages Packages are a way of structuring Python's module namespace by using "dotted module names". 'sorted'.2 'object'. 'type'. 'setattr'. 'set'.py . 'unichr'. creating an artificial stereo effect).au). 'super'. 'pow'. such as string. For example.. . Modules .
delay=0.PY should be imported as a module echo. output. "reverse"] 6. Release 2. sound. class or variable. delay=0.effects.4. output.echo This loads the submodule sound. If it fails to find it. if not.echo import echofilter Again. The import statement uses the following convention: if a package's __init__. 6. this loads the submodule echo. where the filesystem does not always have accurate information about the case of a filename! On these platforms.py could contain the following code: __all__ = ["echo". this operation does not work very well on Windows platforms. Windows 95 has the annoying practice of showing all file names with a capitalized first letter. atten=4) Yet another variation is to import the desired function or variable directly: from sound. when using syntax like import item. like a function. each item except for the last must be a package. (For example.py code defines a list named __all__.7. it is taken to be the list of module names that should be imported when from package import * is encountered. and imports them all.Python Tutorial.effects. The import statement first tests whether the item is defined in the package.echo. the item can be either a submodule (or subpackage) of the package. the last item can be a module or a package but can't be a class or function or variable defined in the previous item. so it can be used as follows: echo. Packages 47 . the file sounds/effects/__init__. "surround". output. It is up to the package author to keep this list up-to-date when a new version of the package is released. but this makes its function echofilter() directly available: echofilter(input. Echo or ECHO.2 import sound.7. The only solution is for the package author to provide an explicit index of the package.7.effects.effects import echo This also loads the submodule echo. Package authors may also decide not to support it.1 Importing * From a Package Now what happens when the user writes from sound.echofilter(input. It must be referenced with its full name. if they don't see a use for importing * from their package. there is no guaranteed way to know whether a file ECHO.) The DOS 8+3 filename restriction adds another interesting problem for long module names. For example.6.echo. Unfortunately. Contrarily. one would hope that this somehow goes out to the filesystem. delay=0. an ImportError exception is raised.subsubitem.4. or some other name defined in the package.effects.effects import *? Ideally. atten=4) An alternative way of importing the submodule is: from sound. and makes it available without its package prefix. atten=4) Note that when using from package import item.subitem.echofilter(input. finds which submodules are present in the package. it assumes it is a module and attempts to load it.
.3 Packages in Multiple Directories Packages support one more special attribute.echo import sound.5.py) and then imports whatever names are defined in the package..effects import * In this example. 48 Chapter 6.vocoder needs to use the echo module in the sound. If the imported module is not found in the current package (the package of which the current module is a submodule).2 Intra-package References The submodules often need to refer to each other. this is the recommended notation unless the importing module needs to use submodules with the same name from different packages. When packages are structured into subpackages (as with the sound package in the example). import formats from .. if the module sound. since it often causes poorly readable code. it is okay to use it to save typing in interactive sessions. This is initialized to be a list containing the name of the directory holding the package's __init__.effects package.surround from sound. the surround module might use the echo module..) Note that in general the practice of importing * from a module or package is frowned upon. 6. This includes any names defined (and submodules explicitly loaded) by __init__.py. the surround module can simply use import echo or from echo import echofilter. the statement from sound. __path__. you can write explicit relative imports with the from module import name form of import statement.effects has been imported (possibly running any initialization code in __init__. Modules .effects package when the from. the echo and surround modules are imported in the current namespace because they are defined in the sound. These explicit relative imports use leading dots to indicate the current and parent packages involved in the relative import.4. However. you can use absolute imports to refer to submodules of siblings packages. and certain modules are designed to export only names that follow certain patterns. It also includes any submodules of the package that were explicitly loaded by previous import statements.py before the code in that file is executed.filters import equalizer Note that both explicit and implicit relative imports are based on the name of the current module.effects.2 This would mean that from sound. import echo from .effects.effects import echo.effects import * would import the three named submodules of the sound package.filters. Remember. there is nothing wrong with using from Package import specific_submodule! In fact.6. Since the name of the main module is always "__main__". such references are so common that the import statement first looks in the containing package before looking in the standard module search path.import statement is executed. In fact. From the surround module for example.effects import * does not import all submodules from the package sound. This variable can be modified. the import statement looks for a top-level module with the given name. it only ensures that the package sound. Consider this code: import sound. you might use: from . For example. For example.Python Tutorial. in addition to the implicit relative imports described above. 6. Starting with Python 2.4. Thus. Release 2.effects into the current namespace. it can use from sound. If __all__ is not defined. (This also works when __all__ is defined. modules intended for use as the main module of a Python application should always use absolute imports.
4. While this feature is not often needed.Python Tutorial. it can be used to extend the set of modules found in a package. Packages 49 . 6.2 doing so affects future searches for modules and subpackages contained in the package.6. Release 2.
2 50 Chapter 6. Modules .Python Tutorial. Release 2.6.
1) '0. The standard module string contains some useful operations for padding strings to a given column width. using string slicing and concatenation operations you can create any layout you can imagine.' >>> repr(s) "'Hello.25 >>> y = 200 * 200 >>> s = 'The value of x is ' + repr(x) + '. these will be discussed shortly. world. have two distinct representations. such as numbers or structures like lists and dictionaries. the standard output file can be referenced as sys. and y is ' + repr(y) + '. (A third way is using the write() method of file objects.10000000000000001' >>> x = 10 * 3. 7. have the same representation using either function.5. See the Library Reference for more information on this.CHAPTER SEVEN INPUT AND OUTPUT There are several ways to present the output of a program.' >>> str(s) 'Hello.. or written to a file for future use.' >>> print s The value of x is 32. the first way is to do all the string handling yourself. This chapter will discuss some of the possibilities. world..1) '0.'" >>> str(0. of course: how do you convert values to strings? Luckily. str() will return the same value as repr().1' >>> repr(0. Strings and floating point numbers. Some examples: >>> s = 'Hello. For objects which don't have a particular representation for human consumption. There are two ways to format your output.) Often you'll want more control over the formatting of your output than simply printing space-separated values. Many values. world. The str() function is meant to return representations of values which are fairly human-readable. Python has ways to convert any value to a string: pass it to the repr() or str() functions. >>> # The repr() of a string adds string quotes and backslashes: 51 . and y is 40000.format() method.stdout. One question remains. while repr() is meant to generate representations which can be read by the interpreter (or will force a SyntaxError if there is not equivalent syntax).. data can be printed in a human-readable form. in particular.1 Fancier Output Formatting So far we've encountered two ways of writing values: expression statements and the print statement. The second way is to use the str..
.ljust(n)[:n].. These methods do not write anything. 'eggs'))) "(32.zfill(5) '00012' 52 Chapter 7. they just return a new string.. x*x*x) .11): . 'eggs'))" Here are two ways to write a table of squares and cubes: >>> for x in range(1. which would be lying about a value.. repr(x*x). 1 1 1 2 4 8 3 9 27 4 16 64 5 25 125 6 36 216 7 49 343 8 64 512 9 81 729 10 100 1000 (Note that in the first example.... zfill(). as in x..rjust(2).2 .) There is another method..rjust(3). world\n' >>> # The argument to repr() may be any Python object: . There are similar methods ljust() and center().rjust(4) .5. one space between each column was added by the way print works: it always adds spaces between its arguments. (If you really want truncation you can always add a slice operation. .. print repr(x). they don't truncate it.6. ('spam'. print repr(x*x*x).. ('spam'..format(x.) This example demonstrates the rjust() method of string objects. y. x*x.Python Tutorial. repr((x. print '{0:2d} {1:3d} {2:4d}'. 11): . Input and Output .. but return it unchanged. this will mess up your column lay-out but that's usually better than the alternative. If the input string is too long. hello = 'hello. 1 1 1 2 4 8 3 9 27 4 16 64 5 25 125 6 36 216 7 49 343 8 64 512 9 81 729 10 100 1000 >>> for x in range(1.. Release 2. which pads a numeric string on the left with zeros.. world\n' >>> hellos = repr(hello) >>> print hellos 'hello. 40000. # Note trailing comma on previous line .. which right-justifies a string in a field of a given width by padding it with spaces on the left. It understands about plus and minus signs: >>> '12'.
The number in the brackets refers to the position of the object passed into the format method. food='spam'. This can be done by simply passing the dict and using square brackets '[]' to access the keys 7.. other='Georg') The story of Bill. This also greater control over how the value is formatted..pi) The value of PI is approximately 3. . 'Jack': 4098.142.format() method looks like this: >>> print 'We are the {0} who say "{1}!"'. it would be nice if you could reference the variables to be formatted by name instead of by position.items(): .. phone) .zfill(7) '-003.format( .3f}. 'eggs') spam and eggs >>> print '{1} and {0}'.. Manfred..'.'. their values are referred to by using the name of the argument.. Passing an integer after the ':' will cause that field to be a minimum number of characters wide. 'eggs') eggs and spam If keyword arguments are used in the format method. An optional ':' and format specifier can follow the field name.format(math.zfill(5) '3. 'Dcab': 7678} >>> for name. 'Manfred'.2 >>> '-3. >>> import math >>> print 'The value of PI is approximately {0:.14'. and Georg. 'Ni') We are the knights who say "Ni!" The brackets and characters within them (called format fields) are replaced with the objects passed into the format method.6. This is useful for making tables pretty. Fancier Output Formatting 53 .14' >>> '3. Positional and keyword arguments can be arbitrarily combined: >>> print 'The story of {0}. print '{0:10} ==> {1:10d}'.format('spam'.format('knights'.format('spam'.'. The following example truncates the Pi to three places after the decimal.format('Bill'.14159265359' Basic usage of the str. >>> print '{0} and {1}'.Python Tutorial. phone in table. {1}.format(name. >>> print 'This {food} is {adjective}.. adjective='absolutely horrible') This spam is absolutely horrible.: >>> table = {'Sjoerd': 4127. Release 2.1. and {other}. Jack ==> 4098 Dcab ==> 7678 Sjoerd ==> 4127 If you have a really long format string that you don't want to split up..14159265359'.
2 Reading and Writing Files open() returns a file object.3f. Input and Output . >>> f = open('/tmp/workfile'.142. so there are also modes like 'rb'. The mode argument is optional. Sjoerd: 4127. For a complete overview of string formating with str. Dcab: 8637678 This is particularly useful in combination with the new built-in vars() function. Dcab: {Dcab:d}'.format(). The second argument is another string containing a few characters describing the way in which the file will be used. Be very careful to use binary mode when reading and writing such files. 'Dcab': 8637678} >>> print ('Jack: {0[Jack]:d}.' % math. and returns the string resulting from this formatting operation. and 'a' opens the file for appending. mode). 7. and is most commonly used with two arguments: open(filename. which returns a dictionary containing all local variables. it doesn't hurt to append a 'b' to the mode. Windows makes a distinction between text and binary files. 'Dcab: {0[Dcab]:d}'. Release 2. 'Dcab': 8637678} >>> print 'Jack: {Jack:d}. but it'll corrupt binary data like that in JPEG or EXE files. 'w') >>> print f <open file '/tmp/workfile'. Sjoerd: {Sjoerd:d}. Dcab: 8637678 This could also be done by passing the table as keyword arguments with the '**' notation.format() should generally be used.pi The value of PI is approximately 3. 'Jack': 4098. ' . Since str.format(**table) Jack: 4098. any data written to the file is automatically added to the end. mode 'w' at 80a0960> The first argument is a string containing the filename. However. 7. see Format String Syntax (in The Python Library Reference).format() is quite new.1 Old string formatting The % operator can also be used for string formatting.. 'Jack': 4098.6. For example: >>> import math >>> print 'The value of PI is approximately %5. the end-of-line characters in text files are automatically altered slightly when data is read or written. 'r' will be assumed if it's omitted. This behind-the-scenes modification to file data is fine for ASCII text files.. 'r+' opens the file for both reading and writing. On Windows. and 'r+b'.Python Tutorial. Sjoerd: 4127.1. 'wb'. 'b' appended to the mode opens the file in binary mode. 54 Chapter 7. a lot of Python code still uses the % operator.2 >>> table = {'Sjoerd': 4127. More information can be found in the String Formatting Operations (in The Python Library Reference) section. On Unix. so you can use it platformindependently for all binary files. mode can be 'r' when the file will only be read. because this old style of formatting will eventually removed from the language str. Sjoerd: {0[Sjoerd]:d}.: >>> table = {'Sjoerd': 4127. 'w' for only writing (an existing file with the same name will be erased). It interprets the left argument much like a sprintf-style format string to be applied to the right argument.format(table)) Jack: 4098.
and returns the lines from that. a newline character (\n) is left at the end of the string.readline() reads a single line from the file. fast.\n' >>> f. the end of the file has been reached.6. >>> f.readlines() ['This is the first line of the file. f. 'Second line of the file\n'] An alternative approach to reading lines is to loop over the file object. >>> f. Since the two approaches manage line buffering differently.readline() returns an empty string.readline() 'This is the first line of the file. a string containing only a single newline. Only complete lines will be returned. Reading and Writing Files 55 . This makes the return value unambiguous. returning None. while a blank line is represented by '\n'.read() 'This is the entire file.write('This is a test\n') To write something other than a string. but without having to load the entire file in memory. This is the first line of the file. it needs to be converted to a string first: 7.read(size). This is memory efficient. >>> f.readlines() returns a list containing all the lines of data in the file. call f. it's your problem if the file is twice as large as your machine's memory.readline() '' f. the entire contents of the file will be read and returned.read() '' f. size is an optional numeric argument.readline() 'Second line of the file\n' >>> f. When size is omitted or negative. Second line of the file The alternative approach is simpler but does not provide as fine-grained control. if f.read() will return an empty string ("").\n' >>> f. This is often used to allow efficient reading of a large file by lines.Python Tutorial. and leads to simpler code: >>> for line in f: print line. it reads that many bytes from the file and enough more to complete a line. f. >>> f.\n'.2 7.write(string) writes the contents of string to the file.2. and is only omitted on the last line of the file if the file doesn't end in a newline. which reads some quantity of data and returns it as a string. If the end of the file has been reached. Release 2.2. they should not be mixed. If given an optional parameter sizehint. To read a file's contents. Otherwise.1 Methods of File Objects The rest of the examples in this section will assume that a file object called f has already been created. at most size bytes are read and returned.
read() >>> f. or class instances. dictionaries.2. call f. from_what).seek(-3. read_data = f.write(s) f.write('0123456789abcdef') f. even if an exception is raised on the way.tell() returns an integer giving the file object's current position in the file. the string representing the object may have been stored in a file or data. such as isatty() and truncate() which are less frequently used. It is also much shorter than writing equivalent try-finally blocks: >>> with open('/tmp/workfile'. 'r') as f: . This is an amazing module that can take almost any Python object (even some forms of Python code!). things get a lot more complicated. the reference point is selected by the from_what argument. when you want to save more complex data types like lists.2 >>> value = ('the answer'. in ? ValueError: I/O operation on closed file It is good practice to use the with keyword when dealing with file objects.read(1) When you're done with a file.seek(5) # Go to the 6th byte in the file f. Reconstructing the object from the string representation is called unpickling. from_what can be omitted and defaults to 0. 7.seek(offset. and convert it to a string representation. using the beginning of the file as the reference point. Numbers take a bit more effort. To change the file object's position. 2) # Go to the 3rd byte before the end f. since the read() method only returns strings. which takes a string like '123' and returns its numeric value 123.close() to close it and free up any system resources taken up by the open file.. attempts to use the file object will automatically fail.Python Tutorial. use f. or sent over a network connection to some distant machine.close(). Python provides a standard module called pickle. However. Input and Output . Rather than have users be constantly writing and debugging code to save complicated data types. Between pickling and unpickling.. line 1. >>> f. and 2 uses the end of the file as the reference point. consult the Library Reference for a complete guide to file objects. After calling f. 56 Chapter 7. A from_what value of 0 measures from the beginning of the file.6.close() >>> f.read(1) f. measured in bytes from the beginning of the file.read() Traceback (most recent call last): File "<stdin>".closed True File objects have some additional methods. which will have to be passed to a function like int().2 The pickle Module Strings can easily be written to and read from a file. 1 uses the current file position. 42) >>> s = str(value) >>> f. this process is called pickling. 'r+') f. This has the advantage that the file is properly closed after its suite finishes. Release 2. >>> >>> >>> >>> '5' >>> >>> 'd' f = open('/tmp/workfile'. The position is computed from adding offset to a reference point.
dump(x. consult the complete documentation for pickle in the Python Library Reference.) pickle is the standard way to make Python objects which can be stored and reused by other programs or by a future invocation of the same program. f) To unpickle the object again. many authors who write Python extensions take care to ensure that new data types such as matrices can be properly pickled and unpickled. the technical term for this is a persistent object. used when pickling many objects or when you don't want to write the pickled data to a file.load(f) (There are other variants of this. the simplest way to pickle the object takes only one line of code: pickle.Python Tutorial.2.6. Because pickle is so widely used. if f is a file object which has been opened for reading: x = pickle.2 If you have an object x. Release 2. 7. and a file object f that's been opened for writing. Reading and Writing Files 57 .
Input and Output .2 58 Chapter 7. Release 2.6.Python Tutorial.
There are (at least) two distinguishable kinds of errors: syntax errors and exceptions. 8. but if you have tried out the examples you have probably seen some.1 Syntax Errors Syntax errors. line 1. Most exceptions are not handled by programs. File name and line number are printed so you know where to look in case the input came from a script. however. it may cause an error when an attempt is made to execute it. are perhaps the most common kind of complaint you get while you are still learning Python: >>> while True print 'Hello world' File "<stdin>". and result in error messages as shown here: >>> 10 * (1/0) Traceback (most recent call last): File "<stdin>". in ? NameError: name 'spam' is not defined >>> '2' + 2 Traceback (most recent call last): 59 . in ? while True print 'Hello world' ^ SyntaxError: invalid syntax The parser repeats the offending line and displays a little 'arrow' pointing at the earliest point in the line where the error was detected. since a colon (':') is missing before it. 8. The error is caused by (or at least detected at) the token preceding the arrow: in the example. in ? ZeroDivisionError: integer division or modulo by zero >>> 4 + spam*3 Traceback (most recent call last): File "<stdin>".CHAPTER EIGHT ERRORS AND EXCEPTIONS Until now error messages haven't been more than mentioned. also known as parsing errors. the error is detected at the keyword print. line 1. Errors detected during execution are called exceptions and are not unconditionally fatal: you will soon learn how to handle them in Python programs.2 Exceptions Even if a statement or expression is syntactically correct. line 1.
At most one handler will be executed. In general it contains a stack traceback listing source lines. line 1. for example: . Try again. note that a user-generated interruption is signalled by raising the KeyboardInterrupt exception. but allows the user to interrupt the program (using Control-C or whatever the operating system supports). in the form of a stack traceback. and the type is printed as part of the message: the types in the example are ZeroDivisionError. The preceding part of the error message shows the context where the exception happened. TypeError. try: . but need not be true for user-defined exceptions (although it is a useful convention). An except clause may name multiple exceptions as a parenthesized tuple... Release 2... not in other handlers of the same try statement. which asks the user for input until a valid integer has been entered. NameError): . The try statement works as follows..6. Look at the following example. except ValueError: .. the rest of the clause is skipped. >>> while True: .. Standard exception names are built-in identifiers (not reserved keywords). to specify handlers for different exceptions.. A try statement may have more than one except clause. it will not display lines read from standard input.. print "Oops! That was no valid number.2 File "<stdin>". The string printed as the exception type is the name of the built-in exception that occurred.. if no handler is found. 8. The rest of the line provides detail based on the type of exception and what caused it.. Built-in Exceptions (in The Python Library Reference) lists the built-in exceptions and their meanings. • If an exception occurs which does not match the exception named in the except clause. it is an unhandled exception and execution stops with a message as shown above. • If an exception occurs during execution of the try clause. the except clause is skipped and execution of the try statement is finished.. and then execution continues after the try statement. • If no exception occurs. Handlers only handle exceptions that occur in the corresponding try clause.. in ? TypeError: cannot concatenate 'str' and 'int' objects The last line of the error message indicates what happened.. • First. x = int(raw_input("Please enter a number: ")) . NameError and TypeError. the except clause is executed. Then if its type matches the exception named after the except keyword. pass 60 Chapter 8. Errors and Exceptions .. Exceptions come in different types.3 Handling Exceptions It is possible to write programs that handle selected exceptions.. the try clause (the statement(s) between the try and except keywords) is executed." . break .Python Tutorial. however.. This is true for all built-in exceptions. except (RuntimeError. it is passed on to outer try statements..
x. It is useful for code that must be executed if the try clause does not raise an exception. x . For example: for arg in sys. which.args # arguments stored in . Instead..format(errno. For convenience. >>> try: .. 'has'.. 'r') except IOError: print 'cannot open'.args is discouraged.. the preferred use is to pass a single argument to an exception (which can be a tuple if multiple arguments are needed) and have it bound to the message attribute. print inst # __str__ allows args to printed directly .. print 'x ='. sys. also known as the exception's argument. print type(inst) # the exception instance . y = inst # __getitem__ allows args to be unpacked directly . except statement has an optional else clause...txt') s = f.strip()) except IOError as (errno..6. The except clause may specify a variable after the exception name (or tuple). y . But use of ..readlines()). Use this with extreme caution. must follow all except clauses. it may have an associated value.. to serve as a wildcard.Python Tutorial. since it is easy to mask a real programming error in this way! It can also be used to print an error message and then re-raise the exception (allowing a caller to handle the exception as well): import sys try: f = open('myfile. 8. print 'y ='. except statement.exc_info()[0] raise The try .close() The use of the else clause is better than adding additional code to the try clause because it avoids accidentally catching an exception that wasn't raised by the code being protected by the try . 'eggs') . raise Exception('spam'. the exception instance defines __getitem__() and __str__() so the arguments can be accessed or printed directly without having to reference .args.argv[1:]: try: f = open(arg.. Release 2.. when present.args .2 The last except clause may omit the exception name(s). The variable is bound to an exception instance with the arguments stored in instance. When an exception occurs.. arg else: print arg.args.. 'lines' f. One may also instantiate an exception first before raising it and add any attributes to it as desired." except: print "Unexpected error:".. len(f.... The presence and type of the argument depend on the exception type... strerror): print "I/O error({0}): {1}".. Handling Exceptions 61 . print inst.3. except Exception as inst: . strerror) except ValueError: print "Could not convert data to an integer.readline() i = int(s..
Exceptions should typically be derived from the Exception class. but also if they occur inside functions that are called (even indirectly) in the try clause..2 <type 'exceptions. Exception handlers don't just handle exceptions if they occur immediately in the try clause. but there seems to be a growing stylistic preference for the latter.Exception'> ('spam'.. If you need to determine whether an exception was raised but don't intend to handle it. it is printed as the last part ('detail') of the message for unhandled exceptions. line 2.... 'HiThere' Traceback (most recent call last): File "<stdin>". An exception flew by! Traceback (most recent call last): File "<stdin>". raise NameError.. print 'Handling run-time error:'.. For example: 62 Chapter 8.. Alternatively. except ZeroDivisionError as detail: .. >>> try: . Handling run-time error: integer division or modulo by zero 8. 'eggs') x = spam y = eggs If an exception has an argument..4 Raising Exceptions The raise statement allows the programmer to force a specified exception to occur.. the above could be written as raise NameError('HiThere').. Release 2. For example: >>> raise NameError. detail . The optional second argument specifies the exception's argument. x = 1/0 .... a simpler form of the raise statement allows you to re-raise the exception: >>> try: ... 'HiThere' . in ? NameError: HiThere 8.Python Tutorial.6. For example: >>> def this_fails(): . Either form works fine. 'eggs') ('spam'.. Errors and Exceptions . except NameError: . print 'An exception flew by!' .5 User-defined Exceptions Programs may name their own exceptions by creating a new exception class.. line 1.. either directly or indirectly. raise ... in ? NameError: HiThere The first argument to raise names the exception to be raised. this_fails() .
print 'My exception occurred..MyError: 'oops!' In this example.2 >>> class MyError(Exception): ..explanation of the error """ def __init__(self. message): self... the default __init__() of Exception has been overridden. Release 2. value): .next = next self.message = message 8.. expression. return repr(self.. previous.. Attributes: expression -. raise MyError(2*2) .Python Tutorial. The new behavior simply creates the value attribute.expression = expression self. def __str__(self): .previous = previous self.value) . in ? __main__.6.message = message class TransitionError(Error): """Raised when an operation attempts a state transition that's not allowed.5. line 1..explanation of why the specific transition is not allowed """ def __init__(self. def __init__(self.. When creating a module that can raise several distinct errors.. next.. e.attempted new state message -. but are usually kept simple.value = value . message): self. except MyError as e: ... a common practice is to create a base class for exceptions defined by that module. and subclass that to create specific exception classes for different error conditions: class Error(Exception): """Base class for exceptions in this module. This replaces the default behavior of creating the args attribute. >>> try: . value:'. self. often only offering a number of attributes that allow information about the error to be extracted by handlers for the exception.. Exception classes can be defined which do anything any other class can do. My exception occurred.""" pass class InputError(Error): """Exception raised for errors in the input... value: 4 >>> raise MyError. User-defined Exceptions 63 .input expression in which the error occurred message -.. 'oops!' Traceback (most recent call last): File "<stdin>"..value . Attributes: previous -.state at beginning of transition next -.
The TypeError raised by dividing two strings is not handled by the except clause and therefore re-raised after the finally clause has been executed. finally: ." similar to the naming of the standard exceptions. try: . Many standard modules define their own exceptions to report errors that may occur in functions they define. line 2.. 8. Goodbye.6 Defining Clean-up Actions The try statement has another optional clause which is intended to define clean-up actions that must be executed under all circumstances. "1") executing finally clause Traceback (most recent call last): File "<stdin>".. 1) result is 2 executing finally clause >>> divide(2... y): .. When an exception has occurred in the try clause and has not been handled by an except clause (or it has occurred in a except or else clause)... whether an exception has occurred or not. line 1.. print "division by zero!" . For example: >>> try: . print "result is".5): >>> def divide(x.. line 3. world!' .... print "executing finally clause" . in ? KeyboardInterrupt A finally clause is always executed before leaving the try statement..6. The finally clause is also executed "on the way out" when any other clause of the try statement is left via a break.2 Most exceptions are defined with names that end in "Error. world! Traceback (most recent call last): File "<stdin>". Errors and Exceptions .... A more complicated example (having except and finally clauses in the same try statement works as of Python 2. in ? File "<stdin>".. in divide TypeError: unsupported operand type(s) for /: 'str' and 'str' As you can see. else: ..... it is re-raised after the finally clause has been executed. except ZeroDivisionError: . result = x / y .. Release 2. result . 64 Chapter 8.. raise KeyboardInterrupt . >>> divide(2.. 0) division by zero! executing finally clause >>> divide("2". the finally clause is executed in any event.Python Tutorial. finally: .. More information on classes is presented in chapter Classes. continue or return statement. print 'Goodbye..
Look at the following example. Predefined Clean-up Actions 65 .6. 8. This is not an issue in simple scripts.Python Tutorial. the finally clause is useful for releasing external resources (such as files or network connections). the file f is always closed. Other objects which provide predefined clean-up actions will indicate this in their documentation.7. The with statement allows objects like files to be used in a way that ensures they are always cleaned up promptly and correctly. regardless of whether or not the operation using the object succeeded or failed. but can be a problem for larger applications.2 In real world applications. regardless of whether the use of the resource was successful. with open("myfile. even if a problem was encountered while processing the lines.txt"): print line The problem with this code is that it leaves the file open for an indeterminate amount of time after the code has finished executing.txt") as f: for line in f: print line After the statement is executed. 8. for line in open("myfile.7 Predefined Clean-up Actions Some objects define standard clean-up actions to be undertaken when the object is no longer needed. Release 2. which tries to open a file and print its contents to the screen.
6.Python Tutorial. Release 2. Errors and Exceptions .2 66 Chapter 8.
For example. (I would use Modula-3 terms. classes themselves are objects. strings. since aliases behave like pointers in some respects. which is provided implicitly by the call. It is a mixture of the class mechanisms found in C++ and Modula-3. and a method can call the method of a base class with the same name. I will make occasional use of Smalltalk and C++ terms.) can be redefined for class instances. As in Modula-3. albeit in the wider sense of the word: in Python. and multiple names (in multiple scopes) can be bound to the same object. and all member functions are virtual. and most types representing entities outside the program (files. 9. tuples). a derived class can override any methods of its base class or classes. built-in types can be used as base classes for extension by the user. and you need to know how scopes and namespaces work to fully understand what's going on. In C++ terminology. and can be safely ignored when dealing with immutable basic types (numbers.2 Python Scopes and Name Spaces Before introducing classes. dictionaries. This is usually not appreciated on a first glance at Python. Let's begin with some definitions. Unlike C++ and Modula-3. Also. However. This is known as aliasing in other languages." The most important features of classes are retained with full power. but I expect that few readers have heard of it. etc. there are no shorthands for referencing the object's members from its methods: the method function is declared with an explicit first argument representing the object. Class definitions play some neat tricks with namespaces. most built-in operators with special syntax (arithmetic operators. This provides semantics for importing and renaming. There are no special constructors or destructors. 67 . all class members (including the data members) are public. all data types are objects. passing an object is cheap since only a pointer is passed by the implementation.CHAPTER NINE CLASSES Python's class mechanism adds classes to the language with a minimum of new syntax and semantics. knowledge about this subject is useful for any advanced Python programmer. and if a function modifies an object passed as an argument.). but rather rely on the politeness of the user not to "break into the definition. classes in Python do not put an absolute barrier between definition and user. Objects can contain an arbitrary amount of private data.1 A Word About Terminology Lacking universally accepted terminology to talk about classes. subscripting etc.) Objects have individuality. I first have to tell you something about Python's scope rules. 9. windows. like in C++ but unlike in Modula-3. Incidentally. As is true for modules. the caller will see the change — this eliminates the need for two different argument passing mechanisms as in Pascal. As in Smalltalk. aliasing has an (intended!) effect on the semantics of Python code involving mutable objects such as lists. This is usually used to the benefit of the program. since its object-oriented semantics are closer to those of Python than C++. however: the class inheritance mechanism allows multiple base classes.
the language definition is evolving towards static name resolution. I use the word attribute for any name following a dot — for example. the global names in a module. 1 68 Chapter 9. Module objects have a secret read-only attribute called __dict__ which returns the dictionary used to implement the module's namespace. In the latter case. but that's normally not noticeable in any way (except for performance). at "compile" time.real. no matter from where or by what alias the function is called. which is searched first. contains the current module's global names. are considered part of a module called __main__. (The built-in names actually also live in a module. the actual search for names is done dynamically. the namespaces of any enclosing functions. then all references and assignments go directly to the middle scope containing the module's global names. Release 2. contains the local names. Class definitions place yet another namespace in the local scope.2 A namespace is a mapping from names to objects. "Directly accessible" here means that an unqualified reference to a name attempts to find the name in the namespace. using this violates the abstraction of namespace implementation. Name spaces are created at different moments and have different lifetimes.) A special quirk of Python is that – if no global statement is in effect – assignments to names always go into the innermost scope. (Actually. modname is a module object and funcname is an attribute of it. Module attributes are writable: you can write modname. and built-in exception names). Usually. searched next. and should be restricted to things like post-mortem debuggers. in the expression z. all variables found outside of the innermost scope are read-only (an attempt to write to such a variable will simply create a new local variable in the innermost scope. and it may change in the future. If a name is declared global.6. The namespace containing the built-in names is created when the Python interpreter starts up. Examples of namespaces are: the set of built-in names (functions such as abs(). for instance. real is an attribute of the object z. Strictly speaking. Otherwise. assignment to attributes is possible. and the outermost scope (searched last) is the namespace containing built-in names. they are used dynamically. two different modules may both define a function "maximize" without confusion — users of the modules must prefix it with the module name. normally. On the other hand. references to names in modules are attribute references: in the expression modname. and deleted when the function returns or raises an exception that is not handled within the function. In a sense the set of attributes of an object also form a namespace. The important thing to know about namespaces is that there is absolutely no relation between names in different namespaces.) The local namespace for a function is created when the function is called. leaving the identically named outer variable unchanged). the middle scope. the local scope references the local names of the (textually) current function. either read from a script file or interactively. this is called __builtin__. del modname. which are searched starting with the nearest enclosing scope. In this case there happens to be a straightforward mapping between the module's attributes and the global names defined in the module: they share the same namespace! 1 Attributes may be read-only or writable. Most namespaces are currently implemented as Python dictionaries. local variables are already determined statically. A scope is a textual region of a Python program where a namespace is directly accessible. and the local names in a function invocation. The global namespace for a module is created when the module definition is read in. so they have their own global namespace. By the way. and is never deleted.the_answer = 42. Classes . forgetting would be a better way to describe what actually happens. at run time — however. The statements executed by the top-level invocation of the interpreter. At any time during execution.the_answer will remove the attribute the_answer from the object named by modname.funcname. Obviously. the name __dict__ is an attribute but not a global name. Although scopes are determined statically. recursive invocations each have their own local namespace. the local scope references the same namespace as the global scope: the module's namespace. module namespaces also last until the interpreter quits.) Of course. Outside functions.Python Tutorial. Assignments do not copy data — they just bind names to objects. It is important to realize that scopes are determined textually: the global scope of a function defined in a module is that module's namespace. For example. so don't rely on dynamic name resolution! (In fact. there are at least three nested scopes whose namespaces are directly accessible: the innermost scope. Writable attributes may also be deleted with the del statement. The same is true for deletions: the Except for one thing.
9. 9.i and MyClass.2 Class Objects Class objects support two kinds of operations: attribute references and instantiation. . all assignments to local variables go into this new namespace. The function definitions inside a class normally have a peculiar form of argument list. three new object types. like function definitions (def statements) must be executed before they have any effect. respectively. so you can change the value of MyClass. the statements inside a class definition will usually be function definitions. we'll learn more about class objects in the next section. returning an integer and a function object. a new namespace is created. and sometimes useful — we'll come back to this later.Python Tutorial. <statement-N> Class definitions.2 statement del x removes the binding of x from the namespace referenced by the local scope. and used as the local scope — thus. a class object is created. When a class definition is entered.) 9.1 Class Definition Syntax The simplest form of class definition looks like this: class ClassName: <statement-1> . and the class object is bound here to the class name given in the class definition header (ClassName in the example). The original local scope (the one in effect just before the class definition was entered) is reinstated. Attribute references use the standard syntax used for all attribute references in Python: obj. __doc__ is also a valid attribute. Valid attribute names are all the names that were in the class's namespace when the class object was created. (You could conceivably place a class definition in a branch of an if statement. This is basically a wrapper around the contents of the namespace created by the class definition. function definitions bind the name of the new function here.) In practice.3. and some new semantics.3. A First Look at Classes 69 . returning the docstring belonging to the class: "A simple example class". if the class definition looked like this: class MyClass: """A simple example class""" i = 12345 def f(self): return 'hello world' then MyClass. In fact. but other statements are allowed. dictated by the calling conventions for methods — again. this is explained later.3. 9.6.3 A First Look at Classes Classes introduce a little bit of new syntax. So.f are valid attribute references.name. or inside a function.i by assignment. When a class definition is left normally (via the end). Class attributes can also be assigned to. all operations that introduce new names use the local scope: in particular. (The global statement can be used to indicate that particular variables live in the global scope. . In particular. Release 2. import statements and function definitions bind the module or function name in the local scope.
. we'll use the term method exclusively to mean methods of class instance objects. and to "data members" in C++. For example. sort. insert. However. >>> class Complex: .r = realpart . So in this example.i (3. Many classes like to create objects with instances customized to a specific initial state.f is a function. the term method is not unique to class instances: other object types can have methods as well. unless explicitly stated otherwise. like local variables. not a function object. (In Python. arguments given to the class instantiation operator are passed on to __init__().counter = x. class instantiation automatically invokes __init__() for the newly-created class instance.5) >>> x..0. if x is the instance of MyClass created above.counter del x. and so on. list objects have methods called append.f — it is a method object.2 Class instantiation uses function notation. the __init__() method may have arguments for greater flexibility.i is not. remove. By definition. For example. So in our example.6. but x. For example. Data attributes need not be declared.Python Tutorial.i = imagpart .. >>> x = Complex(3.counter < 10: x. -4. -4. There are two kinds of valid attribute names.. initialized instance can be obtained by: x = MyClass() Of course. def __init__(self.counter = 1 while x.counter The other kind of instance attribute reference is a method. For example (assuming the above class): x = MyClass() creates a new instance of the class and assigns this object to the local variable x. Classes . since MyClass..data = [] When a class defines an __init__() method.0. A method is a function that "belongs to" an object.) Valid method names of an instance object depend on its class.3 Instance Objects Now what can we do with instance objects? The only operations understood by instance objects are attribute references.i is not. imagpart): .. in the following discussion. a new.. The instantiation operation ("calling" a class object) creates an empty object.f is a valid method reference.5) 9. since MyClass.. the following piece of code will print the value 16. Therefore a class may define a special method named __init__(). self. In that case. x.counter * 2 print x. all attributes of a class that are function objects define corresponding methods of its instances. self. data attributes and methods. data attributes correspond to "instance variables" in Smalltalk.f is not the same thing as MyClass. like this: def __init__(self): self.r. Release 2. 70 Chapter 9. x. without leaving a trace: x. they spring into existence when they are first assigned to. But x. realpart. Just pretend that the class object is a parameterless function that returns a new instance of the class.3.
however.) Clients should use data attributes with care — clients may mess up invariants maintained by the methods by stamping on their data attributes. the call x. 9. Note that clients may add data attributes of their own to an instance object without affecting the validity of the methods.. What happened to the argument? Surely Python raises an exception when a function that requires an argument is called without any — even if the argument isn't actually used. In our example.f(x). you may have guessed the answer: the special thing about methods is that the object is passed as the first argument of the function. What exactly happens when a method is called? You may have noticed that x. it is not necessary to call a method right away: x. the Python implementation.4 Method Objects Usually. If the name denotes a valid class attribute that is a function object. calling a method with a list of n arguments is equivalent to calling the corresponding function with an argument list that is created by inserting the method's object before the first argument. a naming convention can save a lot of headaches here. Possible conventions include capitalizing method names.. a new argument list is constructed from the instance object and the original argument list. Actually.4 Random Remarks Data attributes override method attributes with the same name.f() In the MyClass example. or using verbs for methods and nouns for data attributes. For example: xf = x. classes are not usable to implement pure abstract data types. Data attributes may be referenced by methods as well as by ordinary users ("clients") of an object. written in C. When an instance attribute is referenced that isn't a data attribute. it is wise to use some kind of convention that minimizes the chance of conflicts. prefixing data attribute names with a small unique string (perhaps just an underscore). and the function object is called with this new argument list.3. its class is searched. (On the other hand. I find that this actually increases the readability of methods: there is no chance of confusing local variables and instance variables when glancing through a method. However.f() was called without an argument above. nothing in Python makes it possible to enforce data hiding — it is all based upon convention. This is nothing more than a convention: the name self has absolutely no special meaning to Python.2 9. this will return the string 'hello world'. that by not following the convention your code may be less 9. In fact. which may cause hard-to-find bugs in large programs. Release 2. as long as name conflicts are avoided — again.6. Often. a method object is created by packing (pointers to) the instance object and the function object just found together in an abstract object: this is the method object. the first argument of a method is called self. Random Remarks 71 . If you still don't understand how methods work. can completely hide implementation details and control access to an object if necessary. and can be stored away and called at a later time.f() is exactly equivalent to MyClass. a method is called right after it is bound: x. When the method object is called with an argument list.f while True: print xf() will continue to print hello world until the end of time. In general. even though the function definition for f() specified an argument. (Note.f is a method object. In other words.4. to avoid accidental name conflicts. a look at the implementation can perhaps clarify matters. this can be used by extensions to Python written in C.Python Tutorial. it is unpacked again. There is no shorthand for referencing data attributes (or other methods!) from within methods.
the class containing the method is itself defined in this global scope. Methods may call other methods by using method attributes of the self argument: class Bag: def __init__(self): self. <statement-N> 72 Chapter 9. Usually. a language feature would not be worthy of the name "class" without supporting inheritance. g and h are all attributes of class C that refer to function objects.append(x) def addtwice(self. It is not necessary that the function definition is textually enclosed in the class definition: assigning a function object to a local variable in the class is also ok. Classes .__class__. x+y) class C: f = f1 def g(self): return 'hello world' h = g Now f. The syntax for a derived class definition looks like this: class DerivedClassName(BaseClassName): <statement-1> . x.Python Tutorial.2 readable to other Python programmers.add(x) Methods may reference global names in the same way as ordinary functions. and consequently they are all methods of instances of C — h being exactly equivalent to g.) Any function object that is a class attribute defines a method for instances of that class.data. functions and modules imported into the global scope can be used by methods. It is stored as object. and therefore has a class (also called its type). x): self. Release 2.add(x) self. The global scope associated with a method is the module containing the class definition. Note that this practice usually only serves to confuse the reader of a program.6. there are many legitimate uses of the global scope: for one thing. (The class itself is never used as a global scope!) While one rarely encounters a good reason for using global data in a method. and in the next section we'll find some good reasons why a method would want to reference its own class! Each value is an object. For example: # Function defined outside the class def f1(self.5 Inheritance Of course. and it is also conceivable that a class browser program might be written that relies upon such a convention. . x): self.data = [] def add(self. as well as functions and classes defined in it. y): return min(x. . 9.
A class definition with multiple base classes looks like this: class DerivedClassName(Base1. • Use issubclass() to check class inheritance: issubclass(bool. when the base class is defined in another module: class DerivedClassName(modname. This is used for resolving attribute references: if a requested attribute is not found in the class. However. Because methods have no special privileges when calling other methods of the same object. (Note that this only works if the base class is defined or imported directly in the global scope. Release 2. In place of a base class name. str) is False since unicode is not a subclass of str (they only share a common ancestor. the base class is remembered. However. and only if it is not found there.5. Thus. . basestring).6.) 9. it is searched in Base1. int) is True since bool is a subclass of int. This can be useful. Base3): <statement-1> . (For C++ programmers: all methods in Python are effectively virtual. 9. descending down the chain of base classes if necessary. this would require you to know whether a particular attribute of Base1 is actually defined in Base1 or in one of its base classes before you can figure out the consequences of a name conflict with an attribute of Base2. the only rule is depth-first. the search proceeds to look in the base class. if an attribute is not found in DerivedClassName. Inheritance 73 .5. arguments).2 The name BaseClassName must be defined in a scope containing the derived class definition. (To some people breadth first — searching Base2 and Base3 before the base classes of Base1 — looks more natural. it is searched in Base2.methodname(self. a method of a base class that calls another method defined in the same base class may end up calling a method of a derived class that overrides it. Method references are resolved as follows: the corresponding class attribute is searched. Base2. and so on. This is occasionally useful to clients as well. and the method reference is valid if this yields a function object. Derived classes may override methods of their base classes. for example.Python Tutorial. There is a simple way to call the base class method directly: just call BaseClassName.) An overriding method in a derived class may in fact want to extend rather than simply replace the base class method of the same name.) Python has two builtin functions that work with inheritance: • Use isinstance() to check an object's type: isinstance(obj. When the class object is constructed. . <statement-N> For old-style classes. issubclass(unicode. This rule is applied recursively if the base class itself is derived from some other class. int) will be True only if obj. left-to-right.1 Multiple Inheritance Python supports a limited form of multiple inheritance as well. The depth-first rule makes no differences between direct and inherited attributes of Base1. then (recursively) in the base classes of Base1. There's nothing special about instantiation of derived classes: DerivedClassName() creates a new instance of the class.__class__ is int or some class derived from int. other arbitrary expressions are also allowed.BaseClassName): Execution of a derived class definition proceeds the same as for a base class.
you 74 Chapter 9. so it can be used to define class-private instance and class variables. the dynamic algorithm linearizes the search order in a way that preserves the left-to-right ordering specified in each class. see http://www. this is similar to the effect of the global statement. This mangling is done without regard to the syntactic position of the identifier. private to this class on instances of other classes. methods. Taken together. all new-style classes inherit from object.Python Tutorial. For more detail. setattr() and delattr(). as well as when referencing __dict__ directly. For example. With new-style classes. eval() or execfile() does not consider the classname of the invoking class to be the current class.dept = 'computer lab' john. An empty class definition will do nicely: class Employee: pass john = Employee() # Create an empty employee record # Fill the fields of the record john. Any identifier of the form __spam (at least two leading underscores.) Notice that code passed to exec. the method resolution order changes dynamically to support cooperative calls to super(). Release 2.7 Odds and Ends Sometimes it is useful to have a data type similar to the Pascal "record" or C "struct". 9.python. Classes . the effect of which is likewise restricted to code that is byte-compiled together.salary = 1000 A piece of Python code that expects a particular abstract data type can often be passed a class that emulates the methods of that data type instead. or when the class name consists of only underscores.6 Private Variables There is limited support for class-private identifiers. 9. so any case of multiple inheritance provides more than one path to reach object. To keep the base classes from being accessed more than once. and that's one reason why this loophole is not closed. such as in the debugger.org/download/releases/2. For instance. The same restriction applies to getattr(). it still is possible for a determined soul to access or modify a variable that is considered private. if you have a function that formats some data from a file object. that calls each parent only once. or mucking with instance variables by code outside the class. (Buglet: derivation of a class with the same name as the base class makes use of private variables of the base class possible. Truncation may occur when the mangled name would be longer than 255 characters. without having to worry about instance variables defined by derived classes.2 For new-style classes. Name mangling is intended to give classes an easy way to define "private" instance variables and methods. This approach is known in some other multiple-inheritance languages as call-next-method and is more powerful than the super call found in single-inheritance languages. This can even be useful in special circumstances. no mangling occurs. and even variables stored in instances. where classname is the current class name with leading underscore(s) stripped. variables stored in globals.6. these properties make it possible to design reliable and extensible classes with multiple inheritance. and that is monotonic (meaning that a class can be subclassed without affecting the precedence order of its parents). Outside classes. at most one trailing underscore) is textually replaced with _classname__spam. dynamic ordering is necessary because all cases of multiple inheritance exhibit one or more diamond relationships (where one at least one of the parent classes can be accessed through multiple paths from the bottommost class).3/mro/. bundling together a few named data items.name = 'John Doe' john. Note that the mangling rules are designed mostly to avoid accidents.
C. and m. 9. The second form is a shorthand for: raise instance.Python Tutorial. Exceptions Are Classes Too 75 .im_self is the instance object with the method m(). B. D in that order: class B: pass class C(B): pass class D(C): pass for c in [B.__class__. D]: try: raise c() except D: print "D" except C: print "C" except B: print "B" Note that if the except clauses were reversed (with except B first). When an error message is printed for an unhandled exception. the following code will print B. C.6.2 can define a class with methods read() and readline() that get the data from a string buffer instead. it would have printed B.im_func is the function object corresponding to the method. then a colon and a space.9 Iterators By now you have probably noticed that most container objects can be looped over using a for statement: 9. Using this mechanism it is possible to create extensible hierarchies of exceptions. too: m. There are two new valid (semantic) forms for the raise statement: raise Class.8 Exceptions Are Classes Too User-defined exceptions are identified by classes as well. Release 2.8. and finally the instance converted to a string using the built-in function str(). instance A class in an except clause is compatible with an exception if it is the same class or a base class thereof (but not the other way around — an except clause listing a derived class is not compatible with a base class). For example. the exception's class name is printed. B — the first matching except clause is triggered. and pass it as an argument. 9. instance must be an instance of Class or of a class derived from it. Instance method objects have attributes. instance raise instance In the first form.
Release 2. concise.next() 'b' >>> it.Python Tutorial. When there are no more elements. and convenient.data[self. print char 76 Chapter 9.next() Traceback (most recent call last): File "<stdin>".next() 'c' >>> it. This example shows how it all works: >>> s = 'abc' >>> it = iter(s) >>> it <iterator object at 0x00A1DB50> >>> it.index = len(data) def __iter__(self): return self def next(self): if self.index == 0: raise StopIteration self. The use of iterators pervades and unifies Python.next() StopIteration Having seen the mechanics behind the iterator protocol. data): self. The function returns an iterator object that defines the method next() which accesses elements in the container one at a time. 3]: print element for element in (1.next() 'a' >>> it.index . 2.index = self.data = data self. in ? it. it is easy to add iterator behavior to your classes. Classes .2 for element in [1.txt"): print line This style of access is clear. If the class defines next(). line 1.6. Define a __iter__() method which returns an object with a next() method.index] >>> for char in Reverse('spam'): .. the for statement calls iter() on the container object.1 return self.. then __iter__() can just return self: class Reverse: "Iterator for looping over a sequence backwards" def __init__(self. next() raises a StopIteration exception which tells the for loop to terminate. 2. Behind the scenes. 3): print element for key in {'one':1. 'two':2}: print key for char in "123": print char for line in open("myfile.
6. What makes generators so compact is that the __iter__() and next() methods are created automatically. -1): yield data[index] >>> for char in reverse('golf'): . Generators # sum of squares # dot product 77 . 30] >>> yvec = [7.10. f l o g Anything that can be done with generators can also be done with class based iterators as described in the previous section. Release 2. 5.. 20. the generator resumes where it left-off (it remembers all the data values and which statement was last executed). They are written like regular functions but use the yield statement whenever they want to return data. Generator expressions are more compact but less versatile than full generator definitions and tend to be more memory friendly than equivalent list comprehensions. This made the function easier to write and much more clear than an approach using instance variables like self. print char .data. Examples: >>> sum(i*i for i in range(10)) 285 >>> xvec = [10. 9. Another key feature is that the local variables and execution state are automatically saved between calls.2 .. these features make it easy to create iterators with no more effort than writing a regular function.Python Tutorial. These expressions are designed for situations where the generator is used right away by an enclosing function. when generators terminate.10 Generators Generators are a simple and powerful tool for creating iterators..y in zip(xvec. 3] >>> sum(x*y for x.. Each time next() is called. yvec)) 260 9. In addition to automatic method creation and saving program state. they automatically raise StopIteration.11 Generator Expressions Some simple generators can be coded succinctly as expressions using a syntax similar to list comprehensions but with parentheses instead of brackets... m a p s 9. In combination.index and self. An example shows that generators can be trivially easy to create: def reverse(data): for index in range(len(data)-1. -1.
'o'. sin(x*pi/180)) for x in range(0.-1.-1)) ['f'. student.name) for student in graduates) >>> data = 'golf' >>> list(data[i] for i in range(len(data)-1.split()) >>> valedictorian = max((student. 91)) >>> unique_words = set(word for line in page for word in line. sin >>> sine_table = dict((x.gpa.Python Tutorial. 'g'] 78 Chapter 9.2 >>> from math import pi. 'l'. Classes . Release 2.6.
'installdir') 10. The builtin dir() and help() functions are useful as interactive aids for working with large modules like os: >>> import os >>> dir(os) <returns a list of all module functions> >>> help(os) <returns an extensive manual page created from the module's docstrings> For daily file and directory management tasks.system('time 0:02') 0 >>> os.open() from shadowing the builtin open() function which operates much differently.copyfile('data.1 Operating System Interface The os module provides dozens of functions for interacting with the operating system: >>> import os >>> os.getcwd() # Return the current working directory 'C:\\Python26' >>> os.db'. 'quote.chdir('/server/accesslogs') Be sure to use the import os style instead of from os import *.CHAPTER TEN BRIEF TOUR OF THE STANDARD LIBRARY 10.glob('*.db') >>> shutil.py'] 79 .py'. This will keep os. 'random.py'. 'archive.py') ['primes. the shutil module provides a higher level interface that is easier to use: >>> import shutil >>> shutil.move('/build/executables'.2 File Wildcards The glob module provides a function for making file lists from directory wildcard searches: >>> import glob >>> glob.
'fell'. 'cat in the the hat') 'cat in the hat' When only simple capabilities are needed. 'fastest'] >>> re. log file not found starting a new one\n') Warning.Python Tutorial. The latter is useful for emitting warnings and error messages to make them visible even when stdout has been redirected: >>> sys.findall(r'\bf[a-z]*'. 10.2 10. 2) 10.3 Command Line Arguments Common utility scripts often need to process command line arguments. stdout.stderr.log(1024. These arguments are stored in the sys module's argv attribute as a list.0) 0.sub(r'(\b[a-z]+) \1'. regular expressions offer succinct. 'which foot or hand fell fastest') ['foot'.py one two three at the command line: >>> import sys >>> print sys. For complex matching and manipulation.write('Warning.exit().argv ['demo. r'\1'.py'. Brief Tour of the Standard Library . 'two') 'tea for two' 10. 'one'. and stderr. 10. log file not found starting a new one The most direct way to terminate a script is to use sys. optimized solutions: >>> import re >>> re.replace('too'.6 Mathematics The math module gives access to the underlying C library functions for floating point math: >>> import math >>> math.0 80 Chapter 10.argv using the conventions of the Unix getopt() function.70710678118654757 >>> math. 'three'] The getopt module processes sys.5 String Pattern Matching The re module provides regular expression tools for advanced string processing.4 Error Output Redirection and Program Termination The sys module also has attributes for stdin. For instance the following output results from running python demo. Release 2. More powerful and flexible command line processing is provided by the optparse module.pi / 4.cos(math.6. 'two'. string methods are preferred because they are easier to read and debug: >>> 'tea for too'.
"""To: jcaesar@example.. 'jcaesar@example. . 25. """) server..8 Dates and Times The datetime module supplies classes for manipulating dates and times in both simple and complex ways. . if 'EST' in line or 'EDT' in line: # look for Eastern Time .pl'): . 2) >>> now.today() >>> now datetime..urlopen('http://tycho.' 10.navy. the focus of the implementation is on efficient member extraction for output formatting and manipulation.sendmail('soothsayer@example.. 33] >>> random.7 Internet Access There are a number of modules for accessing the internet and processing internet protocols. 02 Dec 2003 is a Tuesday on the 02 day of December.usno. Internet Access 81 . 41.17970987693706186 >>> random. While date and time arithmetic is supported.quit() (Note that the second example needs a mailserver running on localhost.org'.2 The random module provides tools for making random selections: >>> import random >>> random.Python Tutorial.. # dates are easily constructed and formatted >>> from datetime import date >>> now = date..date(2003. 18. 12. 10) # sampling without replacement [30. ..org Beware the Ides of March. %d %b %Y is a %A on the %d day of %B..org From: email@example.com() # random float 0. 'pear'. 4... 81.") '12-02-03. The module also supports objects that are timezone aware. .6.7. 8. 83. print line <BR>Nov. 09:43:32 PM EST >>> >>> >>> . >>> import smtplib server = smtplib.. Two of the simplest are urllib2 for retrieving data from urls and smtplib for sending mail: >>> import urllib2 >>> for line in urllib2.sample(xrange(100).) 10..org'.SMTP('localhost') server..strftime("%m-%d-%y. Release 2. 'banana']) 'apple' >>> random..randrange(6) # random integer chosen from range(6) 4 10. 16. 50.mil/cgi-bin/timer.choice(['apple'.
'a=1.Python Tutorial. Brief Tour of the Standard Library . a=b.timeit() 0. 31) >>> age = now .54962537085770791 In contrast to timeit's fine level of granularity. b=2').decompress(t) 'witch which has which witches wrist watch' >>> zlib.days 14368 10. The timeit module quickly demonstrates a modest performance advantage: >>> from timeit import Timer >>> Timer('t=a.6. >>> import zlib >>> s = 'witch which has which witches wrist watch' >>> len(s) 41 >>> t = zlib.compress(s) >>> len(t) 37 >>> zlib.9 Data Compression Common data archiving and compression formats are directly supported by modules including: zlib. bz2.57535828626024577 >>> Timer('a.crc32(s) 226805979 10.10 Performance Measurement Some Python users develop a deep interest in knowing the relative performance of different approaches to the same problem. 'a=1. b=2').b = b. Test construction is as simple as cutting-and-pasting a typical call along with its results into the docstring.timeit() 0. the profile and pstats modules provide tools for identifying time critical sections in larger blocks of code. Python provides a measurement tool that answers those questions immediately. This improves the documentation by providing the user with an example and it allows the doctest module to make sure the code remains true to the documentation: 82 Chapter 10. 7. Release 2.11 Quality Control One approach for developing high quality software is to write tests for each function as it is developed and to run those tests frequently during the development process. For example. The doctest module provides a tool for scanning a module and validating tests embedded in a program's docstrings. b=t'. 10.a'. zipfile and tarfile. it may be tempting to use the tuple packing and unpacking feature instead of the traditional approach to swapping arguments.2 # dates support calendar arithmetic >>> birthday = date(1964. gzip.birthday >>> age.
including MIME and other RFC 2822-based message documents. and the codecs package. no direct knowledge or handling of XML is needed. 1).0) self. the csv module supports direct reads and writes in a common database format. 20.main() # Calling from the command line invokes all tests 10. 70) unittest.12 Batteries Included Python has a "batteries included" philosophy. >>> print average([20. This is best seen through the sophisticated and robust capabilities of its larger packages. 10. 0. 30.6.assertEqual(average([20. 5. []) self. • The email package is a library for managing email messages. For example: • The xmlrpclib and SimpleXMLRPCServer modules make implementing remote procedure calls into an almost trivial task. • Internationalization is supported by a number of modules including gettext. 40. • The xml. the email package has a complete toolset for building or decoding complex message structures (including attachments) and for implementing internet encoding and header protocols.assertRaises(ZeroDivisionError.2 def average(values): """Computes the arithmetic mean of a list of numbers. Unlike smtplib and poplib which actually send and receive messages.0 """ return sum(values. these modules and packages greatly simplify data interchange between python applications and other tools. 4.0) / len(values) import doctest doctest.3) self. average. Release 2. average. 30.sax packages provide robust support for parsing this popular data interchange format.Python Tutorial. 30. 70]).12.assertEqual(round(average([1.testmod() # automatically validate the embedded tests The unittest module is not as effortless as the doctest module. 70]) 40.TestCase): def test_average(self): self. locale. 7]). Likewise. Despite the modules names.dom and xml. Together. but it allows a more comprehensive set of tests to be maintained in a separate file: import unittest class TestStatisticalFunctions(unittest.assertRaises(TypeError. Batteries Included 83 .
Release 2.2 84 Chapter 10. Brief Tour of the Standard Library .Python Tutorial.6.
'blue']]] . 'white'.. 'cyan']. 'cyan']. ..... 'c'. the "pretty printer" adds line breaks and indentation to more clearly reveal data structure: >>> import pprint >>> t = [[[['black'. .CHAPTER ELEVEN BRIEF TOUR OF THE STANDARD LIBRARY – PART II This second tour covers more advanced modules that support professional programming needs. . . 'yellow']. >>> import textwrap doc = """The wrap() method is just like fill() except that it returns a list of strings instead of one big string with newlines to separate the wrapped lines. >>> pprint.pprint(t. ['green'. width=40) 85 . 'yellow']. [['magenta'. 'd'. [['magenta'. 'red']].. 'g'.1 Output Formatting The repr module provides a version of repr() customized for abbreviated displays of large or deeply nested containers: >>> import repr >>> repr..fill(doc. ['green'.. When the result is longer than one line...repr(set('supercalifragilisticexpialidocious')) "set(['a'.])" The pprint module offers more sophisticated control over printing both built-in and user defined objects in a way that is readable by the interpreter.""" print textwrap. 11. These modules rarely occur in small scripts... 'blue']]] The textwrap module formats paragraphs of text to fit a given screen width: >>> >>> . width=30) [[[['black'. 'white'. 'red']]. 'f'. 'e'.
setlocale(locale.234.567' >>> locale. Writing $$ creates a single escaped $: >>> from string import Template >>> t = Template('${village}folk send $$10 to $cause. a batch renaming utility for a photo browser may elect to use percent signs for placeholders such as the current date. x.substitute(d) Traceback (most recent call last): .localeconv() # get a mapping of conventions >>> x = 1234567.8 >>> locale.LC_ALL.1252' >>> conv = locale.jpg'. x). . grouping=True) '$1. KeyError: 'owner' >>> t.') >>> t. image sequence number. os.Python Tutorial.567.234. For example. .substitute(village='Nottingham'.80' 11.*f".. The locale module accesses a database of culture specific data formats. conv['frac_digits']. grouping=True) '1. Surrounding the placeholder with braces allows it to be followed by more alphanumeric letters with no intervening spaces.2 Templating The string module includes a versatile Template class with a simplified syntax suitable for editing by end-users. The format uses placeholder names formed by $ with valid Python identifiers (alphanumeric characters and underscores)..2 The wrap() method is just like fill() except that it returns a list of strings instead of one big string with newlines to separate the wrapped lines.jpg'] 86 Chapter 11. The grouping attribute of locale's format function provides a direct way of formatting numbers with group separators: >>> import locale >>> locale.jpg'. user supplied data may be incomplete and the safe_substitute() method may be more appropriate — it will leave placeholders unchanged if data is missing: >>> t = Template('Return the $item to $owner. (conv['currency_symbol']. 'img_1077.path >>> photofiles = ['img_1074. cause='the ditch fund') 'Nottinghamfolk send $10 to the ditch fund. . For mail-merge style applications.') >>> d = dict(item='unladen swallow') >>> t. or file format: >>> import time.format("%d".' The substitute() method raises a KeyError when a placeholder is not supplied in a dictionary or a keyword argument. Brief Tour of the Standard Library – Part II .format("%s%. 'English_United States. This allows users to customize their applications without having to alter the application. Release 2. 'img_1076.safe_substitute(d) 'Return the unladen swallow to $owner.6.' Template subclasses can specify a custom delimiter.1252') 'English_United States.
comp_size.path. extra_size = fields start += 16 filename = data[start:start+filenamesize] start += filenamesize extra = data[start:start+extra_size] print filename. newname) img_1074.3 Working with Binary Data Record Layouts The struct module provides pack() and unpack() functions for working with variable length binary record formats.jpg ') Another application for templating is separating program logic from the details of multiple output formats. A related use case is running I/O in parallel with computations in another thread. print '{0} --> {1}'.6. hex(crc32).. n=i. The following code shows how the high level threading module can run tasks in background while the main program continues to run: 11. Threads can be used to improve the responsiveness of applications that accept user input while other tasks run in the background. uncomp_size start += extra_size + comp_size # skip to the next header 11. Working with Binary Data Record Layouts 87 ....jpg --> Ashley_0. The "<" indicates that they are standard size and in little-endian byte order: import struct data = open('myfile. data[start:start+16]) crc32. Pack codes "H" and "I" represent two and four byte unsigned numbers respectively.jpg --> Ashley_2. filenamesize.2 >>> class BatchRename(Template): . ext = os. 'rb')..splitext(filename) .. base. comp_size. This makes it possible to substitute custom templates for XML files.unpack('<IIIHH'. 11.jpg --> Ashley_1.Python Tutorial.substitute(d=date.4 Multi-threading Threading is a technique for decoupling tasks which are not sequentially dependent.jpg img_1077.3. filename in enumerate(photofiles): .jpg img_1076.read() start = 0 for i in range(3): # show the first 3 file headers start += 14 fields = struct.strftime('%d%b%y') >>> for i. plain text reports..format(filename. uncomp_size..zip'. Release 2. delimiter = '%' >>> fmt = raw_input('Enter rename style (%d-date %n-seqnum %f-format): Enter rename style (%d-date %n-seqnum %f-format): Ashley_%n%f >>> t = BatchRename(fmt) >>> date = time. newname = t. The following example shows how to loop through header information in a ZIP file without using the zipfile module. and HTML web reports. f=ext) .
or to an HTTP Server. more readable. and more reliable.__init__(self) self.Queue objects for inter-thread communication and coordination are easier to design.' background. informational and debugging messages are suppressed and the output is sent to standard error. New filters can select different routing based on message priority: DEBUG.close() print 'Finished background zip of: '. the preferred approach to task coordination is to concentrate all access to a resource in a single thread and then use the Queue module to feed that thread with requests from other threads.warning('Warning:config file %s not found'.shutting down') This produces the following output: WARNING:root:Warning:config file server.5 Logging The logging module offers a full featured and flexible logging system.debug('Debugging information') logging. 'server.shutting down By default.conf') logging. 'myarchive. 'w'. Brief Tour of the Standard Library – Part II .write(self.2 import threading.zip') background. Release 2.stderr: import logging logging. While those tools are powerful.infile = infile self.critical('Critical error -.infile) f.conf not found ERROR:root:Error occurred CRITICAL:root:Critical error -.info('Informational message') logging.infile background = AsyncZip('mydata. To that end.Thread): def __init__(self.ZipFile(self. ERROR.join() # Wait for the background task to finish print 'Main program waited until background was done. events. minor design errors can result in problems that are difficult to reproduce. the threading module provides a number of synchronization primitives including locks.6. and CRITICAL.txt'. and semaphores.Thread. outfile): threading. 88 Chapter 11. log messages are sent to a file or to sys. 11. So. The logging system can be configured directly from Python or can be loaded from a user editable configuration file for customized logging without altering the application.' The principal challenge of multi-threaded applications is coordinating threads that share data or other resources. Other output options include routing messages through email. zipfile. self.Python Tutorial. At its simplest.error('Error occurred') logging. datagrams. Applications using Queue. condition variables.start() print 'The main program continues to run in foreground. WARNING.outfile.ZIP_DEFLATED) f.outfile = outfile def run(self): f = zipfile. sockets. zipfile class AsyncZip(threading. INFO. infile.
. These objects are well suited for implementing queues and breadth first tree searches: 11. in <module> d['primary'] # entry was automatically removed File "C:/python26/lib/weakref.6. Release 2. def __repr__(self): .Python Tutorial.. in __getitem__ o = self. 10. The memory is freed shortly after the last reference to it has been eliminated. >>> a = A(10) # create a reference >>> d = weakref. [4000.2 11. return str(self.py". However. def __init__(self. [10.value) . 700.7 Tools for Working with Lists Many data structure needs can be met with the built-in list type. just tracking them creates a reference that makes them permanent.. line 46... The array module provides an array() object that is like a list that stores only homogeneous data and stores it more compactly. line 1. The following example shows an array of numbers stored as two byte unsigned binary numbers (typecode "H") rather than the usual 16 bytes per entry for regular lists of python int objects: >>> from array import array >>> a = array('H'.collect() # run garbage collection right away 0 >>> d['primary'] # entry was automatically removed Traceback (most recent call last): File "<stdin>". Unfortunately. The weakref module provides tools for tracking objects without creating a reference. 22222]) >>> sum(a) 26932 >>> a[1:3] array('H'..6 Weak References Python does automatic memory management (reference counting for most objects and garbage collection to eliminate cycles).data[key]() KeyError: 'primary' 11.. This approach works fine for most applications but occasionally there is a need to track objects only as long as they are being used by something else.. sometimes there is a need for alternative implementations with different performance trade-offs. When the object is no longer needed.value = value . it is automatically removed from a weakref table and a callback is triggered for weakref objects.. gc >>> class A: .. value): . self.WeakValueDictionary() >>> d['primary'] = a # does not create a reference >>> d['primary'] # fetch the object if it is still alive 10 >>> del a # remove the one reference >>> gc. Typical applications include caching objects that are expensive to create: >>> import weakref. 700]) The collections module provides a deque() object that is like a list with faster appends and pops from the left side but slower lookups in the middle.6. Weak References 89 .
'python')] The heapq module provides functions for implementing heaps based on regular lists. 'lua'). control over precision. The difference becomes significant if the results are rounded to the nearest cent: >>> from decimal import * >>> Decimal('0.append("task4") >>> print "Handling".70') * Decimal('1. tracking of significant decimal places.2 >>> from collections import deque >>> d = deque(["task1". heappop. Compared to the builtin float implementation of binary floating point. The lowest valued entry is always kept at position zero. This is useful for applications which repeatedly access the smallest element but do not want to run a full list sort: >>> from heapq import heapify. 0] >>> heapify(data) # rearrange the list into heap order >>> heappush(data.73499999999999999 90 Chapter 11.append(m) In addition to alternative list implementations. 'perl').05 0. "task3"]) >>> d. "task2". (500. (500. calculating a 5% tax on a 70 cent phone charge gives different results in decimal floating point and binary floating point. 5. (300. heappush >>> data = [1. Brief Tour of the Standard Library – Part II . (400.6. or for applications where the user expects the results to match calculations done by hand.8 Decimal Floating Point Arithmetic The decimal module offers a Decimal datatype for decimal floating point arithmetic.05') Decimal("0. 'tcl'). 0.70 * 1. Release 2.7350") >>> . 3. 2. (400. For example. 'tcl'). 4. 'ruby').insort(scores. the new class is especially helpful for financial applications and other uses which require exact decimal representation. 'ruby')) >>> scores [(100. (200. the library also offers other tools such as the bisect module with functions for manipulating sorted lists: >>> import bisect >>> scores = [(100. -5) # add a new entry >>> [heappop(data) for i in range(3)] # fetch the three smallest entries [-5.popleft() for m in gen_moves(node): if is_goal(m): return m unsearched. d. 'python')] >>> bisect.popleft() Handling task1 unsearched = deque([starting_node]) def breadth_first_search(unsearched): node = unsearched. 7. 'perl'). 'lua'). (200. control over rounding to meet legal or regulatory requirements. 6.Python Tutorial. (300. 8. 9. 1] 11.
00 % 0.00") >>> 1. automatically inferring four place significance from multiplicands with two place significance.Python Tutorial. Exact representation enables the Decimal class to perform modulo calculations and equality tests that are unsuitable for binary floating point: >>> Decimal('1.6.2 The Decimal result keeps a trailing zero.1')]*10) == Decimal('1.10') Decimal("0.8.prec = 36 >>> Decimal(1) / Decimal(7) Decimal("0.1]*10) == 1.00') % Decimal('.0') True >>> sum([0. Release 2.0 False The decimal module provides arithmetic with as much precision as needed: >>> getcontext().142857142857142857142857142857142857") 11. Decimal Floating Point Arithmetic 91 .10 0.09999999999999995 >>> sum([Decimal('0. Decimal reproduces mathematics as done by hand and avoids issues that can arise when binary floating point cannot exactly represent decimal quantities.
6. Release 2.Python Tutorial. Brief Tour of the Standard Library – Part II .2 92 Chapter 11.
so messages posted to one will automatically be forwarded to the other. and many other tasks. 93 . and announcing new modules.org: The major Python Web site. and Australia.org. a mirror may be faster than the main site.) For Python-related questions and problem reports. It contains code. generate random numbers.python. parse command-line options.org: Fast access to Python's documentation. It's heavy reading.org: The Python Package Index. larger modules. or look for it in the Misc/ directory of the Python source distribution. be sure to check the list of Frequently Asked Questions (also called the FAQ). which gives complete (though terse) reference material about types. and useful scripts. compress data. retrieve documents via HTTP. • Installing Python Modules (in Installing Python Modules) explains how to install external modules written by other Python users. and may already contain the solution for your problem.org/pipermail/.python. There are modules to read Unix mailboxes.lang. The standard Python distribution includes a lot of additional code. but is useful as a complete guide to the language itself.python.python. functions. • http://docs. previously also nicknamed the Cheese Shop. The newsgroup and mailing list are gatewayed. • http://aspn. Skimming through the Library Reference will give you an idea of what's available. Some other documents in the set are: • The Python Standard Library (in The Python Library Reference): You should browse through this manual. and pointers to Pythonrelated pages around the Web. The FAQ answers many of the questions that come up again and again. Before posting. This Web site is mirrored in various places around the world. or send them to the mailing list at python-list@python. documentation. you can register it here so that others can find it.CHAPTER TWELVE WHAT NOW? Reading this tutorial has probably reinforced your interest in using Python — you should be eager to apply Python to solving your real-world problems. More Python resources: • http://www. and the modules in the standard library.activestate. There are around 120 postings a day (with peaks up to several hundred). depending on your geographical location. Particularly notable contributions are collected in a book also titled Python Cookbook (O'Reilly & Associates. is an index of user-created Python modules that are available for download.com/ASPN/Python/Cookbook/: The Python Cookbook is a sizable collection of code examples. ISBN 0-596-00797-3. such as Europe. • The Python Language Reference (in The Python Language Reference): A detailed explanation of Python's syntax and semantics. Where should you go to learn more? This tutorial is part of Python's documentation set.python. Once you begin releasing code. you can post to the newsgroup comp. Japan. asking (and answering) questions. write CGI programs. suggesting new features. • http://pypi. Mailing list archives are available at http://mail.
6. Release 2.Python Tutorial.2 94 Chapter 12. What Now? .
Any line in the history buffer can be edited. Pressing the Return key passes the current line to the interpreter. however. 13. and when a new prompt is given you are positioned on a new line at the bottom of this buffer. The most important of these are: C-A (Control-A) moves the cursor to the beginning of the line. which supports Emacs-style and vi-style editing. C-E to the end. This is implemented using the GNU Readline library. C-N moves one down. IDLE. All non-empty input lines issued are saved in a history buffer. C-R starts an incremental reverse search. This chapter does not document the editing facilities of Mark Hammond's PythonWin package or the Tk-based environment. 13. C-P moves one line up (back) in the history buffer.3 Key Bindings The key bindings and some other parameters of the Readline library can be customized by placing commands in an initialization file called ~/. C-F to the right. distributed with Python. input line editing is active whenever the interpreter prints a primary or secondary prompt.CHAPTER THIRTEEN INTERACTIVE INPUT EDITING AND HISTORY SUBSTITUTION Some versions of the Python interpreter support editing of the current input line and history substitution.inputrc. Backspace erases the character to the left of the cursor. The current line can be edited using the conventional Emacs control characters.1 Line Editing If supported. C-underscore undoes the last change you made. Key bindings have the form key-name: function-name 95 . C-K kills (erases) the rest of the line to the right of the cursor. C-B moves it one position to the left. C-D the character to its right. This library has its own documentation which I won't duplicate here. similar to facilities found in the Korn shell and the GNU Bash shell. C-Y yanks back the last killed string.2 History Substitution History substitution works as follows. The interactive editing and history described here are optionally available in the Unix and Cygwin versions of the interpreter. it can be repeated for cumulative effect. C-S starts a forward search. an asterisk appears in front of the prompt to mark a line as modified. the basics are easily explained. 13. The command line history recall which operates within DOS boxes on NT and some other DOS and Windows flavors is yet another beast.
Store the file in ~/. the current local variables. To enable it in the interpreter's interactive mode. which turn out to be needed in most sessions with the interpreter. You may find it convenient to keep some of the imported modules.inputrc. Note that this may execute application-defined code if an object with a __getattr__() method is part of the expression. 1 Python will execute the contents of a file identified by the PYTHONSTARTUP environment variable when you start an interactive interpreter. add the following to your startup file: 1 import rlcompleter.2 or "string": function-name and options can be set with set option-name value For example: # I prefer vi-style editing: set editing-mode vi # Edit using a single line: set horizontal-scroll-mode On # Rebind some keys: Meta-h: backward-kill-word "\C-u": universal-argument "\C-x\C-r": re-read-init-file Note that the default binding for Tab in Python is to insert a Tab character instead of Readline's default filename completion function. A more capable startup file might look like this example.parse_and_bind('tab: complete') This binds the Tab key to the completion function. Interactive Input Editing and History Substitution . Autocomplete is bound to the Esc key by default (you can change it . it will evaluate the expression up to the final '. this makes it harder to type indented continuation lines if you're accustomed to using Tab for that purpose.) Automatic completion of variable and module names is optionally available.Python Tutorial. readline. # # # # # # Add auto-completion and a stored history file of commands to your Python interactive interpreter. Release 2. you can override this by putting Tab: complete in your ~/. so hitting the Tab key twice suggests completions. Note that this deletes the names it creates once they are no longer needed. If you insist.pystartup.0+.6.pystartup" in bash.' and then suggest completions from the attributes of the resulting object.a. and set an environment variable to point to it: "export PYTHONSTARTUP=/home/user/. it looks at Python statement names. and the available module names. this is done since the startup file is executed in the same namespace as the interactive commands. 96 Chapter 13. such as os. Requires Python 2. For dotted expressions such as string. and removing the names avoids creating side effects in the interactive environment. readline readline. (Of course.see readline docs).
Python Tutorial, Release 2.6.2
# # Note that PYTHONSTARTUP does *not* expand "~", so you have to put in the # full path to your home directory. import import import import atexit os readline rlcompleter
historyPath = os.path.expanduser("~/.pyhistory") def save_history(historyPath=historyPath): import readline readline.write_history_file(historyPath) if os.path.exists(historyPath): readline.read_history_file(historyPath) atexit.register(save_history) del os, atexit, readline, rlcompleter, save_history, historyPath
13.4 Commentary
This facility is an enormous step forward compared to earlier versions of the interpreter; however, some wishes are left: It would be nice if the proper indentation were suggested on continuation lines (the parser knows if an indent token is required next). The completion mechanism might use the interpreter's symbol table. A command to check (or even suggest) matching parentheses, quotes, etc., would also be useful.
13.4. Commentary
Python Tutorial, Release 2.6.2
Chapter 13. Interactive Input Editing and History Substitution
Floating-point numbers are represented in computer hardware as base 2 (binary) fractions. For example, the decimal fraction 0.125 has value 1/10 + 2/100 + 5/1000, and in the same way the binary fraction 0.001 has value 0/2 + 0/4 + 1/8. These two fractions have identical values, the only real difference being that the first is written in base 10 fractional notation, and the second in base 2. Unfortunately, most decimal fractions cannot be represented exactly as binary fractions. A consequence is that, in general, the decimal floating-point numbers you enter are only approximated by the binary floating-point numbers actually stored in the machine. The problem is easier to understand at first in base 10. Consider the fraction 1/3. You can approximate that as a base 10 fraction: 0.3 or, better, 0.33 or, better, 0.333 and so on. No matter how many digits you're willing to write down, the result will never be exactly 1/3, but will be an increasingly better approximation of 1/3. In the same way, no matter how many base 2 digits you're willing to use, the decimal value 0.1 cannot be represented exactly as a base 2 fraction. In base 2, 1/10 is the infinitely repeating fraction 0.0001100110011001100110011001100110011001100110011... 99
1 0.1) 0. summing ten values of 0.1 0. though.1 It's important to realize that this is. if Python were to print the true decimal value of the binary approximation stored for 0.10000000000000001 you may be tempted to use the round() function to chop it back to the single digit you expect. but the output may be more pleasant to look at: >>> print str(0.0. You may not. it would have to display >>> 0. But that makes no difference: >>> round(0. Other surprises follow from this one.1" was already the best possible binary approximation to 1/10. Another consequence is that since 0. in a real sense.10000000000000001 The problem is that the binary floating-point value stored for "0. you're simply rounding the display of the true machine value. but rounding to 16 digits is not enough to make that true.1 is not exactly 1/10.2 Stop at any finite number of bits.1 may not yield exactly 1. Note that this is in the very nature of binary floating-point: this is not a bug in Python. 1) 0. or in all output modes).Python Tutorial. This is why you see things like: >>> 0. because the number of bits used by the hardware to store floating-point values can vary across machines. and you get an approximation. so trying to round it again can't make it better: it was already as good as it gets. that is what you'll see if you enter 0. You'll see the same kind of thing in all languages that support your hardware's floating-point arithmetic (although some languages may not display the difference by default. giving 0. either: >>> sum = 0. Python's builtin str() function produces only 12 significant digits.6. and it is not a bug in your code either. an illusion: the value in the machine is not exactly 1/10. after seeing >>> 0.1.10000000000000001 On most machines today. For floats. and you may wish to use that instead. Floating Point Arithmetic: Issues and Limitations .1. On most machines.1 0. It's unusual for eval(str(x)) to reproduce x.0 >>> for i in range(10): 100 Chapter 14. For example. repr(float) rounds the true decimal value to 17 significant digits. and Python only prints a decimal approximation to the true decimal value of the binary approximation stored by the machine.1 at a Python prompt.1000000000000000055511151231257827021181583404541015625 instead! The Python prompt uses the builtin repr() function to obtain a string version of everything it displays.10000000000000001 repr(float) produces 17 significant digits because it turns out that's enough (on most machines) so that eval(repr(x)) == x exactly for all finite floats x. Release 2.
While pathological cases do exist. C. Basic familiarity with binary floating-point representation is assumed. and many others) often won't display the exact decimal number you expect: >>> 0.1" example in detail. the best value for N is 56: >>> 2**52 4503599627370496L >>> 2**53 9007199254740992L >>> 2**56/10 7205759403792793L That is. Rewriting 1 / 10 ~= J / (2**N) as J ~= 2**N / 10 and recalling that J has exactly 53 bits (is >= 2**52 but < 2**53). >>> sum 0.10000000000000001 Why is that? 1/10 is not exactly representable as a binary fraction. The best possible value for J is then that quotient rounded: 14. The problem with "0. 14. 56 is the only value for N that leaves J with exactly 53 bits. for most casual use of floating-point arithmetic you'll see the result you expect in the end if you simply round the display of your final results to the number of decimal digits you expect.. "there are no easy answers.2 . That's more than adequate for most tasks.1 Representation Error This section explains the "0. and on most machines are on the order of no more than 1 part in 2**53 per operation. in the "Representation Error" section. Release 2. Representation error refers to the fact that some (most.1 0..1. As that says near the end. sum += 0..99999999999999989 Binary floating-point arithmetic holds many surprises like this." Still. so on input the computer strives to convert 0.6..format() method's format specifiers in Format String Syntax (in The Python Library Reference). and almost all platforms map Python floats to IEEE-754 "double precision". Fortran.1 . don't be unduly wary of floating-point! The errors in Python float operations are inherited from the floating-point hardware. str() usually suffices. Representation Error 101 .1 to the closest fraction it can of the form J/2***N* where J is an integer containing exactly 53 bits. Almost all machines today (November 2000) use IEEE-754 floating point arithmetic. and that every float operation can suffer a new rounding error. See The Perils of Floating Point for a more complete account of other common surprises. but you do need to keep in mind that it's not decimal arithmetic. 754 doubles contain 53 bits of precision.1" is explained in precise detail below. and for finer control see the str. Java. and shows how you can perform an exact analysis of cases like this yourself.Python Tutorial. This is the chief reason why Python (or Perl. actually) decimal fractions cannot be represented exactly as binary (base 2) fractions. C++.
But in no case can it be exactly 1/10! So the computer never "sees" 1/10: what it sees is the exact fraction given above.0 If we multiply that fraction by 10**30. the best approximation is obtained by rounding up: >>> q+1 7205759403792794L Therefore the best possible approximation to 1/10 in 754 double precision is that over 2**56. if we had not rounded up. Release 2. this is actually a little bit larger than 1/10. r = divmod(2**56.10000000000000001 that Python displays (well.Python Tutorial. we can see the (truncated) value of its 30 most significant decimal digits: >>> 7205759403792794 * 10**30 / 2**56 100000000000000005551115123125L meaning that the exact number stored in the computer is approximately equal to the decimal value 0.2 >>> q. the quotient would have been a little bit smaller than 1/10. or 7205759403792794 / 72057594037927936 Note that since we rounded up.1 * 2**56 7205759403792794. will display on any 754-conforming platform that does best-possible input and output conversions in its C library — yours may not!). Rounding that to 17 significant digits gives the 0. 10) >>> r 6L Since the remainder is more than half of 10.6. the best 754 double approximation it can get: >>> . Floating Point Arithmetic: Issues and Limitations .100000000000000005551115123125. 102 Chapter 14.
. square brackets or curly braces). attribute A value associated with an object which is referenced by name using dotted expressions.x code to Python 3. each argument is of a different type (one int. a standalone entry point is provided as Tools/scripts/2to3. Python's creator. and both must be converted to the same type before they can be added or it will raise a TypeError. Positional and keyword arguments may be variable-length: * accepts or passes (if in the function definition or call) several positional arguments in a list. but in 3+4. while ** does the same for keyword arguments in a dictionary. assigned to a named local variable in the function body.APPENDIX A GLOSSARY >>> The default Python prompt of the interactive shell. if an object o has an attribute a it would be referenced as o. one float). 2to3 A tool that tries to convert Python 2. Class definitions normally contain method definitions which operate on instances of the class. You can create your own ABC with the abc module. For example. coercion The implicit conversion of an instance of one type to another during an operation which involves two arguments of the same type.15) converts the floating point number to the integer 3. a. and the evaluated value is passed to the local variable. classic class Any class which does not inherit from object..pyo files so that executing the same file is faster the second time (recompilation from source to bytecode can be avoided).k. 2to3 is available in the standard library as lib2to3. The default Python prompt of the interactive shell when entering code for an indented code block or within a pair of matching left and right delimiters (parentheses.0. and streams (in the io module). Any expression may be used within the argument list. the internal representation of a Python program in the interpreter. class A template for creating user-defined objects. Coercion between two operands can be performed with the 103 . numbers (in the numbers module).a. The bytecode is also cached in . A function or method may have both positional arguments and keyword arguments in its definition. .a. Guido van Rossum. This "intermediate language" is said to run on a virtual machine that executes the machine code corresponding to each bytecode. Python comes with many builtin ABCs for data structures (in the collections module). bytecode Python source code is compiled into bytecode. For example. int(3. BDFL Benevolent Dictator For Life. argument A value passed to a function or method. abstract base class Abstract Base Classes (abbreviated ABCs) complement duck-typing by providing a way to define interfaces when other techniques like hasattr() would be clumsy.x code by handling most of the incompatibilites which can be detected by parsing the source and traversing the parse tree. See new-style class. Often seen for code examples which can be executed interactively in the interpreter. Classic classes will be removed in Python 3.Automated Python 2 to 3 code translation (in The Python Library Reference).pyc and . See 2to3 .5.
however.Python Tutorial. which are written with this latter notation.5).6. __set__(). Called a hash in Perl. float(3)+4. See PEP 343. it must be a duck. If you're not aware of a need for them. static methods..g.") By emphasizing interfaces rather than specific types. docstring A string literal which appears as the first expression in a class. While ignored when the suite is executed.. Python has builtin support for complex numbers. it is recognized by the compiler and put into the __doc__ attribute of the enclosing class. Duck-typing avoids tests using type() or isinstance().g. properties. but if b is a descriptor. The term "CPython" is used in contexts when necessary to distinguish this implementation from others such as Jython or IronPython. 3+1j. CPython The canonical implementation of the Python programming language. see Implementing Descriptors (in The Python Language Reference).add(3. The decorator syntax is merely syntactic sugar. Without coercion. that duck-typing can be complemented with abstract base classes. function or module. Glossary . 104 Appendix A.b to get. the imaginary part is written with a j suffix.. and reference to super classes. it's almost certain you can safely ignore them.add(*coerce(3. context manager An object which controls the environment seen in a with statement by defining __enter__() and __exit__() methods. all arguments of even compatible types would have to be normalized to the same value by the programmer. the following two function definitions are semantically equivalent: def f(. Imaginary numbers are real multiples of the imaginary unit (the square root of -1). The use of dict closely resembles that for list. it typically employs hasattr() tests or EAFP programming. (Note. f = staticmethod(f) @staticmethod def f(. use cmath. 4.5 is equivalent to calling operator. thus. See the documentation for function definition (in The Python Language Reference) for more about decorators. e. its special binding behavior is triggered upon attribute lookup. Common examples for decorators are classmethod() and staticmethod(). Use of complex numbers is a fairly advanced mathematical feature. the respective descriptor method gets called. dictionary An associative array. descriptor Any new-style object which defines the methods __get__(). function or module.): . not just integers.) Instead. duck-typing A pythonic programming style which determines an object's type by inspection of its method or attribute signature rather than by explicit relationship to some type object ("If it looks like a duck and quacks like a duck. Release 2. well-designed code improves its flexibility by allowing polymorphic substitution. complex number An extension of the familiar real number system in which all numbers are expressed as a sum of a real part and an imaginary part.... 4. 3+4. Since it is available via introspection. e. often written i in mathematics or j in engineering. methods. set or delete an attribute looks up the object named b in the class dictionary for a. using a.2 coerce builtin function. class methods. Normally. where arbitrary keys are mapped to values. To get access to complex equivalents of the math module. it is the canonical place for documentation of the object.5.. usually applied as a function transformation using the @wrapper syntax. but the keys can be any object with a __hash__() function. decorator A function returning another function. For more information about descriptors' methods.. When a class attribute is a descriptor.5)) and results in operator.0. Understanding descriptors is a key to a deep understanding of Python because they are the basis for many features including functions.): .5 rather than just 3+4. or __delete__()...
generator A function which returns an iterator. generator expression An expression that returns a generator. The technique contrasts with the LBYL style common to many other languages such as C.. The combined expression generates values for an enclosing function: >>> sum(i*i for i in range(10)) 285 GIL See global interpreter lock. If the module in which it is executed had enabled true division by executing: from __future__ import division the expression 11/4 would evaluate to 2. See also argument and method. Python performs garbage collection via reference counting and a cyclic garbage collector that is able to detect and break reference cycles. By importing the __future__ module and evaluating its variables. This clean and fast style is characterized by the presence of many try and except statements. attribute access. It must implement a method named find_module(). It looks like a normal expression followed by a for expression defining a loop variable.75. # sum of squares 0. an expression is an accumulation of expression elements like literals. global interpreter lock The lock used by Python threads to assure that only one thread executes in the CPython virtual machine at a time. 8192) garbage collection The process of freeing memory when it is not used anymore. Efforts have been made in the past to create a "free-threaded" interpreter (one which locks shared data at a much finer granularity). finder An object that tries to find the loader for a module. not all language constructs are expressions. 0. It looks like a normal function except that values are returned to the caller using a yield statement instead of a return statement. the expression 11/4 currently evaluates to 2. 0. at the expense of much of the parallelism afforded by multi-processor machines. you can see when a new feature was first added to the language and when it will become the default: >>> import __future__ >>> __future__. See PEP 302 for details. 2. and an optional if expression. For example. In other words. It can also be passed zero or more arguments which may be used in the execution of the body. 'alpha'. 81 105 . (3. expression A piece of syntax which can be evaluated to some value. 0). names.Python Tutorial. Generator functions often contain one or more for or while loops which yield elements back to the caller. This common Python coding style assumes the existence of valid keys or attributes and catches exceptions if the assumption proves false. 2). function A series of statements which returns some value to a caller. extension module A module written in C or C++. 4. 1. such as print or if. .division _Feature((2. but so far none have been successful because performance suffered in the common single-processor case. There are also statements which cannot be used as expressions. operators or function calls which all return a value.2 EAFP Easier to ask for forgiveness than permission.. range. __future__ A pseudo module which programmers can use to enable new language features which are not compatible with the current interpreter. Locking the entire interpreter makes it easier for the interpreter to be multi-threaded. This simplifies the CPython implementation by assuring that no two processes can access the same memory at the same time. Assignments are also statements. Release 2. 0. not expressions. The function execution is stopped at the yield keyword (returning the result) and is resumed there when the next element is requested by calling the next() method of the returned iterator.6. using Python's C API to interact with the core and with user code. In contrast to many other languages. 'alpha'.
Also called floor division.). Attempting this with an iterator will just return the same exhausted iterator object used in the previous iteration pass.Python Tutorial. Good for beginners. creating a temporary unnamed variable to hold the iterator for the duration of the loop. 106 Appendix A. They play an important role in places where a constant hash value is needed. it also serves as clear example code for those wanting to implement a moderately sophisticated. immediately execute them and see their results. Just launch python with no arguments (possibly by selecting it from your computer's main menu).2 hashable An object is hashable if it has a hash value which never changes during its lifetime (it needs a __hash__() method). for example as a key in a dictionary. map(). Such an object cannot be altered. This iterator is good for one pass over the set of values. interactive Python has an interactive interpreter which means you can enter statements and expressions at the interpreter prompt. iterable A container object capable of returning its members one at a time. possibly with a decimal fraction. both a finder and loader object. if one of the operands is another numeric type (such as a float). str. However. The for statement does that automatically for you. At this point.6. and their hash value is their id(). Iterables can be used in a for loop and in many other places where a sequence is needed (zip(). IDLE An Integrated Development Environment for Python. Repeated calls to the iterator's next() method return successive items in the stream. integer division Mathematical division discarding any remainder. they all compare unequal. and can be compared to other objects (it needs an __eq__() or __cmp__() method). When no more data are available a StopIteration exception is raised instead. See also iterator. the iterator object is exhausted and any further calls to its next() method just raise StopIteration again. For example. Integer division can be forced by using the // operator instead of the / operator. because these data structures use the hash value internally. When an iterable object is passed as an argument to the builtin function iter().. immutable An object with a fixed value. the expression 11/4 currently evaluates to 2 in contrast to the 2. the result will be coerced (see coercion) to a common type. Immutable objects include numbers. This means that source files can be run directly without explicitly creating an executable which is then run. When dividing two integers the outcome will always be another integer (having the floor function applied to it). making it appear like an empty container. while no mutable containers (such as lists or dictionaries) are. iterator An object representing a stream of data.75 returned by float division. All of Python's immutable built-in objects are hashable. it returns an iterator for the object. Interpreted languages typically have a shorter development/debug cycle than compiled ones. Objects which are instances of user-defined classes are hashable by default. and generator. See also __future__. Hashability makes an object usable as a dictionary key and a set member. A new object has to be created if a different value has to be stored. IDLE is a basic editor and interpreter environment which ships with the standard distribution of Python. it is usually not necessary to call iter() or deal with iterator objects yourself. Hashable objects which compare equal must have the same hash value. See also interactive.. It is a very powerful way to test out new ideas or inspect modules and packages (remember help(x)). multi-platform GUI application. Release 2. and tuple) and some non-sequence types like dict and file and objects of any classes you define with an __iter__() or __getitem__() method. More information can be found in Iterator Types (in The Python Library Reference). A container object (such as a list) produces a fresh new iterator each time you pass it to the iter() function or use it in a for loop. For example. interpreted Python is an interpreted language. sequence. importer An object that both finds and loads a module. When using iterables. an integer divided by a float will result in a float value. One notable exception is code which attempts multiple iteration passes. as opposed to a compiled one. though the distinction can be blurry because of the presence of the bytecode compiler. strings and tuples. Examples of iterables include all sequence types (such as list. though their programs generally also run more slowly. . Iterators are required to have an __iter__() method that returns the iterator object itself so every iterator is also iterable and may be used in most places where other iterables are accepted. Glossary .
adding thread-safety. respectively. For instance. This coding style explicitly tests for pre-conditions before making calls or lookups. writing random. named tuple Any tuple-like class whose indexable elements are also accessible using named attributes (for example. Despite its name it is more akin to an array in other languages than to a linked list since access to elements are O(1). It must define a method named load_module(). list A built-in Python sequence. or it can be created with a regular class definition.2 keyword argument Arguments which are preceded with a variable_name= in the call. What makes Python special is that it is possible to create custom metaclasses.Python Tutorial. Namespaces support modularity by preventing naming conflicts. The metaclass is responsible for taking those three arguments and creating the class. ** is used to accept or pass a dictionary of keyword arguments. metaclass The class of a class. The latter approach automatically provides extra features such as a selfdocumenting representation like Employee(name='jones'. 107 . In contrast. For instance.struct_time. Likewise.open() are distinguished by their namespaces. A named tuple can be a built-in type such as time. global variables read and write to the global namespace. loader An object that loads a module.open() and os.. time. Class definitions create a class name.tm_year).izip() makes it clear that those functions are implemented by the random and itertools modules. mutable Mutable objects can change their value but keep their id(). the functions __builtin__. More information can be found in Customizing class creation (in The Python Language Reference). nested scope The ability to refer to a variable in an enclosing definition.6. the method will get the instance object as its first argument (which is usually called self). method A function which is defined inside a class body. elegant solutions. The syntax to create a lambda function is lambda [arguments]: expression LBYL Look before you leap. metaclasses can provide powerful. Namespaces are implemented as dictionaries. lambda An anonymous inline function consisting of a single expression which is evaluated when the function is called. title='programmer'). If called as an attribute of an instance of that class. mapping A container object (such as dict) which supports arbitrary key lookups using the special method __getitem__().localtime() returns a tuple-like object where the year is accessible either with an index such as t[0] or with a named attribute like t.seed() or itertools. and many other tasks.) in the range from 0 to 255.namedtuple(). Note that nested scopes work only for reference and not for assignment which will always write to the innermost scope. See function and nested scope. namespace The place where a variable is stored. all elements in range(256) are processed. If omitted. result = ["0x%02x" % x for x in range(256) if x % 2 == 0] generates a list of strings containing even hex numbers (0x. a class dictionary. a function defined inside another function can refer to variables in the outer function. The variable name designates the local name in the function to which the value is assigned. They have been used for logging attribute access. Namespaces also aid readability and maintainability by making it clear which module implements a function. This style contrasts with the EAFP approach and is characterized by the presence of many if statements. The if clause is optional. Release 2. and a list of base classes. Most users never need this tool. There are the local. list comprehension A compact way to process all or part of the elements in a sequence and return a list with the results. Most object oriented programming languages provide a default implementation. See argument. implementing singletons. A full featured named tuple can also be created with the factory function collections. See PEP 302 for details. For instance. A loader is typically returned by a finder. but when the need arises. local variables both read and write in the innermost scope. tracking object creation. See also immutable. global and builtin namespaces as well as nested namespaces in objects (in methods).
Pythonic method: for piece in food: print piece reference count The number of references to an object. See argument. The bracket (subscript) notation uses slice objects internally (or in older versions.2 new-style class Any class which inherits from object. Such methods have names starting and ending with double underscores. Reference counting is generally not visible to Python code. The sys module defines a getrefcount() function that programmers can call to return the reference count for a particular object. but it is a key element of the CPython implementation. Glossary .6. Also the ultimate base class of any new-style class. statement A statement is part of a suite (a "block" of code). slice An object usually containing a portion of a sequence. rather than implementing code using concepts common to other languages. Many other languages don't have this type of construct. When the reference count of an object drops to zero. the technique is somewhat tricky to get right and is best reserved for rare cases where there are large numbers of instances in a memory-critical application. For example. positional argument The arguments assigned to local names inside a function or method. __getslice__() and __setslice__()). This includes all built-in types like list and dict. Note that dict also supports __getitem__() and __len__(). object Any data with state (attributes or value) and defined behavior (methods). determined by the order in which they were given in the call. but is considered a mapping rather than a sequence because the lookups use arbitrary immutable keys rather than integers. Only new-style classes can use Python's newer. it is deallocated. a common idiom in Python is to loop over all elements of an iterable using a for statement. Pythonic An idea or piece of code which closely follows the most common idioms of the Python language. sequence An iterable which supports efficient element access using integer indices via the __getitem__() special method and defines a len() method that returns the length of the sequence. A slice is created using the subscript notation. properties. * is used to either accept multiple positional arguments (when in the definition).) This is also abbreviated "Py3k". Some built-in sequence types are list. Though popular. while or print. versatile features like __slots__. Special methods are documented in Special method names (in The Python Language Reference). Release 2. special method A method that is called implicitly by Python to execute a certain operation on a type. and __getattribute__(). __slots__ A declaration inside a new-style class that saves memory by pre-declaring space for instance attributes and eliminating instance dictionaries.Python Tutorial. More information can be found in New-style and classic classes (in The Python Language Reference). A statement is either an expression or a one of several constructs with a keyword. and unicode. descriptors. 108 Appendix A. or pass several arguments as a list to a function.0 (coined long ago when the release of version 3 was something in the distant future. such as in variable_name[1:3:5]. such as addition. so people unfamiliar with Python sometimes use a numerical counter instead: for i in range(len(food)): print food[i] As opposed to the cleaner. tuple. 3. str. Python 3000 Nickname for the next major Python version. such as if. [] with colons between numbers when several are given.
every object has a type.Python Tutorial. They allow you to include unescaped single and double quotes within a string and they can span multiple lines without the use of the continuation character. While they don't provide any functionality not available with single-quoted strings. making them especially useful when writing docstrings. they are useful for a number of reasons. 109 . type The type of a Python object determines what kind of object it is.6. An object's type is accessible as its __class__ attribute or can be retrieved with type(obj).2 triple-quoted string A string which is bound by three instances of either a quotation mark (") or an apostrophe ('). virtual machine A computer defined entirely in software. Release 2. Python's virtual machine executes the bytecode emitted by the bytecode compiler. The listing can be found by typing "import this" at the interactive prompt. Zen of Python Listing of Python design principles and philosophies that are helpful in understanding and using the language.
2 110 Appendix A.6.Python Tutorial. Release 2. Glossary .
Janko Hauser. Michal Bozon. Ben Gertzfield. Amoroso. Lee Busby. Marc-André Lemburg. Jim Ahlstrom. you can submit comments and suggest changes directly on the documentation pages. and we'll be glad to correct the problem. Lorenzo M. Grant Griffin. Hetland. Heidi Annexstad. Catucci. Ulf A. Travis B. A. Thomas Lamb. Gabriel Genellina. Steve Holden. Manus Hand. Ian Bruntlett. Shelley Gooch. Anthony Baxter. Greg Kochanski. Stefan Hoffmeister. Gilles Civario. the creator of the original Python documentation toolset and writer of much of the content. Peter Deutsch. Chris Barker. Randall Hopper. Albert Hofkamp.. Fredrik Lundh. Martin von Löwis. Jeff MacDonald. Matthew Cowles. Andrew Dalke. Jim Kerr. Robert Lehmann. Pedro Diaz Jimenez. Nadim Ghaznavi. Brian Hooper. Pehr Anderson. Piers Lauder. Steve Alexander. B. Jonathan Giddy. Robert Kern.. Jim Fulton. Thomas Holenstein. Jonathan Black. In the online version of these documents. Gerrit Holl. Lele Gaifax. We're always looking for volunteers wanting to help with the docs. John 111 . Blame Andy Eskilsson. Mark Hammond. Carl Cerecke. Fred L. Robin Boerdijk. It is probably not complete – if you feel that you or anyone else should be on this list. Thomas Heller. Jesús Cea Avión. Aahz. L. Mauro Cicognini.APPENDIX B ABOUT THESE DOCUMENTS These documents are generated from reStructuredText sources by Sphinx. Don Bashford. Dave Kuhlman. Jensen. Harald Hanche-Olsen. Michael Hudson. Daniel Kozan. please let us know (send email to docs@python. Peter Funk. Andrew M. Erno Kuusela. Jack Jansen. Eric Huss. Lindgren.org mailing list. Roger Irwin. Fred Allen. Alexander Belopolsky. Carey Evans. Glyph Lefkowitz. Gregor Hoffleit. Josip Dzolonga. Mike Clarkson. Guido Kollerie. Daniel Barclay. Dan Finnie. Development of the documentation and its toolchain takes place on the docs@python. Andreas Jung. so feel free to send a mail there! Many thanks go to: • Fred L. Keith Briggs. Matthew Gallagher. Mirko Liss. Nathaniel Gray. • Fredrik Lundh for his Alternative Python Reference project from which Sphinx got many good ideas. • the Docutils project for creating reStructuredText and the Docutils suite. Magnus L. Jeff Epler. Gerhard Häring. Robert Donohue. Carl Feynman. Kent Johnson. Anders Hammarquist. Oliver Andrich. Everett Lipman. Peter A. Drake. Tim Hatch. Stefan Franke. Michael Abbott. Aaron Brancotti. Detlef Lannert. Martijn Faassen. Bernhard Herzog. Lucas de Jonge. Ross Light. Ben Darnell. Jeremy Hylton. See Reporting Bugs in Python for information how to report bugs in Python itself. Steve Clift. Jr. a document processor specifically written for the Python documentation. Jr. Konrad Hinsen. Drake. Thomas Guettler. Rob Hooft. Bennett Benson. Jeremy Craven. Kuchling. Dave Cole. Koren. Jan Kim.org). Michael Ernst. Georg Brandl. Philip H. Hernán Martínez Foffani.1 Contributors to the Python Documentation This section lists people who have contributed in some way to the Python documentation. Hartwell.
Chris Ryland. Otkidach. Kalle Svensson. Martijn Vries. Tim Peters. About these documents . Blake Winton. Edward K. Release 2. Moshe Zadka. Harri Pasanen. Bernhard Reiter. Justin Sheehy. Donald Wallace Rouse II. Rebecca McCreary. Ng Pheng Siong. Ville Vainio. Dale Nagata. Michael Sloan. Michael Simcich. Paolo Milani. Shriphani Palakodety. Reuben Sumner. Zooko O'Whielacronx. Thomas Wouters. Greg Stein. Eddy Welbourne. Tage Stabell-Kulo. François Pinard.2 Machin. Guido van Rossum. Chris Phoenix. Barry Warsaw. Skip Montanaro. Milan Zamazal. Gregory P. Greg Ward. Bo Peng. Collin Winter. Mark Russell. Ka-Ping Yee. Jim Tittsler. Charles G. Jim Roskind. David Turner. Daniel May. Paul Moore. Bob Savage. It is only with the input and contributions of the Python community that Python has such wonderful documentation – Thank You! 112 Appendix B. Smith.Python Tutorial. Roy Smith. Sean Reifschneider. Joakim Sernbrant. Rory Yorke. Benjamin Peterson. Nicholas Spies. Nick Russo.6. Anthony Starks. Gerry Wiener. Joonas Paalasmaa. Neil Schemenauer. Cheng Zhang. Ross Moore. Barry Scott. Paul Prescod. Vladimir Marangozov. Hugh Sasse. Scott Schram. Frank Stajano. Laura Matson. Mark Summerfield. Armin Ronacher. Corran Webster. Tomas Oppelstrup. Glyn Webster. Jeff Wheeler. Peter Stoehr. Raymond. Vincent Marchetti. Steven Work. Andrew MacIntyre. Doug Mennella. Eric S. Waldman. Ream. Ionel Simionescu. Dan Wolfe. Pettit. William Park. Armin Rigo. Sjoerd Mullender. Denis S.. Charlie Shepherd. Wes Rishel. Constantina S. Christopher Petrilli. Justin D. Bob Weiner. Koray Oner. Timothy Wild. Mats Wichmann. Clay Spence.
com to form the BeOpen PythonLabs team.cnri.com/). see http://www. All Python releases are Open Source (see http://www. Guido and the Python core development team moved to BeOpen. although it includes many contributions from others. In 2001. see http://www. In October of the same year. see http://www.zope.org/ for the Open Source Definition).reston. In May 2000. a non-profit organization created specifically to own Python-related Intellectual Property. the Python Software Foundation (PSF. 113 .nl/) in the Netherlands as a successor of a language called ABC. Guido continued his work on Python at the Corporation for National Research Initiatives (CNRI. Virginia where he released several versions of the software. the table below summarizes the various releases.va.python. most.1 History of the software Python was created in the early 1990s by Guido van Rossum at Stichting Mathematisch Centrum (CWI.org/psf/) was formed. Zope Corporation is a sponsoring member of the PSF. the PythonLabs team moved to Digital Creations (now Zope Corporation. Python releases have also been GPL-compatible.opensource.us/) in Reston.APPENDIX C HISTORY AND LICENSE C. Historically. Guido remains Python's principal author.cwi. but not all. see http://www. In 1995.
22.214.171.124 Year 1991-1995 1995-1999 2000 2000 2001 2001 2001 2001 2001 2002 2002 2002 2002 2002-2003 2002-2003 2002-2003 2003 2003 2004 2005 2004 2005 2005 2006 2006 2006 2007 2008 2008 2008 2008 Owner CWI CNRI CNRI BeOpen.2 2.6.com CNRI PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF PSF GPL compatible? yes yes no no no no yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes Note: GPL-compatible doesn't mean that we're distributing Python under the GPL.4 2.5.1 2.2 1.6.2 1. the others don't.0+1.5 2. however. "Copyright © 2001-2009 Python Software Foundation. All Python licenses.2 2.2 2. Subject to the terms and conditions of this License Agreement. royalty-free.2.1.1 2.1 2.6 1. C.2.5 2..4 220.127.116.11.9.e.2 alone or in any derivative version.3.2 Release 0.2 2.5. prepare derivative works.1 2.3 2.0 1. History and License .3 2.1+2.1 2.2 2.1.2 2. PSF hereby grants Licensee a nonexclusive.126.96.36.199 2.6.2 2.6 2. that PSF's License Agreement and PSF's notice of copyright.3 2. 114 Appendix C.5.1 2. unlike the GPL.5 2.5.1 2. provided.6 18.104.22.168 2.1 Derived from n/a 184.108.40.206 2.1.2 2.6.1. analyze.5.6.3 2.2.0+1. test. and the Individual or Organization ("Licensee") accessing and otherwise using Python 2.2 18.104.22.168.4 2.2 2.2 2.1. world-wide license to reproduce.3 2. let you distribute a modified version without making your changes open source.1 2.6.0 thru 1.3 2.Python Tutorial.1 2. Release 2. i. and otherwise use Python 2. This LICENSE AGREEMENT is between the Python Software Foundation ("PSF").4.6 2.4.3. The GPL-compatible licenses make it possible to combine Python with other software that is released under the GPL.4 2.2 1.1 2. Thanks to the many outside volunteers who have worked under Guido's direction to make these releases possible.4 2.2.2 1. distribute.4 2.2 2.5.1 2.3.1 2.4. perform and/or display publicly.2 alone or in any derivative version prepared by Licensee.5 2.2 1.1 2.4.1 184.108.40.206 thru 1.3 2.3 2.3 2.2 software in source or binary form and its associated documentation. All Rights Reserved" are retained in Python 2.3.2 Terms and conditions for accessing or otherwise using Python PSF LICENSE AGREEMENT FOR PYTHON 2.1 2.1 2.
6. BEOPEN MAKES NO AND DISCLAIMS ANY REPRESENTATION OR WARRANTY OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE OR THAT THE USE OF THE SOFTWARE WILL NOT INFRINGE ANY THIRD PARTY RIGHTS. partnership. PSF SHALL NOT BE LIABLE TO LICENSEE OR ANY OTHER USERS OF PYTHON 2. OR OTHERWISE USING PYTHON 2. By copying. PSF MAKES NO AND DISCLAIMS ANY REPRESENTATION OR WARRANTY OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE OR THAT THE USE OF PYTHON 2. In the event Licensee prepares a derivative work that is based on or incorporates Python 2. analyze.2.2.6. and otherwise use the Software alone or in any derivative version. Santa Clara. having an office at 160 Saratoga Avenue. Release 2. BEOPEN.2 FOR ANY INCIDENTAL. and the Individual or Organization ("Licensee") accessing and otherwise using this software in source or binary form and its associated documentation ("the Software").2 3. Terms and conditions for accessing or otherwise using Python 115 . Subject to the terms and conditions of this BeOpen Python License Agreement.0 BEOPEN PYTHON OPEN SOURCE LICENSE AGREEMENT VERSION 1 1. 5. BeOpen is making the Software available to Licensee on an "AS IS" basis.COM LICENSE AGREEMENT FOR PYTHON 2. BUT NOT LIMITATION. This License Agreement shall be governed by and interpreted in all respects by the law of the State of California. BeOpen hereby grants Licensee a non-exclusive. 5.2 or any part thereof. This License Agreement will automatically terminate upon a material breach of its terms and conditions. 8. BEOPEN MAKES NO REPRESENTATIONS OR WARRANTIES. distribute. test. BEOPEN SHALL NOT BE LIABLE TO LICENSEE OR ANY OTHER USERS OF THE SOFTWARE FOR ANY INCIDENTAL. partnership. alone or in any derivative version prepared by Licensee.Python Tutorial. 6. EXPRESS OR IMPLIED. or joint venture between PSF and Licensee.2 available to Licensee on an "AS IS" basis. or any third party. Licensee agrees to be bound by the terms and conditions of this License Agreement. the "BeOpen Python" logos available at http://www. provided. or joint venture between BeOpen and Licensee. royalty-free.com/logos.6. 4.2 WILL NOT INFRINGE ANY THIRD PARTY RIGHTS. PSF is making Python 2. that the BeOpen Python License is retained in the Software. SPECIAL. PSF MAKES NO REPRESENTATIONS OR WARRANTIES. EVEN IF ADVISED OF THE POSSIBILITY THEREOF.html may be used according to the permissions granted on that web page.6.6. Nothing in this License Agreement shall be deemed to create any relationship of agency. however. EXPRESS OR IMPLIED. perform and/or display publicly. 6. DISTRIBUTING. 7. world-wide license to reproduce.6. BUT NOT LIMITATION. SPECIAL. This License Agreement will automatically terminate upon a material breach of its terms and conditions. This LICENSE AGREEMENT is between BeOpen. and wants to make the derivative work available to others as provided herein. 3. Nothing in this License Agreement shall be deemed to create any relationship of agency. OR CONSEQUENTIAL DAMAGES OR LOSS AS A RESULT OF MODIFYING. prepare derivative works. This License Agreement does not grant permission to use PSF trademarks or trade name in a trademark sense to endorse or promote products or services of Licensee. excluding conflict of law provisions. C. OR ANY DERIVATIVE THEREOF. EVEN IF ADVISED OF THE POSSIBILITY THEREOF. OR CONSEQUENTIAL DAMAGES OR LOSS AS A RESULT OF USING. installing or otherwise using Python 2. MODIFYING OR DISTRIBUTING THE SOFTWARE.2. 4. then Licensee hereby agrees to include in any such work a brief summary of the changes made to Python 2. As an exception.6.com ("BeOpen").pythonlabs.2. 2. BY WAY OF EXAMPLE. or any third party. CA 95051. OR ANY DERIVATIVE THEREOF. BY WAY OF EXAMPLE.6. This License Agreement does not grant permission to use BeOpen trademarks or trade names in a trademark sense to endorse or promote products or services of Licensee.
and distribute this software and its documentation for any purpose and without fee is hereby granted. BUT NOT LIMITATION. persistent identifier (known as a handle): 1895. then Licensee hereby agrees to include in any such work a brief summary of the changes made to Python 1. installing or otherwise using Python 1. By clicking on the "ACCEPT" button where indicated. federal law does not apply.220.127.116.11. perform and/or display publicly. however.1 that incorporate non-separable material that was previously distributed under the GNU General Public License (GPL). installing or otherwise using the software.6.1 may be located on the Internet using the following unique. All rights reserved. 2.1 software in source or binary form and its associated documentation.1 alone or in any derivative version. EVEN IF ADVISED OF THE POSSIBILITY THEREOF.2 Copyright © 1991 . world-wide license to reproduce. in lieu of CNRI's License Agreement. including without limitation the federal copyright law. Alternately.1 alone or in any derivative version prepared by Licensee. EXPRESS OR IMPLIED. CNRI MAKES NO AND DISCLAIMS ANY REPRESENTATION OR WARRANTY OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE OR THAT THE USE OF PYTHON 1. partnership. royalty-free.0 THROUGH 1. Licensee agrees to be bound by the terms and conditions of this License Agreement. modify.handle. SPECIAL. 5. Licensee may substitute the following text (omitting the quotes): "Python 1. This License Agreement will automatically terminate upon a material breach of its terms and conditions.2 7. copy.1 is made available subject to the terms and conditions in CNRI's License Agreement. Reston. CNRI LICENSE AGREEMENT FOR PYTHON 1. provided. excluding Virginia's conflict of law provisions. Subject to the terms and conditions of this License Agreement.1 WILL NOT INFRINGE ANY THIRD PARTY RIGHTS. This License Agreement does not grant permission to use CNRI trademarks or trade name in a trademark sense to endorse or promote products or services of Licensee. The Netherlands.9.e. provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation. "Copyright © 1995-2001 Corporation for National Research Initiatives.. and 7 of this License Agreement.1 FOR ANY INCIDENTAL.1 available to Licensee on an "AS IS" basis. This License Agreement shall be governed by the federal intellectual property law of the United States. Notwithstanding the foregoing.6. CNRI is making Python 1. analyze.6.net/1895. by the law of the Commonwealth of Virginia. Licensee agrees to be bound by the terms and conditions of this License Agreement. This LICENSE AGREEMENT is between the Corporation for National Research Initiatives. 5. This Agreement together with Python 1. and otherwise use Python 1.S.6. This Agreement may also be obtained from a proxy server on the Internet using the following URL: http://hdl. test. and the Individual or Organization ("Licensee") accessing and otherwise using Python 1.1995." 3. CNRI SHALL NOT BE LIABLE TO LICENSEE OR ANY OTHER USERS OF PYTHON 1. to the extent such U. In the event Licensee prepares a derivative work that is based on or incorporates Python 1. and that the name of Stichting Mathematisch Centrum or 116 Appendix C.1. i. DISTRIBUTING.6. 8. Nothing in this License Agreement shall be deemed to create any relationship of agency. with regard to derivative works based on Python 1.1 1. prepare derivative works. CNRI hereby grants Licensee a nonexclusive. CNRI MAKES NO REPRESENTATIONS OR WARRANTIES.6. having an office at 1895 Preston White Drive. that CNRI's License Agreement and CNRI's notice of copyright.1.6.1 or any part thereof. Permission to use. and. VA 20191 ("CNRI"). By copying. Stichting Mathematisch Centrum Amsterdam. History and License . OR CONSEQUENTIAL DAMAGES OR LOSS AS A RESULT OF MODIFYING. All Rights Reserved" are retained in Python 1.22/1013. and wants to make the derivative work available to others as provided herein. OR OTHERWISE USING PYTHON 220.127.116.11/1013. or any third party. distribute. Release 2. OR ANY DERIVATIVE THEREOF. 4. 7.6. BY WAY OF EXAMPLE. ACCEPT CWI LICENSE AGREEMENT FOR PYTHON 0.Python Tutorial. or joint venture between CNRI and Licensee. or by copying.6. the law of the Commonwealth of Virginia shall govern this License Agreement only as to issues arising under or with respect to Paragraphs 4.6. 6.
WHETHER IN AN ACTION OF CONTRACT. written prior permission. INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE. OR CONSEQUENTIAL DAMAGES (INCLUDING. The following are the verbatim comments from the original code: A C-program for MT19937. STICHTING MATHEMATISCH CENTRUM DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE. Coded by Takuji Nishimura and Makoto Matsumoto. Redistributions of source code must retain the above copyright notice. BUT NOT LIMITED TO. BUT NOT LIMITED TO.html.Python Tutorial.2 CWI not be used in advertising or publicity pertaining to distribution of the software without specific.3. INCLUDING. THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. Licenses and Acknowledgements for Incorporated Software 117 .2002. PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES. Release 2. OR PROFITS. but growing list of licenses and acknowledgements for third-party software incorporated in the Python distribution. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES. Makoto Matsumoto and Takuji Nishimura. with initialization improved 2002/1/26. C. SPECIAL. The names of its contributors may not be used to endorse or promote products derived from this software without specific prior written permission.1 Mersenne Twister The _random module includes code based on a download from http://www.jp/ matumoto/MT2002/emt19937ar. INCIDENTAL. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT. 2.3 Licenses and Acknowledgements for Incorporated Software This section is an incomplete. INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. initialize the state by using init_genrand(seed) or init_by_array(init_key. with or without modification. Before using. All rights reserved. NEGLIGENCE OR OTHER TORTIOUS ACTION. Redistribution and use in source and binary forms. OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF C. are permitted provided that the following conditions are met: 1. 3. Redistributions in binary form must reproduce the above copyright notice. ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. LOSS OF USE. IN NO EVENT SHALL STICHTING MATHEMATISCH CENTRUM BE LIABLE FOR ANY SPECIAL.6. DATA.ac. this list of conditions and the following disclaimer. EXEMPLARY. INDIRECT. Copyright (C) 1997 .math. key_length).keio. DATA OR PROFITS.3. C.
Python Tutorial, Release 2.6.2
Any feedback is very welcome. http://www.math.keio.ac.jp/matumoto/emt.html email: email@example.com
C.3.2 Sockets
The socket module uses the functions, getaddrinfo(), and getnameinfo(), which are coded in separate source files from the WIDE Project, http://www.wide.ad.jp/. Copyright (C) 1995, 1996, 1997, and 1998 WIDE Project. All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. 3. Neither the name of the project nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY THE PROJECT AND CONTRIBUTORS ''AS IS'' AND GAI_ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE PROJECT OR CONTRIBUTORS BE LIABLE FOR GAI_ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON GAI_ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN GAI_ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
C.3.3 Floating point exception control
The source for the fpectl module includes the following notice: --------------------------------------------------------------------/ Copyright (c) 1996. \ | The Regents of the University of California. | | All rights reserved. | | | | Permission to use, copy, modify, and distribute this software for | | any purpose without fee is hereby granted, provided that this en| 118 Appendix C. History and License
Python Tutorial, Release 2.6.2
| | | | | | | | | | | | | | | | | | | | | | | | | |
tire notice is included in all copies of any software which is or includes a copy or modification of this software and in all copies of the supporting documentation for such software. This work was produced at the University of California, Lawrence Livermore National Laboratory under contract no. W-7405-ENG-48 between the U.S. Department of Energy and The Regents of the University of California for the operation of UC LLNL. DISCLAIMER This software was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately-owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product \ endorsement purposes. ---------------------------------------------------------------------
| | | | | | | | | | | | | | | | | | | | | | | | | | /
C.3.4 MD5 message digest algorithm
The source code for the md5 module contains the following notice: Copyright (C) 1999, 2002 Aladdin Enterprises. All rights reserved.
This software is provided 'as-is', without any express or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software. Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions: 1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required. 2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software. 3. This notice may not be removed or altered from any source distribution. L. Peter Deutsch firstname.lastname@example.com C.3. Licenses and Acknowledgements for Incorporated Software 119
Python Tutorial, Release 2.6.2
Independent implementation of MD5 (RFC 1321). This code implements the MD5 Algorithm defined in RFC 1321, whose text is available at http://www.ietf.org/rfc/rfc1321.txt The code is derived from the text of the RFC, including the test suite (section A.5) but excluding the rest of Appendix A. It does not include any code or documentation that is identified in the RFC as being copyrighted. The original and principal author of md5.h is L. Peter Deutsch <firstname.lastname@example.com>. Other authors are noted in the change history that follows (in reverse chronological order): 2002-04-13 lpd Removed support for non-ANSI compilers; removed references to Ghostscript; clarified derivation from RFC 1321; now handles byte order either statically or dynamically. 1999-11-04 lpd Edited comments slightly for automatic TOC extraction. 1999-10-18 lpd Fixed typo in header comment (ansi2knr rather than md5); added conditionalization for C++ compilation from Martin Purschke <email@example.com>. 1999-05-03 lpd Original version.
C.3.5 Asynchronous socket services
The asynchat and asyncore modules contain the following notice: Copyright 1996 by Sam Rushing All Rights Reserved Permission to use, copy, modify, and distribute this software and its documentation for any purpose and without fee is hereby granted, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation, and that the name of Sam Rushing not be used in advertising or publicity pertaining to distribution of the software without specific, written prior permission. SAM RUSHING DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL SAM RUSHING BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
C.3.6 Cookie management
The Cookie module contains the following notice:
Appendix C. History and License
edu> All Rights Reserved Permission to use. Written by James Roskind Permission to use. WHETHER IN AN ACTION OF CONTRACT. and that both that copyright notice and this permission notice appear in supporting documentation. ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.6. ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.3. Licenses and Acknowledgements for Incorporated Software 121 . NEGLIGENCE OR OTHER TORTIOUS ACTION. INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE. by InfoSeek Corporation.7 Profiling The profile and pstats modules contain the following notice: Copyright 1994. written prior permission. DATA OR PROFITS. provided that the above copyright notice appears in all copies. WHETHER IN AN ACTION OF CONTRACT. modify.3. copy. IN NO EVENT SHALL INFOSEEK CORPORATION BE LIABLE FOR ANY SPECIAL.mit. INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. all rights reserved. and that the name of Timothy O'Malley not be used in advertising or publicity pertaining to distribution of the software without specific. IN NO EVENT SHALL Timothy O'Malley BE LIABLE FOR ANY SPECIAL. copy. C. This permission is explicitly restricted to the copying and modification of the software to remain in Python. C. compiled Python. modify. INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. DATA OR PROFITS.2 Copyright 2000 by Timothy O'Malley <timo@alum. and that the name of InfoSeek not be used in advertising or publicity pertaining to distribution of the software without specific.Python Tutorial. Timothy O'Malley DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE. NEGLIGENCE OR OTHER TORTIOUS ACTION. and distribute this software and its documentation for any purpose and without fee is hereby granted. and distribute this Python software and its associated documentation for any purpose (subject to the restriction in the following sentence) without fee is hereby granted. INFOSEEK CORPORATION DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE. INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE. or other languages (such as C) wherein the modified or derived code is exclusively imported into a Python module. provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation. written prior permission. Release 2.
modify. INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE. all rights reserved. United States of America. Autonomous Zones Industries.com Copyright 2000.. Stichting Mathematisch Centrum. copy. Release 2. reserved and offered to the public under the terms of the Python 2. and distribute this software and its documentation for any purpose and without fee is hereby granted.3. Inc. Inc. provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation. written prior permission.3.. All Rights Reserved Permission to use.. Inc. and distribute this Python software and its associated documentation for any purpose without fee is hereby granted. Automatrix. and that the name of neither Automatrix.. DATA OR PROFITS. Bioreason or Mojam Media be used in advertising or publicity pertaining to distribution of the software without specific.2 license. and that both that copyright notice and this permission notice appear in supporting documentation. INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. WHETHER IN AN ACTION OF CONTRACT.9 UUencode and UUdecode functions The uu module contains the following notice: Copyright 1994 by Lance Ellinghouse Cathedral City. C. and that the name of Lance Ellinghouse not be used in advertising or publicity pertaining to distribution of the software without specific. Inc. 122 Appendix C. Mojam Media. LANCE ELLINGHOUSE DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE. Bioreason. err. NEGLIGENCE OR OTHER TORTIOUS ACTION. IN NO EVENT SHALL LANCE ELLINGHOUSE CENTRUM BE LIABLE FOR ANY SPECIAL..com/ mailto:zooko@zooko. modify. California Republic. ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. Author: Zooko O'Whielacronx http://zooko. all rights.6. provided that the above copyright notice appears in all copies. History and License . copy...Python Tutorial.8 Execution tracing The trace module contains the following notice: portions copyright 2001. all rights reserved. Author: Skip Montanaro Copyright 1999. Permission to use..2 C. written prior permission. Author: Andrew Dalke Copyright 1995-1997. Author: Skip Montanaro Copyright 1991-1995. all rights reserved. all rights reserved.
understood. modify. NEGLIGENCE OR OTHER TORTIOUS ACTION. copy. INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE. Permission is hereby granted.2 Modified by Jack Jansen. C. and will comply with the following terms and conditions: Permission to use. July 1995: . The C version is still 5 times faster. and/or copying this software and/or its associated documentation. modify.10 XML Remote Procedure Calls The xmlrpclib module contains the following notice: The XML-RPC client interface is Copyright (c) 1999-2002 by Secret Labs AB Copyright (c) 1999-2002 by Fredrik Lundh By obtaining.6. INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. This results in a 1000-fold speedup. and that the name of Secret Labs AB or the author not be used in advertising or publicity pertaining to distribution of the software without specific. SECRET LABS AB AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE. distribute.3. provided that the above copyright notice appears in all copies. WHETHER IN AN ACTION OF CONTRACT. using. written prior permission. and distribute this software and its associated documentation for any purpose and without fee is hereby granted. to deal in the Software without restriction. and that both that copyright notice and this permission notice appear in supporting documentation. you agree that you have read. free of charge. Licenses and Acknowledgements for Incorporated Software 123 . DATA OR PROFITS. to any person obtaining a copy of this software and associated documentation files (the "Software"). IN NO EVENT SHALL SECRET LABS AB OR THE AUTHOR BE LIABLE FOR ANY SPECIAL. merge. and/or sell copies of the Software. CWI. and to permit persons to whom the Software is furnished to do so. publish.11 test_epoll The test_epoll contains the following notice: Copyright (c) 2001-2006 Twisted Matrix Laboratories. copy. though. ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. including without limitation the rights to use. sublicense.Python Tutorial.Arguments more compliant with python standard C. subject to the following conditions: The above copyright notice and this permission notice shall be C.3. .Use binascii module to do the actual line-by-line conversion between ascii and binary.3. Release 2.
INDIRECT. this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. INCLUDING. BUT NOT LIMITED TO. 2007 Christian Heimes All rights reserved. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM. Redistributions in binary form must reproduce the above copyright notice. INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY. 2. INCIDENTAL. FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. TORT OR OTHERWISE.2 included in all copies or substantial portions of the Software. BUT NOT LIMITED TO. DAMAGES OR OTHER LIABILITY. OR CONSEQUENTIAL DAMAGES (INCLUDING. DATA. WITHOUT WARRANTY OF ANY KIND.3. with or without modification. WHETHER IN AN ACTION OF CONTRACT. PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES. EXEMPLARY. are permitted provided that the following conditions are met: 1. WHETHER IN CONTRACT. OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY. OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. Release 2.12 Select kqueue The select and contains the following notice for the kqueue interface: Copyright (c) 2000 Doug White.6. ARISING FROM. Redistributions of source code must retain the above copyright notice. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT. C. this list of conditions and the following disclaimer. THE SOFTWARE IS PROVIDED "AS IS". OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE. STRICT LIABILITY. 124 Appendix C. SPECIAL. 2006 James Knight. History and License . LOSS OF USE. THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES. EXPRESS OR IMPLIED. OR PROFITS.Python Tutorial. THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. Redistribution and use in source and binary forms. EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
Copyright © 1991-1995 Stichting Mathematisch Centrum. Copyright © 2000 BeOpen. All rights reserved. 125 . Copyright © 1995-2000 Corporation for National Research Initiatives.APPENDIX D COPYRIGHT Python and this documentation is: Copyright © 2001-2008 Python Software Foundation. See History and License for complete license and permissions information.com. All rights reserved. All rights reserved. All rights reserved.
Python Tutorial. Release 2.2 126 Appendix D. Copyright .6.
44 PYTHONSTARTUP. 54 finder. 104 127 . 104 context manager. 105 extension module. 106 integer division. 105 global interpreter lock. 16 bytecode. 106 D decorator. 79 I IDLE. 104 docstring. 103 attribute. 47 __builtin__ module. 105 __slots__. 105 B BDFL.. 105 generator expression. 103 coding style. 21 function. 108 >>>. 106 importer. 103 F file object. 7. 103 __all__. 106 immutable. 104 dictionary. 104 docstrings. 105 for statement. 105 C class. 45 __future__. 103 descriptor. 104 environment variable PATH. 28 . 104 CPython. 103 G garbage collection. 54 unicode.INDEX Symbols * statement.. 79 open. 29 documentation strings. 24. 103 compileall module. 43 PYTHONPATH. 103 built-in function help. 103 argument. 44 complex number. 24.. 103 classic class. 29 coercion. 29 duck-typing. 105 A abstract base class. 103 2to3. 104 E EAFP. 105 GIL. 105 generator. 43. 28 ** statement. 104 H hashable. 96 expression. 105 help built-in function. 7.
documentation. 108 PYTHONPATH. 107 list comprehension. 106 PEP 343. 108 Python 3000. 107 128 Index . 70 module __builtin__. 107 loader. 44 mutable. 108 rlcompleter module. 29 sys module. 56 positional argument. 108 slice. 106 interpreted. 106 iterable.6. 108 type. 43 sequence. 109 U unicode built-in function. 109 P PATH. 43. 51 strings. 107 namespace. 107 method. 104 PEP 8. 21 string module. 24. 106 iterator. 45 compileall. 43 path module search. 108 statement. 43 pickle module. 107 list.Python Tutorial. 108 *. 109 Z Zen of Python. 96 search path. 7. 56 readline. 54 method. 107 object. 107 nested scope. 108 Python Enhancement Proposals PEP 302. 7. module. 96 R readline module. 107 T triple-quoted string. 107 new-style class. Release 2. 70 open built-in function. 96 L lambda. 51 sys. 29 Pythonic. 28 **. 54 V virtual machine. 96 rlcompleter. 16 O object. 43 string. 44 M mapping. 44 pickle. 107 metaclass. 107 LBYL. 108 special method. 106 K keyword argument. 108 file. 29 style coding. 96 reference count. 107 N named tuple. 107 S search path. 105.2 interactive. 44 PYTHONSTARTUP. 28 for.
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subset: commoncrawl