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  It's not too long. please read the Translation Howto. what . and I'm also a contributor of TurboGears web framework. I found 'A Byte of Python' hit the sweet point for both newbies and experienced programmers. channel estimation and multi-user detection of multicarrier CDMA system. 'A Byte of Python' elaborates the python essentials with affordable size. it's really easy-understanding. Fred Lin . An exciting feature of this translation is that it also contains the executable chinese python sources side by side with the original python sources. thanks to many tireless volunteers! If you want to help these translations. This project is mainly aimed for education. China PR.?. com/ p/ zhpy/ wiki/ ByteOfZhpy (http:/ / code. Chinese Juan Shen (orion-underscore-val-at-163-dot-com) has volunteered to translate the book to Chinese. It is available at http:/ / code. 'A Byte of Python' is my tutorial to learn Python. As a python evangelist (:-p).Python en:Translations 7 Python en:Translations There are many translations of the book available in different human languages. . actually. which are achieved by my new 'zhpy' (python in chinese) project (launch from Aug 07). com/ p/ zhpy/ wiki/ ByteOfZhpy). Chinese Traditional Fred Lin (gasolin-at-gmail-dot-com) has volunteered to translate the book to Chinese Traditional. I think 'A Byte of Python' should be strongly recommendable for newbies as their first Python tutorial. and soon a lot of rewrite were made to fit the current wiki version and the quality of reading. Beijing University of Technology. The translation are originally based on simplified chinese version. I learned Python just half a year before. The recent chinese traditional version also featured with executable chinese python sources.I am a postgraduate at Wireless Telecommunication Graduate School. Just dedicate my translation to the potential millions of Python users in China. google.I'm working as a network firmware engineer at Delta Network.H. or zippy) build a layer upon python to translate or interact with python in chinese(Traditional or Simplified). 'It's my favorite programming language now'. My current research interest is on the synchronization. If you plan to start a new translation. Just as what is ensured in Swaroop's book. google. easy-to-use and productive. It's clear and effective to lead you into a world of Python in the shortest time. please see the list of volunteers and languages below and decide if you want to start a new translation or help in existing translation projects. but efficiently covers almost all important things in Python. with the help of Python Numeric. but as you can see. zhpy(pronounce (Z. I need some material to promote python language. Python is my major programming language for daily simulation and research job.
  de). where the user interface is build using Java frameworks like Struts.it (now under construction) for Nuclear Magnetic Resonance applications and Congress Organization and Managements. The main language I use as a professional is Java.Python en:Translations 8 Italian Enrico Morelli (mr-dot-mlucci-at-gmail-dot-com) and Massimo (morelli-at-cerm-dot-unifi-dot-it) have volunteered to translate the book to Italian.nmr. . Enrico as service engineer and system administrator for our CED and parallel / clustered systems.it web site for Gentoo/Linux distrubution and www. and 'Dive into Python'. since most of my programming is about web applications. I (Massimo) as service engineer and system administrator for Nuclear Magnetic Resonance Spectrometers. since it is not too long. which makes translating the text a generation the output in various formats a charm. That's all! We are impressed by the smart language used on your Book and we think this is essential for approaching the Python to new users (we are thinking about hundred of students and researcher working on our labs). I like the simple DocBook structure. I've found the book 'How to Think Like a Computer Scientist: Learning with Python'. and at the same time verbose enough to teach a newbie.Chemistry Department. The new translation is in progress and start with "Prefazione". Besides this. de (http:/ / abop-german.gentoo. we had experience working with Linux platforms since ten years. berlios. The first is good for beginners but to long to translate. German Lutz Horn (lutz-dot-horn-at-gmx-dot-de). In Italy we are responsible and administrator for www. The second is not suitable for beginners. Especially text analysis and conversion is very easy with Python. We are programming on python since about seven years. it/ Programmazione/ byteofpython).we are working at the University of Florence (Italy) . I was very impressed with the use of blocks in this language.it/Programmazione/byteofpython (http:/ / www. berlios. I've searched for some kind of introduction to programming. Lutz Horn : I'm 32 years old and have a degree of Mathematics from University of Heidelberg. gentoo. Their translation is located at http:/ / abop-german. Currently I'm working as a software engineer on a publicly funded project to build a web portal for all things related to computer science in Germany.gentoo. Bernd Hengelein (bernd-dot-hengelein-at-gmail-dot-com) and Christoph Zwerschke (cito-at-online-dot-de) have volunteered to translate the book to German. I'm not very familiar with GUI toolkits. Massimo Lucci and Enrico Morelli . I think 'A Byte of Python' falls nicely between these. but I try to do as much as possible with Python behind the scenes. suitable to teach a complete non-programmer. Generally I like the dynamic nature of languages like Python and Ruby since it allows me to do things not possible in more static languages like Java. Currently I try to make more use of the functional programming features of Python and of generators. Germany. written to the point. After taking a short look into Ruby. Lucci The Italian translation is present at www.
  so most high school graduates will not even have the vocabulary to understand what the tutorial is about. After reading half of the book. I hope the translation will help people who have found themself in the same situation as me (especially young people). when the "Commodore C64" ruled the nurseries. We just started with the intro and preface but we will keep you informed about the progress we make. php/ UkÄ Å_Pythona).I'm a Computer Science and Information Technology teacher. Although C++ is the main language I (have to) use for my daily work. and you can check the table of contents for more details. I read somewhere in the net about a guy who said that he likes python. Last year I fell in love with Python. I am 34 years old and playing with computers since the 1980's. Most tutorials and books are written in very technical English. which is a wonderful language. I am constantly looking for new things to learn. com/ ) and currently translating the book to Norwegian (bokmål). Ok. bielsko. a blogger (http:/ / forbedre. blogspot. org/ wiki/ Sandvika_videregÃ¥ende_skole) in Norway. At the time I decided to learn python. Luckily. When I came across your book the spontaneous idea of a german translation crossed my mind. . lo5. When I discovered this book. all my problems were solved.. the learning process was much harder. both for its possibilities and its beauty. pl/ index. but because I speak a small language. Indonesian Daniel (daniel-dot-mirror-at-gmail-dot-com) is translating the book to Indonesian at http:/ / python. I noticed that there is very little good documentation in german available.Python en:Translations Bernd Hengelein : Lutz and me are going to do the german translation together. wikipedia. because the code looks so beautiful. After studying computer science I started working as a software engineer. cgi/ ByteofPython Polish Dominik Kozaczko (dkozaczko-at-gmail-dot-com) has volunteered to translate the book to Polish. Eirik Vågeskar: I have always wanted to program. The translation is in progress. now some personal things about me. "A Byte of Python" used simple non-technical language to explain a programming language that is just as simple. In my opinion he's absolutly right. and these two things make learning Python fun. Dominik Kozaczko . and maybe help spread interest for the language among people with less technical knowledge. id/ moin. Translation is in progress and it's main page is available here: Ukąś Pythona (http:/ / wiki. or. I decided that the book was worth translating. mercury. Lutz had the same idea and we can now divide the work.
  and complete enough. I thought some other people in my country could take benefit from it too.I am a developer and also a teacher of programming (normally for people without any previous experience). and starts with the chapter "Taula de continguts".Python en:Translations 10 Catalan Moises Gomez (moisesgomezgiron-at-gmail-dot-com) has volunteered to translate the book to Catalan. here in Romania. Just what I needed. has French Gregory (coulix-at-ozforces-dot-com-dot-au) has volunteered to translate the book to French. Some time ago I needed to learn how to program in Python.. Paul-Sebastian Manole . I started liking Python so I decided to help translate the latest version of Swaroop's book in Romanian. Python. Moisès Gómez . The web told me there was no better way to do so but read A Byte of Python. and Swaroop's work was really helpful. That's how popular this book is (congratulations to the author for writing such an easy to read book). I'm more of a self-taught programmer and decided to learn a new language. Portuguese Fidel Viegas (fidel-dot-viegas-at-gmail-dot-com) has volunteered to translate the book to Portuguese. But English language can be a barrier. com/ notes/ Python_ro). please join me. I selected the Catalan language assuming that others will translate it to the more widespread Spanish.. concise. why not try to translate it? And I did for a previous version of BoP. So. swaroopch. After this experience. Romanian Paul-Sebastian Manole (brokenthorn-at-gmail-dot-com) has volunteered to translate this book to Romanian. Danish Lars Petersen (lars-at-ioflux-dot-net) has volunteered to translate the book to Danish. The translation is in progress. Although I could be the one with the first initiative. Brazilian Portuguese Rodrigo Amaral (http:/ / rodrigoamaral. The translation is being done here (http:/ / www. net) (rodrigoamaral-at-gmail-dot-com) volunteered to translate the book to Brazilian Portuguese. I my country there are two official languages. I'm just one volunteer so if you can help.I'm a second year Computer Science student at Spiru Haret University. . Clear.
  Swedish Mikael Jacobsson (leochingkwake-at-gmail-dot-com) has volunteered to translate the book to Swedish. I've begun to translate "A Byte of Python" with the help of Maximiliano Soler. Russian and Ukranian Averkiev Andrey (averkiyev-at-ukr-dot-net) has volunteered to translate the book to Russian.' and '-underscore-' with '_' in the email addresses mentioned on this page..Net technologies at work but strictly Python or Ruby in my personal projects. 11 Arabic Alaa Abadi (alaanassir-at-gmail-dot-com) has volunteered to translate the book to Arabic. I knew Python many years ago and I got stuck inmediately. Gustavo Echeverria: I work as a software engineer in Argentina. Then I volunteered to translate the book to Spanish. you can read the spanish (argentinian) translation starting by the table of contents (tabla de contenidos). ISA . I use mostly C# and . Not so long after knowing Python I discovered this book and it helped me to learn the language. Note Replace '-at-' with '@' . '-dot-' with '. Now.Python en:Translations Spanish. . The translation is in progress. after receiving some requests. and perhaps Ukranian (time permitting). Dashes in other places in the email address remain as-is.
  I started searching for a good book on Python. Rodrigoamaral. and more importantly. Vages. So. Morellik. If you do have previous programming experience. com/ mediawiki/ index. Then.Python en:Translations 12 Previous Next Source: http:/ / www. However. php? oldid=2278 Contributors: Geopop65. Thorns. After a lot of rewrites. talked about how Python has become his favorite programming language. I settled for the documentation that came with Python. I had to choose between Python and Perl bindings for the Qt library. I got excited about it and suddenly got the idea of writing some stuff on Python. About six months after my first brush with Python. swaroopch. I decided that Python was the language for me. It is mainly targeted at newbies. If you have previous programming experience. you will be interested in the differences between Python and your favorite programming language . then you can learn Python from this book.I have highlighted many such differences.echeverria. It is useful for experienced programmers as well. Python is soon going to become your favorite programming language! History Lesson I first started with Python when I needed to write an installer for a software I had written called 'Diamond' so that I could make the installation easy. It did give a good idea about Python but was not complete. Leochingkwake. I started writing a few pages but it quickly became 30 pages long. . Who This Book Is For This book serves as a guide or tutorial to the Python programming language. Gustavo. I became serious about making it more useful in a book form. Waterox888.0 Linux and I was playing around with KWord. it was too brief and small. This book aims to help you learn this wonderful language and show how to get things done quickly and painlessly . Swaroop. the famous and respected hacker. it has reached a stage where it has become a useful guide to learning the Python language. Moises. I couldn't find any! I did find some O'Reilly books but they were either too expensive or were more like a reference manual than a guide.in effect 'The Perfect Anti-venom to your programming problems'. 20 anonymous edits Python en:Preface Python is probably one of the few programming languages which is both simple and powerful. Raymond. I did some research on the web and I came across an article where Eric S. but it was unsuitable for newbies. I consider this book to be my contribution and tribute to the open source community. The aim is that if all you know about computers is how to save text files. I installed the (then) latest Red Hat 9. This is good for both and beginners as well as experts. then you can also learn Python from this book. I managed with it since I had previous programming experience. So. Then. A little warning though. is fun to program with. I also found out that the PyQt bindings were more mature compared to Perl-Qt.
  e.Python en:Preface This book started out as my personal notes on Python and I still consider it in the same way. 0/ ) license. com/ contact/ ) or the respective translators with your comments and suggestions. swaroopch. com/ notes/ Python where you can read the whole book online. although I've taken a lot of effort to make it more palatable to others :) In the true spirit of open source. The book needs the help of its readers such as yourselves to point out any parts of the book which are not good. • Nothing in this license impairs or restricts the author's moral rights.0 language itself is still not finalized/released.0 release (expected in August/September 2008).e. distribute and transmit this book • You are free to Remix i. in the spirit of the open source philosophy of "Release Early. swaroopch. Since the Python 3. If you alter. not comprehensible or are simply wrong. transform. org/ licenses/ by-nc-sa/ 3. Please write to the main author (http:/ / www. Attribution must be shown by linking back to http:/ / www. to adapt this book • Under the following conditions: • Attribution.0 Unported (http:/ / creativecommons. to copy. you may distribute the resulting work only under the same or similar license to this one. and also send me feedback. 13 Status Of The Book Changes since the last major revision in March 2005 is updating for the Python 3. License 1. • Share Alike. It would be helpful if readers also gave feedback on how much depth this book should go into. the updated book has been released and is constantly being updated. Release Often". It's a constant tussle to balance this book between a beginner's needs and the tendency towards 'completeness' of information. swaroopch. buy a printed hard copy (http:/ / www. download the latest versions of the book. com/ buybook). • For any reuse or distribution. swaroopch. You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of this book). • Any of the above conditions can be waived if you get permission from the copyright holder. This book is licensed under the Creative Commons Attribution-Noncommercial-Share Alike 3. • This means: • You are free to Share i. However. this book is constantly undergoing changes. or build upon this work. you must make clear to others the license terms of this book. . criticisms and feedback from enthusiastic readers which has helped me improve this book a lot. I have received lots of constructive suggestions. Official Website The official website of the book is http:/ / www. com/ notes/ Python and clearly indicating that the original text can be fetched from this location. 2.
  Swaroop. php) unless otherwise noted. com/ buybook) or making a donation. All the code/scripts provided in this book is licensed under the 3-clause BSD License (http:/ /. Hoare Success in life is a matter not so much of talent and opportunity as of concentration and perseverance. then please do inform me. or simply needs improvement. swaroopch. -. com/ mediawiki/ index. -. R. org/ licenses/ bsd-license. You can reach me via my user page. Wendte Previous Next Source: http:/ / www. so that I can make suitable improvements. swaroopch. please consider purchasing a printed copy (http:/ / www. php? oldid=987 Contributors: Gasolin.C. Volunteer contributions to this original book must be under this same license and the copyright must be assigned to the main author of this book. 14 Feedback I have put in a lot of effort to make this book as interesting and as accurate as possible. the other is to make it so complicated that there are no obvious deficiencies. 2 anonymous edits . opensource. Buy the Book If you wish to support the continued development of this book.Python en:Preface 3. However. A. 4. if you find some material to be inconsistent or incorrect. W. Something To Think About There are two ways of constructing a software design: one way is to make it so simple that there are obviously no deficiencies.
  In simple terms. as already mentioned. Reading a good Python program feels almost like reading English. and use pieces of it in new free programs. etc.it has been created and is constantly improved by a community who just want to see a better Python. All your Python programs can work on any of these platforms without requiring any changes at all if you are careful enough to avoid any . powerful programming language. together with its interpreted nature. Features of Python Simple Python is a simple and minimalistic language. FLOSS is based on the concept of a community which shares knowledge.e.Python en:Introduction 15 Python en:Introduction Introduction Python is one of those rare languages which can claim to be both simple and powerful. Python has an extraordinarily simple syntax. Free and Open Source Python is an example of a FLOSS (Free/Libré and Open Source Software). Easy to Learn As you will see. It has efficient high-level data structures and a simple but effective approach to object-oriented programming. I will discuss most of these features in more detail in the next section. you can freely distribute copies of this software. Python's elegant syntax and dynamic typing. read its source code. It allows you to concentrate on the solution to the problem rather than the language itself. you never need to bother about the low-level details such as managing the memory used by your program. make it an ideal language for scripting and rapid application development in many areas on most platforms. The official introduction to Python is: Python is an easy to learn. although very strict English! This pseudo-code nature of Python is one of its greatest strengths. make changes to it. Portable Due to its open-source nature. He doesn't particularly like snakes that kill animals for food by winding their long bodies around them and crushing them. You will find that you will be pleasantly surprised on how easy it is to concentrate on the solution to the problem rather than the syntax and structure of the language you are programming in. Python is extremely easy to get started with. This is one of the reasons why Python is so good . changed to make it work on) many platforms. Python has been ported to (i. named the language after the BBC show "Monty Python's Flying Circus". Note Guido van Rossum. High-level Language When you write programs in Python. the creator of the Python language.
  FreeBSD. you can code that part of your program in C or C++ and then use it from your Python program. Acorn RISC OS. It can help you do various things involving regular expressions. WAV files.e. on the other hand. QNX. Twisted (http:/ / www. htm) and many more. Internally. Python converts the source code into an intermediate form called bytecodes and then translates this into the native language of your computer and then runs it. This is called the 'Batteries Included' philosophy of Python. Extensible If you need a critical piece of code to run very fast or want to have some piece of algorithm not to be open. PlayStation. In procedure-oriented languages. making sure that the proper libraries are linked and loaded. Windows CE and even PocketPC ! Interpreted This requires a bit of explanation. XML. Besides. Python. Windows. Psion. A program written in a compiled language like C or C++ is converted from the source language i. Sharp Zaurus. Python has a very powerful but simplistic way of doing OOP. 16 . GUI (graphical user interfaces). etc. 0s and 1s) using a compiler with various flags and options. Embeddable You can embed Python within your C/C++ programs to give 'scripting' capabilities for your program's users. When you run the program. wxpython. C or C++ into a language that is spoken by your computer (binary code i.Python en:Introduction system-dependent features. documentation generation. cryptography. the program is built around procedures or functions which are nothing but reusable pieces of programs. Macintosh. databases. especially when compared to big languages like C++ or Java. threading. VMS. unit testing. the program is built around objects which combine data and functionality. all this is always available wherever Python is installed. since you can just copy your Python program onto another computer and it just works! Object Oriented Python supports procedure-oriented programming as well as object-oriented programming. org) . XML-RPC. makes using Python much easier since you don't have to worry about compiling the program. CGI. com/ products/ pil/ index. You just run the program directly from the source code. etc. Python Imaging Library (http:/ / www. VxWorks. email. You can use Python on Linux. web browsers. the linker/loader software copies the program from hard disk to memory and starts running it. the standard library. OS/390. does not need compilation to binary. Palm OS. In object-oriented languages. Solaris. BeOS. AROS. twistedmatrix. Tk. Extensive Libraries The Python Standard Library is huge indeed. there are various other high-quality libraries such as wxPython (http:/ / www. pythonware. This also makes your Python programs much more portable. Amiga. and other system-dependent stuff. OS/2. AS/400. com/ products/ twisted). Remember.e. FTP. HTML. All this. actually. z/OS.
  I do admire Perl and I do use it on a daily basis for various things but whenever I write a program. It has the right combination of performance and features that make writing programs in Python both fun and easy. you would have answered this question yourself! In other words. Perl is another extremely popular open source interpreted programming language. then I would definitely recommend you to continue using it. Sadly. python. then I would recommend Python. this is a humongous collection of Perl modules and it is simply mind-boggling because of its sheer size and depth . One of the reasons that Perl has more libraries than Python is that it has been around for a much longer time than Python. This article was the real inspiration for my first brush with Python. If you have ever tried writing a large program in Perl.you can do virtually anything you can do with a computer using these modules. they all praise the beauty of the language. However this seems to be changing with the growing Python Package Index (http:/ / pypi. linuxjournal. He says that no language has made him more productive than Python. the upcoming Perl 6 does not seem to be making any improvements regarding this. org) library . He says that Python has become his favorite programming language (http:/ / www. perl. However. Perl programs are easy when they are small and it excels at small hacks and scripts to 'get work done'. He says that Python is perhaps the only language that focuses on making things easier for the . org/ pypi). is its huge CPAN (http:/ / cpan.the Comprehensive Perl Archive Network. that it feels like it is one big (but one hell of a) hack. Perl has undergone so many hacks and changes. Raymond is the author of "The Cathedral and the Bazaar" and is also the person who coined the term Open Source. For other people who have not used it and are trying to judge whether to learn Python or to learn Ruby. Python programs are definitely simpler. clearer. What Programmers Say You may find it interesting to read what great hackers like ESR have to say about Python: • Eric S. The only and very significant advantage that I feel Perl has. I personally found it hard to grok the Ruby language. I always start thinking in terms of Python because it has become so natural for me. If you already like and use Ruby. easier to write and hence more understandable and maintainable. 17 Why not Perl? If you didn't know already. they quickly become unwieldy once you start writing bigger programs and I am speaking this out of my experience writing large Perl programs at Yahoo! When compared to Perl. As the name suggests. purely from an ease-of-learning perspective. Unfortunately. Ruby is another popular open source interpreted programming language. Why not Ruby? If you didn't know already. but for people who understand Ruby. com/ article. • Bruce Eckel is the author of the famous Thinking in Java and Thinking in C++ books. php?sid=3882). I am not as lucky.Python en:Introduction Python is indeed an exciting and powerful language.
  html). • Peter Norvig is a well-known Lisp author and Director of Search Quality at Google (thanks to Guido van Rossum for pointing that out).6 and 3. 0/ library/ 2to3. If you already have a lot of Python 2. html) (features significantly different from previous Python 2. swaroopch. He says that Python has always been an integral part of Google. You can actually verify this statement by looking at the Google Jobs (http:/ / www. html) for more details. 2 anonymous edits .0 Release Schedule (http:/ / www. org/ dev/ peps/ pep-3100/ ) • Python News (detailed list of changes) (http:/ / www. org/ dev/ 3. artima. python.0 Plans (http:/ / www. com/ jobs/ index.0 is the new version of the language.Python en:Introduction programmer. 18 About Python 3. org/ dev/ 3. com/ mediawiki/ index. org/ dev/ peps/ pep-0361/ ) • Python 3000 (the official authoritative list of proposed changes) (http:/ / www. html) • Python 2.x to 3. google. Read the complete interview (http:/ / www. com/ weblogs/ viewpost. 6. It is sometimes referred to as Python 3000 or Py3K. artima. The main reason for a major new version of Python is to remove all the small problems and nitpicks that have accumulated over the years and to make the language even more clean.0 (http:/ / docs. python.0 Python 3. txt) Previous Next Source: http:/ / www. python. 0/ whatsnew/ 3. then there is a utility to assist you to convert 2. python. 0/ NEWS. python.x code. org/ dev/ peps/ pep-3000/ ) • Miscellaneous Python 3.x versions and most likely will be included in Python 3. php? oldid=1789 Contributors: JeremyBicha. org/ download/ releases/ 3. More details are at: • Guido van Rossum's introduction (http:/ / (http:/ / docs. jsp?thread=208549) • What's New in Python 2. org/ dev/ whatsnew/ 2.0) • What's New in Python 3. python.x source (http:/ / docs. Swaroop. com/ intv/ aboutme. html) page which lists Python knowledge as a requirement for software engineers. python. 0.
  It will be different for you depending on the settings of your OS. etc. This is highly unlikely but possible. In this case. Fedora. you can download the binaries from somewhere else and then copy to your PC and install it. you have two ways of installing Python on your system. To test if you have Python already installed on your Linux box. . if you get a message like this one: $ python -V bash: Python: command not found Then you don't have Python installed. open a shell program (like konsole or gnome-terminal) and enter the command python -V as shown below. such as apt-get in Ubuntu/Debian and other Debian-based Linux. then it is most likely you already have Python installed on your system.0.0] Install the binary packages using the package management software that comes with your OS. Note that you will need an internet connection to use this method.x already installed. yum in Fedora Linux. or a BSD system such as FreeBSD. If you see some version information like the one shown above. Alternatively. • [This option will be available after the final release of Python 3. Note If you have Python 2. The compilation instructions are provided at the website. You can have both installed at the same time. org/ download/ releases/ 3. hence I will indicate the prompt by just the $ symbol. However. 0/ ) and install it. then you have Python installed already.x installed already. you do not have to remove it to install Python 3.Python en:Installation 19 Python en:Installation If you have Python 2. python. • You can compile Python from the source code (http:/ / www. For Linux and BSD users If you are using a Linux distribution such as Ubuntu. pkg_add in FreeBSD. $ python -V Python 3. then try python3 -V. OpenSUSE or {put your choice here}.0b1 Note $ is the prompt of the shell.
  app and run python -V and follow the advice in the above Linux section. Of course. org/ ftp/ python/ 3. don't uncheck any! Some of these components can be useful for you.BAT : 'PATH=%PATH%. XP. 0b1. the DOS prompt. For Windows NT. From now on. 0/ python-3.0 beta 1 (http:/ / www. For older versions of Windows. which was 3. Of course. This is just 12. Caution When you are given the option of unchecking any "optional" components. Click on the variable named PATH in the 'System Variables' section. we will write our first Python program.e. . this doesn't give the complete picture since almost all Linux users will have Python installed already on their systems by default. you most probably already have Python installed on your system. DOS Prompt If you want to be able to use Python from the Windows command line i. python. add the following line to the file C:\AUTOEXEC. Open the Terminal. The installation is just like any other Windows-based software. especially IDLE. installing Python is as easy as downloading the installer and double-clicking on it. use the appropriate directory name. use the AUTOEXEC. 0/ and download the latest version from this website. msi) as of this writing. For a Windows system. Next. we will assume that you have Python installed on your system.C:\Python30' (without the quotes) and restart the system. 2003 . org/ download/ releases/ 3. An interesting fact is that majority of Python downloads are by Windows users.Python en:Installation 20 For Windows Users Visit http:/ / www. then select Edit and add . Summary For a Linux system. click on Control Panel -> System -> Advanced -> Environment Variables. python. For Mac OS X Users Mac OS X Users will find Python already installed on their system. you can install it using the package management software that comes with your distribution.8 MB which is very compact compared to most other languages or software. For Windows 2000.C:\Python30 to the end of what is already there. then you need to set the PATH variable appropriately. Otherwise.NT file.
  There are two ways of using Python to run your program . press ctrl-z followed by enter key. We use print to (unsurprisingly) print any value that you supply to it.Python en:Installation 21 Previous Next Source: http:/ / www. How to Quit the Interpreter Prompt To exit the prompt. Choosing An Editor Before we move on to writing Python programs in source files. com/ mediawiki/ index. press ctrl-d if you are using IDLE or are using a Linux/BSD shell. This will teach you how to write. $ python Python 3. save and run Python programs. we are supplying the text Hello World and this is promptly printed to the screen.1500 32 bit (Intel)] on win32 Type "help". "credits" or "license" for more information. we need an editor to write the source files. . "copyright". making your journey more comfortable and helps you reach your destination (achieve your goal) in a much faster and safer way. A good editor will help you write Python programs easily. 18:44:17) [MSC v. Now enter print('Hello World') followed by the Enter key.0b2 (r30b2:65106. >>> print('Hello World') Hello World >>> Notice that Python gives you the output of the line immediately! What you just entered is a single Python statement. Here.using the interactive interpreter prompt or using a source file. We will now see how to use both of these methods Using The Interpreter Prompt Start the interpreter on the command line by entering python at the shell prompt. You should see the words Hello World as output. php? oldid=1746 Contributors: Swaroop. Jul 18 2008. If you are using IDLE. For Windows users. 1 anonymous edits Python en:First Steps Introduction We will now see how to run a traditional 'Hello World' program in Python. The choice of an editor is crucial indeed. you can run the interpreter in the command line if you have set the PATH variable appropriately. swaroopch.0 → IDLE (Python GUI). In case of the Windows command prompt. click on Start → Programs → Python 3. You have to choose an editor as you would choose a car you would buy.
  org/ cgi-bin/ moinmoin/ PythonEditors) and make your choice. See the comprehensive list of IDEs that support Python (http:/ / www. then I suggest that you use IDLE. python. html). For further details. IDLE is installed by default with the Windows and Mac OS X Python installers. Once you start writing large Python programs. We will explore how to use IDLE in the next section. these are two of the most powerful editors and you will be benefitted by using them to write your Python programs. Good editors such as IDLE (and also VIM) will automatically help you do this. If you are using Windows. com/ 2008/ 03/ install-idle-in-linux. we will use IDLE. then I highly recommend that you do learn to use either of them as it will be very useful for you in the long run. blogspot. If you are just beginning to program. You can also choose an IDE (Integrated Development Environment) for Python. It is also available for installation for Linux (http:/ / love-python. see the comprehensive list of Python editors (http:/ / www. In case you are willing to take the time to learn Vim or Emacs. Needless to say. sontek. html) and BSDs in their respective repositories. It has a graphical user interface and has buttons to compile and run your python program without a fuss.Python en:First Steps One of the very basic requirements is syntax highlighting where all the different parts of your Python program are colorized so that you can see your program and visualize its running. IDLE does syntax highlighting and a lot more such as allowing you to run your programs within IDLE among other things. IDEs can be very useful indeed. then you must be already using Vim or Emacs. If you are a beginner programmer. For Emacs users There is a good introduction on how to make Emacs a powerful Python IDE by Ryan McGuire (http:/ / www. then you can use Kate which is one of my favorites. com/ 2008/ 05/ 09/ emacs-as-a-powerful-python-ide/ ). then you have a lot of choices for an editor. python. org/ cgi-bin/ moinmoin/ IntegratedDevelopmentEnvironments) for more details. you might want to use geany. enigmacurry. net/ 2008/ 05/ 11/ python-with-a-modular-ide-vim/ ). If you still want to explore other choices of an editor. python. If you are an experienced programmer. In this book.it is a bad choice because it does not do syntax highlighting and also importantly it does not support indentation of the text which is very important in our case as we will see later. I personally use Vim for most of my programs. please choose a proper editor . our IDE and editor of choice. A special note: Do not use Notepad . org/ idle/ doc/ idlemain. please refer the IDLE documentation (http:/ / www. For Vim users There is a good introduction on how to make Vim a powerful Python IDE by John M Anderson (http:/ / blog. 22 .it can make writing Python programs more fun and easy. I repeat once again. If you are using Linux/FreeBSD.
  all it does is just say 'Hello World' when you run it.anything to the right of the # symbol is a comment and is mainly useful as notes for the reader of the program.whenever the first two characters of the source file are #! followed by the location of a program.py print('Hello World') Run this program by opening a shell (Linux terminal or DOS prompt) and entering the command python helloworld. In case you got an error. Python does not use comments except for the special case of the first line here. congratulations! . Then click on File → Save. that person can be yourself after six months! The comments are followed by a Python statement.py If you are using IDLE. Remember.e. use the menu Run → Run Module or the keyboard shortcut F5. this tells your Linux/Unix system that this program should be run with this interpreter when you execute the program. Important Use comments sensibly in your program to explain some important details of your program . Here we call the print function this just prints the text 'Hello World'.note the lowercase p in the former and the uppercase P in the latter. ensure there are no spaces or tabs before the first character in each line . Also. the first program that you write and run is the 'Hello World' program . please type the above program exactly as shown and above and run the program again.py.this is useful for readers of your program so that they can easily understand what the program is doing.py .we will see why this is important later.you have successfully run your first Python program. print is not the same as Print . $ python helloworld. Note that Python is case-sensitive i. It is called the shebang line . This is explained in detail in the next section. it is the 'traditional incantation to the programming gods to help you learn the language better' :) . These are called comments . what . #!/usr/bin/python #Filename: helloworld. The output is as shown below. We will learn about functions in a → later chapter. As Simon Cozens [1] puts it. There is a tradition that whenever you learn a new programming language. Note that you can always run the program on any platform by specifying the interpreter directly on the command line such as the command python helloworld. enter the following program and save it as helloworld.py Hello World If you got the output as shown above.Python en:First Steps 23 Using A Source File Now let's get back to programming. If you are using IDLE. Start your choice of editor. How It Works Let us consider the first two lines of the program. click on File → New Window and enter the following program.
  So far. Whenever you run any program. we supply 'Hello World' which is referred to as a string don't worry. 24 Executable Python Programs This applies only to Linux/Unix users but Windows users might be curious as well about the first line of the program. Just change the first line of the program to the following: #!/usr/bin/env python The env program will in turn look for the Python interpreter which will run the program.Python en:First Steps you should understand now is that whatever you supply in the parentheses will be printed back to the screen. First. you can use the special env program on Linux/Unix systems. we have to give the program executable permission using the chmod command then run the source program. we will explore these terminologies in detail later.py $ .this can be done by running PATH=$PATH:/home/swaroop/mydir where '/home/swaroop/mydir' is the directory I want to add to the PATH variable./ to indicate that the program is located in the current directory. Alternatively. This method is very useful if you want to write useful scripts that you want to run the program anytime. you can rename the file to just helloworld and run it as . In this case. We use the . It is like creating your own commands just like cd or any . the system looks for that program in each of the directories listed in the PATH environment variable and then runs that program./helloworld. What if you don't know where Python is located? Then. We can make this program available everywhere by simply copying this source file to one of the directories listed in PATH. What if we wanted to be able to run the program from anywhere? You can do this by storing the program in one of the directories listed in the PATH environment variable. you can add a directory of your choice to the PATH variable . Then. anywhere. We see that /home/swaroop/bin is one of the directories in the PATH variable where swaroop is the username I am using in my system. There will usually be a similar directory for your username on your system. $ echo $PATH /usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/home/swaroop/bin $ cp helloworld.. we execute the program directly by specifying the location of the source file. $ chmod a+x helloworld. To make things more fun.
  This is very useful especially when using the interpreter prompt. For example.Python en:First Steps other commands that you use in the Linux terminal or DOS prompt. Use help() to learn more about using help itself! In case you need to get help for operators like return. Caution W. save and run Python programs at ease. Summary You should now be able to write. you can obtain information about almost anything in Python. Python. References: [1] The author of the amazing 'Beginning Perl' book → Previous → Next Source: http:/ / www. com/ mediawiki/ index. let's learn some more Python concepts. 25 Getting Help If you need quick information about any function or statement in Python. a program or a script or software all mean the same thing. Note Press q to exit the help.this displays the help for the print function which is used to print things to the screen. Now that you are a Python user. run help(print) . then you need to put those inside quotes such as help('return') so that Python doesn't get confused on what we're trying to do. 9 anonymous edits . php? oldid=2332 Contributors: Swaroop. then you can use the built-in help functionality.t.r. Similarly. swaroopch.
  9. so pay attention to the following part on how to use strings in Python. It is called a literal because it is literal . The E notation indicates powers of 10. • Examples of complex numbers are (-5+4j) and (2.Python en:Basics 26 Python en:Basics Just printing 'Hello World' is not enough. unicode. The number 2 always represents itself and nothing else . html#16). which means almost any language in the world (http:/ /. The default integer type can be any large value. org/ faq/ basic_q. floating point and complex numbers. com/ questions/ 175240/ how-do-i-convert-a-files-format-from-unicode-to-ascii-using-python#175270). 1. I can almost guarantee that you will be using strings in almost every Python program that you write. . all strings are in Unicode. Numbers Numbers in Python are of three types . please see the related discussion at StackOverflow (http:/ / stackoverflow.23. manipulate it and get something out of it.23 and 52.encode("ascii"). then use str. all these are referred to as literal constants. Strings A string is a sequence of characters.4.you want to take some input. In this case. • Examples of floating point numbers (or floats for short) are 3. Literal Constants An example of a literal constant is a number like 5. If a strictly ASCII-encoded byte-stream is needed.3 * 10-4. is it? You want to do more than that .25e-3 or a string like 'This is a string' or "It's a string!". Hence. 52.3 . For more details.6j) Note for Experienced Programmers There is no separate 'long int' type.it is a constant because its value cannot be changed.3E-4 means 52. Note for Experienced Programmers There are no "ASCII-only" strings because Unicode is a superset of ASCII. • An examples of an integer is 2 which is just a whole number. Strings are basically just a bunch of words. By default. The words can be in English or any other language that is supported in the Unicode standard.3E-4. We can achieve this in Python using constants and variables.you use its value literally.
  ." Escape Sequences Suppose. You cannot specify 'What's your name?' because Python will be confused as to where the string starts and ends.\ This is the second sentence. One thing to note is that in a string. All white space i. James Bond. how will you specify this string? For example. the second line. You can use single quotes and double quotes freely within the triple quotes.\t. An example is This is the first line\nThis is the second line. Double Quotes Strings in double quotes work exactly the same way as strings in single quotes. you can specify the string as 'What\'s your name?'. you will have to specify that this single quote does not indicate the end of the string.e. you have to indicate the backslash itself using the escape sequence \\. Another useful escape sequence to know is the tab . Also. your name?. An example is: '''This This is "What's He said ''' is a multi-line string.(""" or '''). Similarly.". you have to use an escape sequence for using a double quote itself in a double quoted string." I asked.e. a single backslash at the end of the line indicates that the string is continued in the next line. using double quotes.Python en:Basics 27 Single Quotes You can specify strings using single quotes such as 'Quote me on this'. So. An example is "What's your name?" Triple Quotes You can specify multi-line strings using triple quotes . you want to have a string which contains a single quote (').\n to indicate the start of a new line. For example: "This is the first sentence. You specify the single quote as \' . the string is What's your name?." is equivalent to "This is the first sentence. Another way of specifying this specific string would be "What's your name?" i. There are many more escape sequences but I have mentioned only the most useful ones here. What if you wanted to specify a two-line string? One way is to use a triple-quoted string as shown previously or you can use an escape sequence for the newline character . "Bond. Now. but no newline is added. This can be done with the help of what is called an escape sequence. This is the first line. This is the second sentence.notice the backslash. spaces and tabs are preserved as-is.
  the format method can be called to substitute those specifications with corresponding arguments to the format method. String Literal Concatenation If you place two string literals side by side.format(name)) Output: $ python str_format. #!/usr/bin/python # Filename: str_format. 'What\'s ' 'your name?' is automatically converted in to "What's your name?". a lot of backwhacking may be required.they do not differ in any way. you cannot change it. There is no real need for it and I am sure you won't miss it. Note for C/C++ Programmers There is no separate char data type in Python. Although this might seem like a bad thing. We will see why this is not a limitation in the various programs that we see later on.Python en:Basics 28 Raw Strings If you need to specify some strings where no special processing such as escape sequences are handled. This is where the format() method is useful. The format Method Sometimes we may want to construct strings from other information. Strings Are Immutable This means that once you have created a string.py Swaroop is 25 years old Why is Swaroop playing with that python? How It Works: A string can use certain specifications and subsequently. An example is r"Newlines are indicated by \n". backreferences can be referred to as '\\1' or r'\1'. Note for Regular Expression Users Always use raw strings when dealing with regular expressions. For example. they are automatically concatenated by Python.py age = 25 name = 'Swaroop' print('{0} is {1} years old'. Note for Perl/PHP Programmers Remember that single-quoted strings and double-quoted strings are the same . it really isn't.format(name. . Otherwise. age)) print('Why is {0} playing with that python?'. For example. then what you need is to specify a raw string by prefixing r or R to the string.
  Variables are exactly what the name implies .format(>> '{0:_^11}'. and "this_is_in_quotes". There are some rules you have to follow for naming identifiers: • The first character of the identifier must be a letter of the alphabet (uppercase ASCII or lowercase ASCII or Unicode character) or an underscore ('_'). you need some method of accessing these variables and hence you give them names.format('hello') # fill with underscores (_) with the text centered (^) to 11 width '___hello___' >>> '{name} wrote {book}'. Variables are just parts of your computer's memory where you store some information.Python en:Basics Observe the first usage where we use {0} and this corresponds to the variable name which is the first argument to the format method. There can be more detailed specifications such as: >>> '{0:. What Python does in the format method is that it substitutes each argument value into the place of the specification. Identifiers are names given to identify something. __my_name. i. Identifier Naming Variables are examples of identifiers. the conversion to string would be done automatically by the format method instead of the explicit conversion here. This is where variables come into the picture. • Examples of valid identifier names are i.3}'. this is spaced out.. underscores ('_') or digits (0-9). .we need some way of storing any information and manipulate them as well. the second specification is {1} corresponding to age which is the second argument to the format method. Note the lowercase n in the former and the uppercase N in the latter. 3101 (http:/ / www. Unlike literal constants. • Identifier names are case-sensitive. 29 Variables Using just literal constants can soon become boring . book='A Byte of Python') # keyword-based 'Swaroop wrote A Byte of Python' Details of this formatting specification is explained in the Python Enhancement Proposal No. python. when using the format method. you can store anything using a variable. • The rest of the identifier name can consist of letters (uppercase ASCII or lowercase ASCII or Unicode character). Similarly. Notice that we could achieved the same using string concatenation: name + ' is ' + str(age) + ' years old' but notice how much uglier and error-prone this is.their value can vary. • Examples of invalid identifier names are 2things. name_23.format(1/3) # decimal (. my-name.) precision of 3 for float '0. For example.
  py or use IDLE to run the programs. I follow the convention of having all Python programs saved with the extension . This is the second line.''' print(s) Output: $ python var. we say 'the object'. In later chapters. Save the following example and run the program. which we have already discussed. Python refers to anything used in a program as an object. 1. How It Works: Here's how this program works. This is meant in the generic sense. This line is called a statement because it states that . Objects Remember. Example: Using Variables And Literal Constants # Filename : var. This is the second line. Instead of saying 'the something'.py. First. Enter the program code given in the example. We will now see how to use variables along with literal constants. the standard procedure to save and run a Python program is as follows: 1. You can also use the executable method as explained earlier. Note for Object Oriented Programming users Python is strongly object-oriented in the sense that everything is an object including numbers. Run the interpreter with the command python program. we assign the literal constant value 5 to the variable i using the assignment operator (=). Save it as a file with the filename mentioned in the comment.Python en:Basics 30 Data Types Variables can hold values of different types called data types. Open your favorite editor. How to write Python programs Henceforth. we will see how to create our own types using classes. The basic types are numbers and strings. strings and functions.py i = 5 print(i) i = i + 1 print(i) s = '''This is a multi-line string.py 5 6 This is a multi-line string. 1. 1.
  we assign the literal string to the variable s and then print it. I have never used or even seen a semicolon in Python program. we get the value 6. \ This continues the string. Implicitly. Python implicitly assumes that each physical line corresponds to a logical line. print(i). An example of a logical line is a statement like print('Hello World') . Then we add 1 to the value stored in i and store it back. Next. s = 'This is a string. just prints the value of the variable to the screen. For example. The idea is to avoid the semicolon as much as possible since leads to more readable code.) which indicates the end of a logical line/statement. i = 5 print(i) is effectively same as i = 5. unsurprisingly. then this also corresponds to a physical line. we connect the variable name i to the value 5. or even i = 5. In fact. we print the value of i using the print statement which. Use more than one physical line for a single logical line only the logical line is really long. If you want to specify more than one logical line on a single physical line. We then print it and expectedly. then you have to explicitly specify this using a semicolon (.if this was on a line by itself (as you see it in an editor). I strongly recommend that you stick to writing a single logical line in single physical line only. Note for static language programmers Variables are used by just assigning them a value. print(i) However. print(i). a if it a An example of writing a logical line spanning many physical lines follows. Similarly. No declaration or data type definition is needed/used. A logical line is what Python sees as a single statement.' print(s) . Python encourages the use of a single statement per line which makes code more readable. and the same can be written as i = 5. This is referred to as explicit line joining.Python en:Basics something should be done and in this case. 31 Logical And Physical Lines A physical line is what you see when you write the program.
  the program was not properly written. This is is called implicit line joining. line 4 print('Value is '. square brackets or curly braces.py". This continues the string. Actually. What this means to you is that you cannot arbitrarily start new blocks of statements (except for the default main block which you have been using all along.Python en:Basics This gives the output: This is a string. which in turn is used to determine the grouping of statements. I strongly recommend that you use a single tab or four . The error indicated by Python tells us that the syntax of the program is invalid i. For example: i = 5 print('Value is '. How to indent Do not use a mixture of tabs and spaces for the indentation as it does not work across different platforms properly. whitespace at the beginning of the line is important. you get the following error: File "whitespace. i) # Error! Notice a single space at the start of the line print('I repeat. Similarly. of course). 32 Indentation Whitespace is important in Python. there is an implicit assumption where you don't need to use a backslash. One thing you should remember is that wrong indentation can give rise to errors. This is called indentation.e. We will see examples of how blocks are important in later chapters. the value is '. i) # Error! Notice a single space at the start of the line ^ IndentationError: unexpected indent Notice that there is a single space at the beginning of the second line. Cases where you can use new blocks will be detailed in later chapters such as the control flow chapter. You can see this in action when we write programs using lists in later chapters. Leading whitespace (spaces and tabs) at the beginning of the logical line is used to determine the indentation level of the logical line. This means that statements which go together must have the same indentation. Each such set of statements is called a block. print\ (i) is the same as print(i) Sometimes. This is the case where the logical line uses parentheses. i) When you run this.
  33 Summary Now that we have gone through many nitty-gritty details. -5. php? oldid=2376 Contributors: Swaroop. use the interactive Python interpreter prompt: >>> 2 + 3 5 >>> 3 * 5 15 >>> Operator + Name Plus Minus Explanation Adds the two objects Either gives a negative number or gives the subtraction of one number from the other Examples 3 + 5 gives 8. Choose either of these two indentation styles. 'a' + 'b' gives 'ab'. 2 and 3 are the operands.24 gives 26. use that indentation style only. choose one and use it consistently i. to test the expression 2 + 3. A simple example of an expression is 2 + 3. Be sure to become comfortable with what you have read in this chapter. 10 anonymous edits Python en:Operators and Expressions Introduction Most statements (logical lines) that you write will contain expressions. Previous Next Source: http:/ / www. Run from __future__ import braces to learn more.2 gives a negative number. Note to static language programmers Python will always use indentation for blocks and will never use braces. 50 . com/ mediawiki/ index. An expression can be broken down into operators and operands. For example. Operators require some data to operate on and such data is called operands. Operators are functionality that do something and can be represented by symbols such as + or by special keywords. Operators We will briefly take a look at the operators and their usage: Note that you can evaluate the expressions given in the examples using the interpreter interactively. More importantly. swaroopch. .e. Vages. In this case.Python en:Basics spaces for each indentation level. we can move on to more interesting stuff such as control flow statements.
  Otherwise. it returns False. If both operands are numbers. y = 'str'. Left to the left by the number of shifting by 2 bits gives 1000 which represents the bits specified.5. == x = 2.Python en:Operators and Expressions 34 2 * 3 gives 6. y = 6. > Greater Than 5 > 3 returns True. Floor Division Returns the floor of the quotient Modulo Returns the remainder of the division % 8 % 3 gives 2. -25. Returns whether x is greater than y 11 >> 1 gives 5. and Boolean AND . x == y returns False. Python will not evaluate y since it knows that the left hand side of the 'and' expression is False which implies that the whole expression will be False irrespective of the other values. it always returns False. x != y returns True. represented in memory by bits or binary digits i. 5 | 3 gives 7 5 ^ 3 gives 6 ~5 gives -6. it returns True. * Multiply Gives the multiplication of the two numbers or returns the string repeated that many times. y = True.5 % 2.e. If x = True. they are first converted to a common type. All comparison operators return True or False. 'la' * 3 gives 'lalala'. not Boolean NOT If x is True. not x returns False. y = 3. x >= 3 returns True. x = 3. Bitwise AND of the numbers Bitwise OR of the numbers Bitwise XOR of the numbers The bit-wise inversion of x is -(x+1) Returns whether x is less than y. Returns x to the power of y Divide x by y ** / // Power Divide 3 ** 4 gives 81 (i. x and y returns False if x is False. x is False.25 gives 1. x == y returns True. x = 'str'. <= Less Than or Equal To Greater Than or Equal To Equal To Returns whether x is less than or equal to y Returns whether x is greater than or equal to y Compares if the objects are equal >= x = 4.e. x == y returns True. x and y returns False since x is False. x = 'str'. x <= y returns True. != Not Equal To Compares if the objects are not equal x = 2. y = 'stR'. 11 is represented in bits by 1011 which when right shifted by 1 bit gives 101 which is the decimal 5. This is called short-circuit evaluation. y = 3. Comparisons can be chained arbitrarily: 3 < 5 < 7 gives True. else it returns evaluation of y x = False. 5 & 3 gives 1. 4 // 3 gives 1. 3 * 3 * 3 * 3) 4 / 3 gives 1. (Each number is decimal 8.3333333333333333. In this case. >> Right Shift & | ^ ~ Bitwise AND Bit-wise OR Bit-wise XOR Bit-wise invert Less Than < 5 < 3 gives False and 3 < 5 gives True. Note the capitalization of these names. << Left Shift Shifts the bits of the number 2 << 2 gives 8. 2 is represented by 10 in bits. 0 and 1) Shifts the bits of the number to the right by the number of bits specified. y = 2.
  >> +. It is far better to use parentheses to group operators and operands appropriately in order to explicitly specify the precedence. Floor Division and Remainder Positive. python. hence there is a shortcut for such expressions: You can write: a = 2. y = False. Division. 0/ reference/ expressions. -x Description Lambda Expression Boolean OR Boolean AND Boolean NOT Membership tests Identity tests Comparisons Bitwise OR Bitwise XOR Bitwise AND Shifts Addition and subtraction Multiplication. html#evaluation-order). !=. Operator lambda or and not x in. *. or Boolean OR If x is True. The following table gives the precedence table for Python. This means that in a given expression. This makes the program more readable. See Changing the Order of Evaluation below for details. org/ dev/ 3. Evaluation Order If you had an expression such as 2 + 3 * 4. <=. % +x. a = a * 3 as: a = 2. //. /. x or y returns True. is not <. a *= 3 Notice that var = var operation expression becomes var operation= expression. it returns True. else it returns evaluation of y Shortcut for math operation and assignment It is common to run a math operation on a variable and then assign the result of the operation back to the variable. Negative . == | ^ & <<. This means that the multiplication operator has higher precedence than the addition operator. Short-circuit evaluation applies here as well. >=.Python en:Operators and Expressions 35 x = True. The following table. >. is provided for the sake of completeness. is the addition done first or the multiplication? Our high school maths tells us that the multiplication should be done first. not in is. from the lowest precedence (least binding) to the highest precedence (most binding). Python will first evaluate the operators and expressions lower in the table before the ones listed higher in the table. taken from the Python reference manual (http:/ / docs.
  e.. Changing the Order Of Evaluation To make the expressions more readable.. Some operators like assignment operators have right to left associativity i. the parentheses should be used reasonably (do not overdo it) and should not be redundant (as in 2 + (3 + 4)). For example.] {key:datum.) [expressions. operators with same precedence are evaluated in a left to right manner. Expressions Example: #!/usr/bin/python # Filename: expression. we can use parentheses. .e. . There is an additional advantage to using parentheses .} The operators which we have not already come across will be explained in later chapters.. area) print('Perimeter is'.have the same precedence... Operators with the same precedence are listed in the same row in the above table. Associativity Operators are usually associated from left to right i. For example.attribute x[index] x[index1:index2] f(arguments . then you can write something like (2 + 3) * 4. if you want addition to be evaluated before multiplication in an expression. a = b = c is treated as a = (b = c). 2 + (3 * 4) is definitely easier to understand than 2 + 3 * 4 which requires knowledge of the operator precedences.Python en:Operators and Expressions 36 Bitwise NOT Exponentiation Attribute reference Subscription Slicing Function call Binding or tuple display List display Dictionary display ~x ** x. As with everything else. For example..) (expressions. 2 + 3 + 4 is evaluated as (2 + 3) + 4.. 2 * (length + breadth)) Output: .. For example. .it helps us to change the order of evaluation. + and .py length = 5 breadth = 2 area = length * breadth print('Area is'.
  com/ mediawiki/ index. php? oldid=1579 Contributors: Swaroop.). Next. We use these to calculate the area and perimeter of the rectangle with the help of expressions. Even though we have not specified a space between 'Area is' and the variable area.Python en:Operators and Expressions $ python expression. swaroopch. Previous Next Source: http:/ / www. 37 Summary We have seen how to use operators. In the second case. Also. 4 anonymous edits . operands and expressions . we will see how to make use of these in our programs using statements. We store the result of the expression length * breadth in the variable area and then print it using the print function. notice how Python 'pretty-prints' the output.these are the basic building blocks of any program. This is an example of how Python makes life easy for the programmer.py Area is 10 Perimeter is 14 How It Works: The length and breadth of the rectangle are stored in variables by the same name.
  you guessed it. after the if statement is executed Output: $ python if. it is a little higher than that') # Another block # You can do whatever you want in a block .if. for and while..Python en:Control Flow 38 Python en:Control Flow Introduction In the programs we have seen till now. we run a block of statements (called the if-block)..py Enter an integer : 22 . else we process another block of statements (called the else-block). What if you wanted to change the flow of how it works? For example. The else clause is optional. There are three control flow statements in Python . it is a little lower than that') # you must have guess > number to reach here print('Done') # This last statement is always executed.py Enter an integer : 50 No. else: print('No. The if statement The if statement is used to check a condition and if the condition is true. there has always been a series of statements and Python faithfully executes them in the same order.py number = 23 guess = int(input('Enter an integer : ')) if guess == number: print('Congratulations. Example: #!/usr/bin/python # Filename: if. it is a little lower than that Done $ python if.') # New block starts here print('(but you do not win any prizes!)') # New block ends here elif guess < number: print('No. you want the program to take some decisions and do different things depending on different situations such as printing 'Good Morning' or 'Good Evening' depending on the time of the day? As you might have guessed. this is achieved using control flow statements.
  I hope you are sticking to the "consistent indentation" rule. Notice that we use indentation levels to tell Python which statements belong to which block. we take guesses from the user and check if it is the number that we have. we inform the user to guess a little higher than that. we print a success message. What we have used here is the elif clause which actually combines two related if else-if else statements into one combined if-elif-else statement. we compare the guess of the user with the number we have chosen. and if so. We set the variable number to any integer we want. as a string.this is called a nested if statement.we are indicating to Python that a block of statements follows. Next. After this. The elif and else statements must also have a colon at the end of the logical line followed by their corresponding block of statements (with proper indentation. In this case.py Enter an integer : 23 Congratulations. it is the main block where execution of the program starts and the next statement is the print('Done') statement. we take the user's guess using the input() function. We supply a string to the built-in input function which prints it to the screen and waits for input from the user. we check if the guess is less than the number. say 23. it is a little higher than that Done $ python if. Then. I have been pointing out a lot of things that you should notice even in this simple program. Are you? Notice how the if statement contains a colon at the end .. Functions are just reusable pieces of programs. Python sees the ends of the program and simply finishes up. the input() function returns what we entered. it is true') After Python has finished executing the complete if statement along with the associated elif and else clauses. of course) You can have another if statement inside the if-block of an if statement and so on . A minimal valid if statement is: if True: print('Yes. This makes the program easier and reduces the amount of indentation required. Then. it moves on to the next statement in the block containing the if statement. Although this is a very simple program. Once we enter something and press enter key. you guessed it. Remember that the elif and else parts are optional. All these are pretty straightforward (and surprisingly simple for those of you from C/C++ backgrounds) and requires you to become 39 . If they are equal. We'll read more about them in the next chapter.Python en:Control Flow No. Actually. (but you do not win any prizes!) Done How It Works: In this program. This is why indentation is so important in Python.
  but after that. Example: #!/usr/bin/python # Filename: while. Enter an integer : 22 No..Python en:Control Flow aware of all these initially.elif. it is a little lower than that. A while statement is an example of what is called a looping statement. use a dictionary to do it quickly) 40 The while Statement The while statement allows you to repeatedly execute a block of statements as long as a condition is true. A while statement can have an optional else clause. Enter an integer : 23 Congratulations. You can use an if.') # Do anything else you want to do here print('Done') Output: $ python while. Note for C/C++ Programmers There is no switch statement in Python. it is a little higher than that.') running = False # this causes the while loop to stop elif guess < number: print('No. The while loop is over.else statement to do the same thing (and in some cases.') else: print('No. Done How It Works: ..py number = 23 running = True while running: guess = int(input('Enter an integer : ')) if guess == number: print('Congratulations. you will become comfortable with it and it'll feel 'natural' to you. you guessed it.') else: print('The while loop is over. it is a little lower than that.py Enter an integer : 50 No. you guessed it. it is a little higher than that.
  this may even be the first time that the condition is checked. What you need to know right now is that a sequence is just an ordered collection of items. After this block is executed. go through each item in a sequence. This aptly demonstrates the use of the while statement.5) .. we check if the variable running is True and then proceed to execute the corresponding while-block.Python en:Control Flow In this program. Note for C/C++ Programmers Remember that you can have an else clause for the while loop. it is always executed unless you break out of the loop with a break statement. we execute the while-block again. What we do here is supply it two numbers and range returns a sequence of numbers starting from the first number and up to the second number. but the advantage is that the user is allowed to keep guessing until he guesses correctly . We move the input and if statements to inside the while loop and set the variable running to True before the while loop. The True and False are called Boolean types and you can consider them to be equivalent to the value 1 and 0 respectively. range(1. If there is an else clause for a while loop. Example: #!/usr/bin/python # Filename: for.e.py 1 2 3 4 The for loop is over How It Works: In this program.there is no need to repeatedly run the program for each guess. we are still playing the guessing game. we are printing a sequence of numbers. First. 5): print(i) else: print('The for loop is over') Output: $ python for. else we continue to execute the optional else-block and then continue to the next statement. as we have done in the previous section.in statement is another looping statement which iterates over a sequence of objects i. The else block is executed when the while loop condition becomes False .py for i in range(1. If it is true. 41 The for loop The for. We will see more about sequences in detail in later chapters. For example. We generate this sequence of numbers using the built-in range function. the condition is again checked which in this case is the running variable.
  i++). then that becomes the step count. C# programmers will note that the for loop in Python is similar to the foreach loop in C#.py while True: s = (input('Enter something : ')) if s == 'quit': break print('Length of the string is'. By default. it is always executed once after the for loop is over unless a break statement is encountered. Remember that the else part is optional.5 . In this case.5.Python en:Control Flow gives the sequence [1. range(1. more expressive and less error prone in Python. For example. When included. Example: #!/usr/bin/python # Filename: break.e. i < 5. As you can see. 2. any corresponding loop else block is not executed. 4]. Java programmers will note that the same is similar to for (int i : IntArray) in Java 1. In C/C++. one at a time. the for loop is simpler. len(s)) print('Done') Output: $ python break. and then executing the block of statements for each value of i. if you want to write for (int i = 0.2) gives [1. 3. Note for C/C++/Java/C# Programmers The Python for loop is radically different from the C/C++ for loop.for i in range(1. but in general we can use any kind of sequence of any kind of objects! We will explore this idea in detail in later chapters.3]. we just print the value in the block of statements.py Enter something : Programming is fun Length of the string is 18 Enter something : When the work is done Length of the string is 21 . An important note is that if you break out of a for or while loop. Here. stop the execution of a looping statement. 4] which is like assigning each number (or object) in the sequence to i..e. even if the loop condition has not become False or the sequence of items has been completely iterated over. it does not include the second number. 3. range takes a step count of 1. Remember that the for. 2. Remember that the range extends up to the second number i. 42 The break Statement The break statement is used to break out of a loop statement i. The for loop then iterates over this range .5).in loop works for any sequence. If we supply a third number to range. we have a list of numbers generated by the built-in range function. then in Python you write just for i in range(0.5) is equivalent to for i in [1.
  We are providing a special condition to stop the program by checking if the user input is 'quit'.py Enter something : a Too small Enter something : 12 Too small .Python en:Control Flow Enter something : if you wanna make your work also fun: Length of the string is 37 Enter something : use Python! Length of the string is 12 Enter something : quit Done How It Works: In this program. Remember that the break statement can be used with the for loop as. Output: $ python test.py while True: s = input('Enter something : ') if s == 'quit': break if len(s) < 3: print('Too small') continue print('Input is of sufficient length') # Do other kinds of processing here. Example: #!/usr/bin/python # Filename: continue... We stop the program by breaking out of the loop and reach the end of the program. The length of the input string can be found out using the built-in len function.
  py def sayHello(): print('Hello World!') # block belonging to the function # End of function . Next follows the block of statements that are part of this function. They allow you to give a name to a block of statements and you can run that block using that name anywhere in your program and any number of times. but we process them only if they are at least 3 characters long. com/ mediawiki/ index. we use the built-in len function to get the length and if the length is less than 3. The function concept is probably the most important building block of any non-trivial software (in any programming language). Note that the continue statement works with the for loop as well.if. An example will show that this is actually very simple: Example: #!/usr/bin/python # Filename: function1. This is known as calling the function.Python en:Control Flow Enter something : abc Input is of sufficient length Enter something : quit How It Works: In this program. 44 Summary We have seen how to use the three control flow statements . so we will explore various aspects of functions in this chapter. we accept input from the user. Previous Next Source: http:/ / www. php? oldid=1664 Contributors: Swaroop. These are some of the most often used parts of Python and hence. 8 anonymous edits Python en:Functions Introduction Functions are reusable pieces of programs. we will see how to create and use functions. Otherwise. we skip the rest of the statements in the block by using the continue statement. Functions are defined using the def keyword. Next. This is followed by an identifier name for the function followed by a pair of parentheses which may enclose some names of variables and the line ends with a colon. becoming comfortable with them is essential. the rest of the statements in the loop are executed and we can do any kind of processing we want to do here. swaroopch. while and for along with their associated break and continue statements. So. We have already used many built-in functions such as the len and range.
  b): if a > b: print(a. Parameters to functions are just input to the function so that we can pass in different values to it and get back corresponding results.Python en:Functions 45 sayHello() # call the function sayHello() # call the function again Output: $ python function1. 'is maximum') elif a == b: print(a.py def printMax(a. 'is maximum') printMax(3. which are values you supply to the function so that the function can do something utilising those values. b) else: print(b.the names given in the function definition are called parameters whereas the values you supply in the function call are called arguments. Note the terminology used . separated by commas. Example: #!/usr/bin/python # Filename: func_param. 'is equal to'. This function takes no parameters and hence there are no variables declared in the parentheses. When we call the function. Function Parameters A function can take parameters. 4) # directly give literal values x = 5 y = 7 printMax(x. y) # give variables as arguments Output: . These parameters are just like variables except that the values of these variables are defined when we call the function and are already assigned values when the function runs. Parameters are specified within the pair of parentheses in the function definition.
  46 Local Variables When you declare variables inside a function definition. the first time that we use the value of the name x.. . we assign the value 2 to x. Example: #!/usr/bin/python # Filename: func_local.e.py x is 50 Changed local x to 2 x is still 50 How It Works: In the function.py x = 50 def func(x): print('x is'. we directly supply the numbers i. x) x = 2 print('Changed local x to'. Python uses the value of the parameter declared in the function. x) func(x) print('x is still'. All variables have the scope of the block they are declared in starting from the point of definition of the name. when we change the value of x in the function. printMax(x. arguments.e. In the second usage. we define a function called printMax where we take two parameters called a and b. This is called the scope of the variable. y) causes value of argument x to be assigned to parameter a and the value of argument y assigned to parameter b. So.Python en:Functions $ python func_param. In the first usage of printMax. Next. In the last print function call. the x defined in the main block remains unaffected. variable names are local to the function. The name x is local to our function.py 4 is maximum 7 is maximum How It Works: Here. x) Output: $ python func_local. we call the function using variables. The printMax function works the same in both the cases.else statement and then print the bigger number. they are not related in any way to other variables with the same names used outside the function i. We find out the greater number using a simple if. we display the value of x in the main block and confirm that it is actually unaffected.
  when we assign a value to x inside the function. You can use the values of such variables defined outside the function (assuming there is no variable with the same name within the function). x) func() print('Value of x is'. It is impossible to assign a value to a variable defined outside a function without the global statement. For example. However. z.py x is 50 Changed global x to 2 Value of x is 2 How It Works: The global statement is used to declare that x is a global variable . You can specify more than one global variable using the same global statement. We do this using the global statement.hence. not inside any kind of scope such as functions or classes). x) Output: $ python func_global. but it is global. Using the global statement makes it amply clear that the variable is defined in an outermost block. that change is reflected when we use the value of x in the main block. then you have to tell Python that the name is not local.e. this is not encouraged and should be avoided since it becomes unclear to the reader of the program as to where that variable's definition is. y. .py x = 50 def func(): global x print('x is'. Example: #!/usr/bin/python # Filename: func_global.Python en:Functions 47 Using The global Statement If you want to assign a value to a name defined at the top level of the program (i. global x. x) x = 2 print('Changed global x to'.
  the default argument value should be immutable . Note that the default argument value should be a constant. Try changing the nonlocal x to global x and also by removing the statement itself and observe the difference in behavior in these two cases.Python en:Functions 48 Using nonlocal statement We have seen how to access variables in the local and global scope above. just remember this. you may want to make some of its parameters as optional and use default values if the user does not want to provide values for such parameters. This is done with the help of default argument values. Since everything in Python is just executable code. You can specify default argument values for parameters by following the parameter name in the function definition with the assignment operator (=) followed by the default value. For now. x) func_outer() Output: $ python func_nonlocal. the 'x' defined in the first line of func_outer is relatively neither in local scope nor in global scope. you can define functions anywhere. Default Argument Values For some functions. We declare that we are using this x by nonlocal x and hence we get access to that variable. Nonlocal scopes are observed when you define functions inside functions. There is another kind of scope called "nonlocal" scope which is in-between these two types of scopes. Let's take an example: #!/usr/bin/python # Filename: func_nonlocal.this is explained in detail in later chapters. More precisely.py def func_outer(): x = 2 print('x is'.py x is 2 Changed local x to 5 How It Works: When we are inside func_inner. x) def func_inner(): nonlocal x x = 5 func_inner() print('Changed local x to'. Example: .
  py Hello WorldWorldWorldWorldWorld How It Works: The function named say is used to print a string as many times as specified. In the first usage of say. b=5. Two. b) is not valid. the string is printed just once.e. 49 Keyword Arguments If you have some functions with many parameters and you want to specify only some of them. 'and b is'.py def func(a. provided that the other parameters have default argument values. a. def func(a. b=5) is valid.we use the name (keyword) instead of the position (which we have been using all along) to specify the arguments to the function. 5) Output: $ python func_default. There are two advantages . 7) . then by default. If we don't supply a value. but def func(a=5. b.py def say(message. This is because the values are assigned to the parameters by position.one. times = 1): print(message * times) say('Hello') say('World'. we supply only the string and it prints the string once. we supply both the string and an argument 5 stating that we want to say the string message 5 times. We achieve this by specifying a default argument value of 1 to the parameter times.this is called keyword arguments . then you can give values for such parameters by naming them . Important Only those parameters which are at the end of the parameter list can be given default argument values i. Example: #!/usr/bin/python # Filename: func_key. For example. In the second usage of say. you cannot have a parameter with a default argument value before a parameter without a default argument value in the order of parameters declared in the function parameter list. c) func(3. using the function is easier since we do not need to worry about the order of the arguments. we can give values to only those parameters which we want.Python en:Functions #!/usr/bin/python # Filename: func_default. 'and c is'. c=10): print('a is'.
  py 166 How It Works: . In the first usage. that we are specifying value for parameter c before that for a even though a is defined before c in the function definition. In the third usage func(c=50. keyword arguments. The variable b gets the default value of 5. the parameter b gets the value 7 and c gets the default value of 10. Then.e. func(3. the parameter c gets the value of 24 due to naming i. followed by two parameters with default argument values. 2. c=24). vegetables=50. fruits=100)) Output: $ python total. *numbers. a=100).py is 3 and b is 7 and c is 10 is 25 and b is 5 and c is 24 is 100 and b is 5 and c is 50 50 How It Works: The function named func has one parameter without default argument values. 3. this can be achieved by using the stars: #!/usr/bin/python # Filename: total. Notice. 7)..py def total(initial=5. 1. c=24) func(c=50. In the second usage func(25. **keywords): count = initial for number in numbers: count += number for key in keywords: count += keywords[key] return count print(total(10. the parameter a gets the value 3. a=100) Output: $ a a a python func_key. we use keyword arguments completely to specify the values.Python en:Functions func(25.
  If you are interested to read about annotations. Summary We have seen so many aspects of functions but note that we still haven't covered all aspects of it. we will see how to use as well as create Python modules. com/ mediawiki/ index. However. Therefore. Remember to press the q key to exit help. org/ dev/ peps/ pep-3107/ ). python. swaroopch. I strongly recommend that you use docstrings for any non-trivial function that you write. 3107 (http:/ / www. please see the Python Enhancement Proposal No. we will skip this feature in our discussion. Since the Python language itself does not interpret these annotations in any way (that functionality is left to third-party libraries to interpret in any way they want). Vages. 7 anonymous edits . Previous Next Source: http:/ / www. we have already covered most of what you'll use regarding Python functions on an everyday basis. php? oldid=2379 Contributors: Swaroop.Python en:Functions your program. 54 Annotations Functions have another advanced feature called annotations which are a nifty way of attaching additional information for each of the parameters as well as the return value. The pydoc command that comes with your Python distribution works similarly to help() using docstrings. Automated tools can retrieve the documentation from your program in this manner. Next.
  the answer is modules.argv: print(i) print('\n\nThe PYTHONPATH is'. 'C:\\Python30'. org/ extending/ ) and when compiled. sys. we import the sys module using the import statement.zip'. This is how we can use the Python standard library as well. 'C:\\Python30\\DLLs'. they can be used from your Python code when using the standard Python interpreter. 'C:\\Python30\\lib'. python.py extension that contains functions and variables.Python en:Modules 55 Python en:Modules Introduction You have seen how you can reuse code in your program by defining functions once. '\n') Output: $ python using_sys. you can write modules in the C programming language (http:/ / docs.e. but the simplest way is to create a file with a . this translates to us telling Python that we want to use this module. Example: #!/usr/bin/python # Filename: using_sys. we will see how to use the standard library modules.py we are arguments The command line arguments are: using_sys. Another method is to write the modules in the native language in which the Python interpreter itself was written. What if you wanted to reuse a number of functions in other programs that you write? As you might have guessed. the system. 'C:\\Python30\\lib\\site-packages'] How It Works: First. 'C:\\Windows\\system32\\python30. A module can be imported by another program to make use of its functionality. Basically.py we are arguments The PYTHONPATH is [''. For example.py import sys print('The command line arguments are:') for i in sys. There are various methods of writing modules. First. 'C:\\Python30\\lib\\plat-win'. The sys module contains functionality related to the Python interpreter and its environment i.path. .
  If the module is found.pyc files are usually created in the same directory as the corresponding .path is empty . Python stores the command line arguments in the sys. a module written in Python.argv[3].pyc file is useful when you import the module the next time from a different program .argv variable is a list of strings (lists are explained in detail in a later chapter. the name of the script running is always the first argument in the sys. . then the . If you are using an IDE to write and run these programs. So. Here. then the statements in the body of that module is run and then the module is made available for you to use.pyc files Importing a module is a relatively costly affair. look for a way to specify command line arguments to the program in the menus. sys. The sys. 56 Byte-compiled .py files.this empty string indicates that the current directory is also part of the sys. Specifically. Otherwise. This .argv variable for us to use. This means that you can directly import modules located in the current directory. the arguments passed to your program using the command line. Note These .getcwd()) to find out the current directory of your program. and hence Python knows where to find it.argv list. it is one of the built-in modules.path variable. these byte-compiled files are platform-independent.e. Remember. then the Python interpreter will search for it in the directories listed in its sys. Notice that Python starts counting from 0 and not 1. print(os. so Python does some tricks to make it faster. in this case we will have 'using_sys.path which is same as the PYTHONPATH environment variable.argv contains the list of command line arguments i. If it was not a compiled module i.pyc which is an intermediate form that Python transforms the program into (remember the introduction section on how Python works?). It clearly indicates that this name is part of the sys module. Another advantage of this approach is that the name does not clash with any argv variable used in your program. when we execute python using_sys. Note that the initialization is done only the first time that we import a module.path contains the list of directory names where modules are imported from. Also.Python en:Modules When Python executes the import sys statement.path. Run import os.argv[1].pyc files will not be created.py with the python command and the other things that follow are arguments passed to the program. the sys.argv[0]. you will have to place your module in one of the directories listed in sys. One way is to create byte-compiled files with the extension . 'are' as sys.it will be much faster since a portion of the processing required in importing a module is already done. In this case. Note that the current directory is the directory from which the program is launched. If Python does not have permission to write to files in that directory. 'we' as sys.argv[2] and 'arguments' as sys. The argv variable in the sys module is accessed using the dotted notation i. we run the module using_sys. Observe that the first string in sys. it looks for the sys module.e.py we are arguments.e.py' as sys.argv. The sys.
  . This is handy in the particular situation of figuring out if the module is being run standalone or being imported. In general. If you want to import all the names used in the sys module. everytime for it). it implies that the module is being run standalone by the user and we can take appropriate actions. you should avoid using this statement and use the import statement instead since your program will avoid name clashes and will be more readable. then you can use the from sys import argv statement.. Example: #!/usr/bin/python # Filename: using_name. This works for any module. then you can use the from sys import * statement..Python en:Modules 57 The from . This can be achieved using the __name__ attribute of the module. when a module is imported for the first time.import . the code in that module is executed...py This program is being run by itself $ python >>> import using_name I am being imported from another module >>> How It Works: Every Python module has it's __name__ defined and if this is '__main__'. A module's __name__ Every module has a name and statements in a module can find out the name of its module. As mentioned previously.statement If you want to directly import the argv variable into your program (to avoid typing the sys.
  ') __version__ = '0.Python en:Modules 58 Making Your Own Modules Creating your own modules is easy.import syntax: #!/usr/bin/python # Filename: mymodule_demo2. you've been doing it all along! This is because every Python program is also a module. As you can see. this is mymodule speaking.1' # End of mymodule.py from mymodule import sayhi. Version 0.py extension. this is mymodule speaking. __version__) .py import mymodule mymodule.py Hi.py The above was a sample module.. __version__ sayhi() print('Version'.1 How It Works: Notice that we use the same dotted notation to access members of the module. You just have to make sure it has a . mymodule.path. Remember that the module should be placed in the same directory as the program that we import it in. #!/usr/bin/python # Filename: mymodule_demo. Here is a version utilising the from. there is nothing particularly special about compared to our usual Python program.__version__) Output: $ python mymodule_demo.py def sayhi(): print('Hi. or the module should be in one of the directories listed in sys.sayhi() print ('Version'. We will next see how to use this module in our other Python programs. Python makes good reuse of the same notation to give the distinctive 'Pythonic' feel to it so that we don't have to keep learning new ways to do things. The following example should make it clear. Example: #!/usr/bin/python # Filename: mymodule.
  'dllhandle' . com/ questions/ 228181/ zen-of-python) which lists examples for each of the principles. Hence. the identifiers include the functions. Zen of Python One of Python's guiding principles is that "Explicit is better than Implicit". 'getwindowsversion'. '__excepthook__'. 'exit'. 'argv'. 'getcheckinterval'. 'callstats'. 'maxunicode '. Notice that if there was already a __version__ name declared in the module that imports mymodule. 'call_tracing'. ' byteorder'. '__name__'. 'getprofile'. 'hexversion'. 'flags'. '__stdout__'. 'getdefaultencoding'. classes and variables defined in that module. When no argument is applied to it. 'path_importer_cache'. 'excepthook'. 'path'. 'copyright'. '_getframe'.py is same as the output of mymodule_demo. '__stdin__'. for the sys module >>> dir(sys) ['__displayhook__'. for a module. '__doc__'. For example.py. 'modules'. 'float_info'. 'maxsize'. in this case. 'builtin_module_names'. You could also use: from mymodule import * This will import all public names such as sayhi but would not import __version__ because it starts with double underscores. 'exc_info'. When you supply a module name to the dir() function. 'displayhook'. it returns the list of the names defined in that module. it is always recommended to prefer the import statement even though it might make your program a little longer. 'intern'. 'exec_prefix'. 'executable'. 'meta_path'. Run import this to learn more and see this discussion (http:/ / stackoverflow. '_clear_type_cache'. 'getsizeof'. 59 The dir function You can use the built-in dir function to list the identifiers that an object defines. it returns the list of names defined in the current module. This is also likely because it is common practice for each module to declare it's version number using this name. there would be a clash. Example: $ python >>> import sys # get list of attributes. 'getrefcount'. '_compact_freelists'.Python en:Modules The output of mymodule_demo2. 'gettrace'. 'dont_write_bytecode'. '__s tderr__'. 'getfil esystemencoding'. '__package__'. . '_current_frames'. 'getrecursionlimit'. 'api_version'. 'path_hooks'.
  'winver'] >>> dir() # get list of attributes for current module ['__builtins__'. 'stdout'. 'settrace'. '__doc__'. In order to observe the dir in action.Python en:Modules 'platfor m'. A note on del . 'ps2'. we define a new variable a and assign it a value and then check dir and we observe that there is an additional value in the list of the same name. or dir(str) for the attributes of the str class. in this case del a. Note that the dir() function works on any object. 'version_in fo'. Next. 'a'. we use the dir function without passing parameters to it. 'version'. 'stdin'. We remove the variable/attribute of the current module using the del statement and the change is reflected again in the output of the dir function. For example. it returns the list of attributes for the current module. 'sys'] >>> How It Works: First. 'sys'] >>> a = 5 # create a new variable 'a' >>> dir() ['__builtins__'. 60 . we see the usage of dir on the imported sys module. '__doc__'. 'setcheckinterval'. 'setprofile'. 'setrecursionlimit '. 'prefix'. 'ps1'. 'sys'] >>> del a # delete/remove a name >>> dir() ['__builtins__'. '__name__'. '__doc__'. you can no longer access the variable a . run dir(print) to learn about the attributes of the print function.this statement is used to delete a variable/name and after the statement has run. '__package__'. 'stderr'. We can see the huge list of attributes that it contains. '__name__'. '__package__'. 'warnoptions'. '__package__'.it is as if it never existed before at all. 'subversion'. '__name__'. By default. Notice that the list of imported modules is also part of this list.
  Variables usually go inside functions.madagascar/ . and these subpackages in turn contain modules like 'india'.bar.py .py Packages are just a convenience to hierarchically organize modules. The standard library that comes with Python is an example of such a set of packages and modules. modules are reusable programs.py .asia/ .py . 'africa'. Let's say you want to create a package called 'world' with subpackages 'asia'.africa/ .__init__. Packages are just folders of modules with a special __init__. com/ mediawiki/ index.__init__. Previous Next Source: http:/ / www. This is how you would structure the folders: .<some folder present in the sys. We have seen how to use these modules and create our own modules. You will see many instances of this in the standard library.path>/ . php? oldid=2371 Contributors: Swaroop. etc. swaroopch. Summary Just like functions are reusable parts of programs.py .py .py file that indicates to Python that this folder is special because it contains Python modules. What if you wanted to organize modules? That's where packages come into the picture. Functions and global variables usually go inside modules. we will learn about some interesting concepts called data structures.Python en:Modules 61 Packages By now. you must have started observing the hierarchy of organizing your programs.__init__. Packages are another hierarchy to organize modules.__init__.world/ . etc. 7 anonymous edits .__init__.py .foo. Next. 'madagascar'.india/ .
  for example. In fact. For example. Note the use of dotted notation for accessing methods of the objects.they are structures which can hold some data together.e. There are four built-in data structures in Python . this type can be altered. List A list is a data structure that holds an ordered collection of items i. we say that a list is a mutable data type i. A list is an example of usage of objects and classes. Example: #!/usr/bin/python # Filename: using_list. dictionary and set. you can think of it as creating an object (i. 'carrot'. 'mango'.e. For example.e.e. This is easy to imagine if you can think of a shopping list where you have a list of items to buy. You can use these variables/names only when you have an object of that class. 'items to purchase. You can use these pieces of functionality only when you have an object of that class. you can read help(int) to understand this better. Fields are also accessed by the dotted notation. Quick Introduction To Objects And Classes Although I've been generally delaying the discussion of objects and classes till now. functions defined for use with respect to that class only.field. tuple. We will explore this topic in detail later in its own chapter. The list of items should be enclosed in square brackets so that Python understands that you are specifying a list. type) int.Python en:Data Structures 62 Python en:Data Structures Introduction Data structures are basically just that .py # This is my shopping list shoplist = ['apple'. mylist. remove or search for items in the list. len(shoplist). A class can also have methods i. 'banana'] print('I have'. a little explanation is needed right now so that you can understand lists better. A class can also have fields which are nothing but variables defined for use with respect to that class only. except that you probably have each item on a separate line in your shopping list whereas in Python you put commas in between them. When we use a variable i and assign a value to it. instance) i of class (i. you can add. Python provides an append method for the list class which allows you to add an item to the end of the list.') print('These items are:'. they are used to store a collection of related data. end=' ') . mylist. say integer 5 to it. Once you have created a list. In other words. We will see how to use each of them and how they make life easier for us. you can store a sequence of items in a list.list.e.append('an item') will add that string to the list mylist. Since we can add and remove items.
  this is different from the way strings work.') shoplist. as already discussed before. 'rice'] I will sort my list now Sorted shopping list is ['apple'.Python en:Data Structures for item in shoplist: print(item. This is what we mean by saying that lists are mutable and that 63 . shoplist[0]) olditem = shoplist[0] del shoplist[0] print('I bought the'. By now.py I have 4 items to purchase. My shopping list is now ['apple'. 'mango'. 'carrot'. 'carrot'. you must have realised that a list is also a sequence. shoplist) Output: $ python using_list. 'rice'] How It Works: The variable shoplist is a shopping list for someone who is going to the market. we add an item to the list using the append method of the list object. Then.in loop to iterate through the items of the list. Then.append('rice') print('My shopping list is now'. shoplist) print('The first item I will buy is'. 'mango'. end=' ') print('\nI also have to buy rice. olditem) print('My shopping list is now'.. The speciality of sequences will be discussed in a later section. 'carrot'. It is important to understand that this method affects the list itself and does not return a modified list . These items are: apple mango carrot banana I also have to buy rice. 'banana'. In shoplist. shoplist) print('I will sort my list now') shoplist. we only store strings of the names of the items to buy but you can add any kind of object to a list including numbers and even other lists. we sort the list by using the sort method of the list.sort() print('Sorted shopping list is'. Next. Notice the use of the end keyword argument to the print function to indicate that we want to end the output with a space instead of the usual line break. 'mango'. 'banana'. We have also used the for. 'rice'] The first item I will buy is apple I bought the apple My shopping list is now ['banana'. we check that the item has been indeed added to the list by printing the contents of the list by simply passing the list to the print statement which prints it neatly.
  'penguin')) Animals brought from old zoo are ('python'. 'elephant'. We achieve this by using the del statement. Tuples are defined by specifying items separated by commas within an optional pair of parentheses.e. new_zoo) print('Animals brought from old zoo are'. 'penguin') # remember the parentheses are optional print('Number of animals in the zoo is'. when we finish buying an item in the market. Here. One major feature of tuples is that they are immutable like strings i. If you want to know all the methods defined by the list object. 64 Tuple Tuples are used to hold together multiple objects. you cannot modify tuples. 'elephant'. We specify that we want to remove the first item from the list and hence we use del shoplist[0] (remember that Python starts counting from 0). see help(list) for details. Next. ('python'.py Number of animals in the zoo is 3 Number of cages in the new zoo is 3 All animals in new zoo are ('monkey'. we want to remove it from the list. zoo) print('Number of cages in the new zoo is'. 'elephant'. 'camel'. len(new_zoo)) print('All animals in new zoo are'.e. new_zoo[2]) print('Last animal brought from old zoo is'. the tuple of values used will not change. 'penguin') Last animal brought from old zoo is penguin Number of animals in the new zoo is 5 How It Works: The variable zoo refers to a tuple of items. 'camel'. len(new_zoo)-1+len(new_zoo[2])) Output: $ python using_tuple. we mention which item of the list we want to remove and the del statement removes it from the list for us. Think of them as similar to lists. .py zoo = ('python'. len(zoo)) new_zoo = ('monkey'. but without the extensive functionality that the list class gives you.Python en:Data Structures strings are immutable. We see that the len function can be used to get the length of the tuple. Tuples are usually used in cases where a statement or a user-defined function can safely assume that the collection of values i. new_zoo[2][2]) print('Number of animals in the new zoo is'. This also indicates that a tuple is a sequence as well. Example: #!/usr/bin/python # Filename: using_tuple.
  the former prints three numbers whereas the latter prints a tuple (which contains three numbers).2.py . Remember that key-value pairs in a dictionary are not ordered in any manner. then you will have to sort them yourself before using it. Pairs of keys and values are specified in a dictionary by using the notation d = {key1 : value1.e. key2 : value2 }.3) and print( (1. ) if you mean you want a tuple containing the item 2. This basically translates to say that you should use only simple objects for keys. especially because it avoids ambiguity. I prefer always having them to make it obvious that it is a tuple. As far as Python is concerned. This is pretty simple once you've understood the idiom. We access the third item in new_zoo by specifying new_zoo[2] and we access the third item within the third item in the new_zoo tuple by specifying new_zoo[2][2].e. etc.e. For example.Python en:Data Structures We are now shifting these animals to a new zoo since the old zoo is being closed. Back to reality. or a list within a tuple. a tuple with a single item is not so simple. Parentheses Although the parentheses is optional. the new_zoo tuple contains some animals which are already there along with the animals brought over from the old zoo. You have to specify it using a comma following the first (and only) item so that Python can differentiate between a tuple and a pair of parentheses surrounding the object in an expression i. 65 Dictionary A dictionary is like an address-book where you can find the address or contact details of a person by knowing only his/her name i. note that a tuple within a tuple does not lose its identity. If you want a particular order. Tuple with 0 or 1 items An empty tuple is constructed by an empty pair of parentheses such as myempty = (). Therefore. they are just objects stored using another object. lists are not flattened as in Perl. This is called the indexing operator. The dictionaries that you will be using are instances/objects of the dict class.. that's all. or a tuple within a list. We can access the items in the tuple by specifying the item's position within a pair of square brackets just like we did for lists. Notice that the key-value pairs are separated by a colon and the pairs are separated themselves by commas and all this is enclosed in a pair of curly braces.3) ) mean two different things .2. Example: #!/usr/bin/python # Filename: using_dict. you have to specify singleton = (2 . we associate keys (name) with values (details). print(1. The same applies to a tuple within a tuple. However. Note for Perl programmers A list within a list does not lose its identity i.
  com'. There is no need to know the value corresponding to the key for this operation.org Guido's address is guido@python.the key followed by the value. we access each key-value pair of the dictionary using the items method of the dictionary which returns a list of tuples where each tuple contains a pair of items . We retrieve this pair and assign it to the variables name and address correspondingly for each pair using the for.format(len(ab))) for name.org How It Works: We create the dictionary ab using the notation already discussed. We can delete key-value pairs using our old friend .org' if 'Guido' in ab: # OR ab.org Contact Larry at larry@wall. We then access key-value pairs by specifying the key using the indexing operator as discussed in the context of lists and tuples.org'. 'larry@wall. 'matz@ruby-lang.in loop and then print these values in the . 'spammer@hotmail.the del statement.com' 66 } print("Swaroop's address is".org'.com Contact Matsumoto at matz@ruby-lang. Observe the simple syntax. address)) # Adding a key-value pair ab['Guido'] = 'guido@python. ab['Swaroop']) # Deleting a key-value pair del ab['Spammer'] print('\nThere are {0} contacts in the address-book\n'.py Swaroop's address is swaroop@swaroopch.Python en:Data Structures # 'ab' is short for 'a'ddress'b'ook ab = { 'Swaroop' 'Larry' 'Matsumoto' 'Spammer' : : : : 'swaroop@swaroopch.items(): print('Contact {0} at {1}'.format(name. address in ab. We simply specify the dictionary and the indexing operator for the key to be removed and pass it to the del statement.has_key('Guido') print("\nGuido's address is".. Next. ab['Guido']) Output: $ python using_dict.com There are 3 contacts in the address-book Contact Swaroop at swaroop@swaroopch.
  Keyword Arguments and Dictionaries On a different note. 67 Sequences Lists. shoplist[2:]) print('Item 1 to -1 is'. The three types of sequences mentioned above . 'mango'. also have a slicing operation which allows us to retrieve a slice of the sequence i. shoplist[1:3]) print('Item 2 to end is'. shoplist[2]) print('Item 3 is'. shoplist[3]) print('Item -1 is'.lists. but what are sequences and what is so special about them? The major features is that they have membership tests (i. We can check if a key-value pair exists using the in operator or even the has_key method of the dict class. Example: #!/usr/bin/python # Filename: seq.e. if you have used keyword arguments in your functions. shoplist[-2]) print('Character 0 is'. We can add new key-value pairs by simply using the indexing operator to access a key and assign that value. shoplist[-1]) print('Item -2 is'. a part of the sequence. 'banana'] name = 'swaroop' # Indexing or 'Subscription' operation print('Item 0 is'. shoplist[0]) print('Item 1 is'. tuples and strings are examples of sequences. name[0]) # Slicing on a list print('Item 1 to 3 is'. You can see the documentation for the complete list of methods of the dict class using help(dict). the in and not in expressions) and indexing operations.e. shoplist[:]) # Slicing on a string . shoplist[1:-1]) print('Item start to end is'. The indexing operation which allows us to fetch a particular item in the sequence directly.the key-value pair is specified by you in the parameter list of the function definition and when you access variables within your function.Python en:Data Structures for-block. tuples and strings. it is just a key access of a dictionary (which is called the symbol table in compiler design terminology). as we have done for Guido in the above case. you have already used dictionaries! Just think about it .py shoplist = ['apple'. 'carrot'. shoplist[1]) print('Item 2 is'.
  Python will fetch you the item corresponding to that position in the sequence. name[1:3]) 2 to end is'. This is also referred to as the subscription operation. If the second number is left out. 'carrot'] Item 2 to end is ['carrot'. The first number (before the colon) in the slicing operation refers to the position from where the slice starts and the second number (after the colon) indicates where the slice will stop at. Python will start at the beginning of the sequence. Therefore. 'banana'] characters 1 to 3 is wa characters 2 to end is aroop characters 1 to -1 is waroo characters start to end is swaroop How It Works: First. Hence. shoplist[1:3] returns a slice of the sequence starting at position 1. The slicing operation is used by specifying the name of the sequence followed by an optional pair of numbers separated by a colon within square brackets. 1 to 3 is'. 'mango'. we see how to use indexes to get individual items of a sequence. Similarly. Whenever you specify a number to a sequence within square brackets as shown above. shoplist[0] fetches the first item and shoplist[3] fetches the fourth item in the shoplist sequence. in which case. the position is calculated from the end of the sequence. name[:]) 68 . name[2:]) 1 to -1 is'. Python will stop at the end of the sequence. shoplist[-1] refers to the last item in the sequence and shoplist[-2] fetches the second last item in the sequence. Note that this is very similar to the indexing operation you have been using till now. name[1:-1]) start to end is'. The index can also be a negative number. 'carrot'] Item start to end is ['apple'. shoplist[:] returns a copy of the whole sequence. If the first number is not specified. Thus. Remember the numbers are optional but the colon isn't. the start position is included but the end position is excluded from the sequence slice. Note that the slice returned starts at the start position and will end just before the end position i. 'carrot'. 'banana'] Item 1 to -1 is ['mango'. Remember that Python starts counting numbers from 0.Python en:Data Structures print('characters print('characters print('characters print('characters Output: $ python seq.py Item 0 is apple Item 1 is mango Item 2 is carrot Item 3 is banana Item -1 is banana Item -2 is carrot Character 0 is s Item 1 to 3 is ['mango'. includes position 2 but stops at position 3 and therefore a slice of two items is returned.e.
  we get the items with position 0. find the intersection between two sets.add('china') >>> bric. the step size is 1): >>> shoplist = ['apple'. shoplist[:-1] will return a slice of the sequence which excludes the last item of the sequence but contains everything else. 'banana'] >>> shoplist[::2] ['apple'. The great thing about sequences is that you can access tuples. >>> bri = set(['brazil'. These are used when the existence of an object in a collection is more important than the order or how many times it occurs. you can test for membership. Try various combinations of such slice specifications using the Python interpreter interactively i. For example. 'apple'] Notice that when the step is 2. When the step size is 3. we get the items with position 0. 'mango'.issuperset(bri) True >>> bri. 'carrot'] >>> shoplist[::3] ['apple'.e. 'mango'. 'carrot'. 'russia'.copy() >>> bric.Python en:Data Structures You can also do slicing with negative positions. etc. You can also provide a third argument for the slice. Negative numbers are used for positions from the end of the sequence. 'mango'. 'carrot'.. 'banana'] >>> shoplist[::1] ['apple'. 3. the prompt so that you can see the results immediately. and so on. 'banana'] >>> shoplist[::-1] ['banana'. which is the step for the slicing (by default. . 'india'} How It Works: The example is pretty much self-explanatory because it involves basic set theory mathematics taught in school. 'carrot'. 'india']) >>> 'india' in bri True >>> 'usa' in bri False >>> bric = bri. 2. lists and strings all in the same way! 69 Set Sets are unordered collections of simple objects. Using sets.. .intersection(bric) {'brazil'.remove('russia') >>> bri & bric # OR bri. whether it is a subset of another set.
  shoplist) print('mylist is'.py print('Simple Assignment') shoplist = ['apple'. 'mango'.py Simple Assignment shoplist is ['mango'. 'banana'] How It Works: Most of the explanation is available in the comments. This is called as binding of the name to the object. 'banana'] mylist = shoplist # mylist is just another name pointing to the same object! del shoplist[0] # I purchased the first item. so I remove it from the list print('shoplist is'. then you have to use the slicing operation to make a copy.'. 'carrot'.Python en:Data Structures 70 References When you create an object and assign it to a variable. 'banana'] mylist is ['mango'. both of them will refer . Generally. 'carrot'. 'banana'] Copy by making a full slice shoplist is ['mango'. the variable name points to that part of your computer's memory where the object is stored. If you just assign the variable name to another name. you don't need to be worried about this. but there is a subtle effect due to references which you need to be aware of: Example: #!/usr/bin/python # Filename: reference. 'banana'] mylist is ['carrot'. the variable only refers to the object and does not represent the object itself! That is. 'carrot'. shoplist) print('mylist is'. Remember that if you want to make a copy of a list or such kinds of sequences or complex objects (not simple objects such as integers). 'carrot'. mylist) # notice that now the two lists are different Output: $ python reference.
  For a complete list of such methods. 'India'. Example: #!/usr/bin/python # Filename: str_methods. What more can there be to know? Well.startswith('Swa'): print('Yes. The str class also has a neat method to join the items of a sequence with the string acting as a delimiter between each item of the sequence and returns a bigger string generated from this. it contains the string "war"') delimiter = '_*_' mylist = ['Brazil'.join(mylist)) Output: $ python str_methods.py name = 'Swaroop' # This is a string object if name. . 'Russia'. Note for Perl programmers Remember that an assignment statement for lists does not create a copy. it contains the string "a"') if name. did you know that strings are also objects and have methods which do everything from checking part of a string to stripping spaces! The strings that you use in program are all objects of the class str. 'China'] print(delimiter. The in operator is used to check if a given string is a part of the string. the string starts with "Swa"') if 'a' in name: print('Yes.py Yes. we see a lot of the string methods in action.Python en:Data Structures to the same object and this could be trouble if you are not careful.find('war') != -1: print('Yes. The find method is used to do find the position of the given string in the string or returns -1 if it is not successful to find the substring. You have to use slicing operation to make a copy of the sequence. it contains the string "a" Yes. see help(str). the string starts with "Swa" Yes. The startswith method is used to find out whether the string starts with the given string. Some useful methods of this class are demonstrated in the next example. it contains the string "war" Brazil_*_Russia_*_India_*_China How It Works: Here. 71 More About Strings We have already discussed strings in detail earlier.
  2. sourceforge. In this case. I have created the following list on how I want it to work. we will next see how to design and write a real-world Python program. Windows users can install (http:/ / gnuwin32. we design our program. The idea is to learn how to write a Python script on your own. php) from the GnuWin32 project page (http:/ / gnuwin32. 5. htm) and add C:\Program Files\GnuWin32\bin to your system PATH environment variable. backup must be stored in a main backup directory. Note that you can use any archiving command you want as long as it has a command line interface so that we can pass arguments to it from our script. com/ mediawiki/ index. The The The The files and directories to be backed up are specified in a list. 1. this is a simple problem. The Problem The problem is "I want a program which creates a backup of all my important files". similar to what we did for recognizing the python command itself. files are backed up into a zip file. Now that we have a lot of the basics of Python in place. If you do the design. 3. We make a list of things about how our program should work. so that is perfectly okay. . net/ downlinks/ zip. Previous Next Source: http:/ / www. there is not enough information for us to get started with the solution. you may not come up with the same kind of analysis since every person has their own way of doing things. how do we specify which files are to be backed up? How are they stored? Where are they stored? After analyzing the problem properly. name of the zip archive is the current date and time. swaroopch. We use the standard zip command available by default in any standard Linux/Unix distribution. Although. php? oldid=1582 Contributors: Swaroop.Python en:Data Structures 72 Summary We have explored the various built-in data structures of Python in detail. These data structures will be essential for writing programs of reasonable size. by designing and writing a program which does something useful. sourceforge. For example. A little more analysis is required. 5 anonymous edits Python en:Problem Solving We have explored various parts of the Python language and now we will take a look at how all these parts fit together. 4. net/ packages/ zip.
  If the above program does not work for you. we are in the testing phase where we test that our program works properly. 'C:\\Code'] # Notice we had to use double quotes inside the string for names with spaces in it.py import os import time # 1. The files and directories to be backed up are specified in a list. then check the Python program if it exactly matches the program written above. Now copy/paste the printed zip_command to the shell prompt and see if it runs properly on its own. The backup must be stored in a main backup directory target_dir = 'E:\\Backup' # Remember to change this to what you will be using # 3.zip Now. How It Works: You will notice how we have converted our design into code in a step-by-step manner. If this command succeeds.system(zip_command) == 0: print('Successful backup to'. The files are backed up into a zip file. # 2. # 4.sep + time.system call and run the program.Python en:Problem Solving 73 The Solution As the design of our program is now reasonably stable.zip' # 5. target) else: print('Backup FAILED') Output: $ python backup_ver1. check the zip command manual on what could be wrong. put a print(zip_command) just before the os. . #!/usr/bin/python # Filename: backup_ver1. ' '. we can write the code which is an implementation of our solution.format(target.join(source)) # Run the backup if os.e.py Successful backup to E:\Backup\20080702185040. remove the bugs (errors) from the program.strftime('%Y%m%d%H%M%S') + '. then we have to debug our program i. If it doesn't behave as expected. source = ['"C:\\My Documents"'. If this command fails. We use the zip command to put the files in a zip archive zip_command = "zip -qr {0} {1}". The name of the zip archive is the current date and time target = target_dir + os.
  it will be '/' in Linux. python. Then. it will be '\\' in Windows and ':' in Mac OS. we create a string zip_command which contains the command that we are going to execute. We create the name of the target zip file using the addition operator which concatenates the strings i. The options are followed by the name of the zip archive to create followed by the list of files and directories to backup. in the shell . The -r option specifies that the zip command should work recursively for directories i. Depending on the outcome of the command. The %Y specification will be replaced by the year without the century. You can check if this command works by running it on the shell (Linux terminal or DOS prompt). Linux/Unix users are advised to use the executable method as discussed earlier so that they can run the backup script anytime anywhere.e.Python en:Problem Solving We make use of the os and time modules by first importing them. This is called the operation phase or the deployment phase of the software. we can use it whenever we want to take a backup of the files. else it returns an error number. 0/ library/ time. Unix. etc. html#time. but (usually) first programs do not work exactly as you expect. Appropriately.sep variable . Then. The zip command that we are using has some options and parameters passed. The time. it should include all the subdirectories and files.this gives the directory separator according to your operating system i. Using os. The above program works properly. strftime).it returns 0 if the command was successfully. For example. The name of the zip archive that we are going to create is the current date and time which we find out using the time.zip extension and will be stored in the target_dir directory. The target directory is where store all the backup files and this is specified in the target_dir variable.strftime() function takes a specification such as the one we have used in the above program. Notice the use of os. use 'C:\\Documents' or r'C:\Documents'. We convert the source list into a string using the join method of strings which we have already seen how to use. The -q option is used to indicate that the zip command should work quietly. 74 . Then. That's it.system function which runs the command as if it was run from the system i. it joins the two strings together and returns a new one.e. we have created a script to take a backup of our important files! Note to Windows Users Instead of double backslash escape sequences. It will also have the . there might be problems if you have not designed the program properly or if you have made a mistake in typing the code. you will have to go back to the design phase or you will have to debug your program. However. Now that we have a working backup script. we finally run the command using the os. do not use 'C:\Documents' since you end up using an unknown escape sequence \D. The %m specification will be replaced by the month as a decimal number between 01 and 12 and so on. org/ dev/ 3. we specify the files and directories to be backed up in the source list.sep instead of these characters directly will make our program portable and work across these systems.e. The complete list of such specifications can be found in the Python Reference Manual (http:/ / docs. For example.e. we print the appropriate message that the backup has failed or succeeded.strftime() function. you can also use raw strings. The two options are combined and specified in a shortcut as -qr.
  #!/usr/bin/python # Filename: backup_ver2. The files are backed up into a zip file. Third advantage is that separate directories will help you to easily check if you have taken a backup for each day since the directory would be created only if you have taken a backup for that day.py import os import time # 1. source = ['"C:\\My Documents"'.system(zip_command) == 0: print('Successful backup to'. We use the zip command to put the files in a zip archive zip_command = "zip -qr {0} {1}".sep + now + '. 'C:\\Code'] # Notice we had to use double quotes inside the string for names with spaces in it. The backup must be stored in a main backup directory target_dir = 'E:\\Backup' # Remember to change this to what you will be using # 3.strftime('%Y%m%d') # The current time is the name of the zip archive now = time. The files and directories to be backed up are specified in a list. today) # The name of the zip file target = today + os.format(target.mkdir(today) # make directory print('Successfully created directory'. # 4. ' '.exists(today): os.join(source)) # Run the backup if os. Second advantage is that the length of the filenames are much shorter.using the time as the name of the file within a directory with the current date as a directory within the main backup directory. One of the refinements I felt was useful is a better file-naming mechanism .path. The current day is the name of the subdirectory in the main directory today = target_dir + os.Python en:Problem Solving 75 Second Version The first version of our script works. First advantage is that your backups are stored in a hierarchical manner and therefore it is much easier to manage.sep + time. # 2. target) . However.strftime('%H%M%S') # Create the subdirectory if it isn't already there if not os.zip' # 5. we can make some refinements to it so that it can work better on a daily basis. This is called the maintenance phase of the software.
  I am finding it hard to differentiate what the backups were for! For example. source = ['"C:\\My Documents"'. The files are backed up into a zip file. This can be easily achieved by attaching a user-supplied comment to the name of the zip archive. The files and directories to be backed up are specified in a list. The current day is the name of the subdirectory in the main directory today = target_dir + os. The backup must be stored in a main backup directory target_dir = 'E:\\Backup' # Remember to change this to what you will be using # 3. The changes is that we check if there is a directory with the current day as name inside the main backup directory using the os.zip $ python backup_ver2. # 4. 76 Third Version The second version works fine when I do many backups. please follow along because there's a lesson in here.sep + time. # 2.py import os import time # 1. Note The following program does not work.strftime('%H%M%S') . so do not be alarmed.path.Python en:Problem Solving else: print('Backup FAILED') Output: $ python backup_ver2.py Successfully created directory E:\Backup\20080702 Successful backup to E:\Backup\20080702\202311.py Successful backup to E:\Backup\20080702\202325.exists function. If it doesn't exist. then I want to associate what those changes are with the name of the zip archive. #!/usr/bin/python # Filename: backup_ver3.zip How It Works: Most of the program remains the same.mkdir function.strftime('%Y%m%d') # The current time is the name of the zip archive now = time. 'C:\\Code'] # Notice we had to use double quotes inside the string for names with spaces in it. but when there are lots of backups. I might have made some major changes to a program or presentation. we create it using the os.
  sep + now + '_' + comment.mkdir(today) # make directory print('Successfully created directory'.py".zip' else: target = today + os.system(zip_command) == 0: print('Successful backup to'.join(source)) # Run the backup if os. we make this correction to our program.exists(today): os. .path. We use the zip command to put the files in a zip archive zip_command = "zip -qr {0} {1}".sep + now + '_' + ^ SyntaxError: invalid syntax How This (does not) Work: This program does not work! Python says there is a syntax error which means that the script does not satisfy the structure that Python expects to see. When we observe the error given by Python. So we start debugging our program from that line.format(target. it also tells us the place where it detected the error as well. we see that the single logical line has been split into two physical lines but we have not specified that these two physical lines belong together.. today) # 5.sep + now + '. Python has found the addition operator (+) without any operand in that logical line and hence it doesn't know how to continue.replace(' '. '_') + '. ' '. So. On careful observation.zip' # Create the subdirectory if it isn't already there if not os. Basically. target) else: print('Backup FAILED') Output: $ python backup_ver3. line 25 target = today + os. Remember that we can specify that the logical line continues in the next physical line by the use of a backslash at the end of the physical line.py File "backup_ver3.
  The files are backed up into a zip file.join(source)) # Run the backup if os.sep + time. 'C:\\Code'] # Notice we had to use double quotes inside the string for names with spaces in it. source = ['"C:\\My Documents"'. The backup must be stored in a main backup directory target_dir = 'E:\\Backup' # Remember to change this to what you will be using # 3. '_') + '.strftime('%H%M%S') #.mkdir(today) # make directory print('Successfully created directory'.sep + now + '_' + \ comment.exists(today): os.zip' else: target = today + os.Python en:Problem Solving 78 Fourth Version #!/usr/bin/python # Filename: backup_ver4. # 2.format(target.strftime('%Y%m%d') # The current time is the name of the zip archive now = time.py import os import time # 1.path.replace(' '.zip' # Create the subdirectory if it isn't already there if not os. ' '.sep + now + '.system(zip_command) == 0: print('Successful backup to'. The files and directories to be backed up are specified in a list. We use the zip command to put the files in a zip archive zip_command = "zip -qr {0} {1}". target) else: print('Backup FAILED') Output: . today) # 5. # 4.
  org/ dev/ 3.system way of creating archives and instead using the zipfile or tarfile built-in module to create these archives.py Enter a comment --> added new examples Successful backup to E:\Backup\20080702\202836_added_new_examples.system call? . but there is always room for improvement. 0/ library/ zipfile. so that the example is simple enough to be understood by everybody but real enough to be useful. We can get these names from the sys. For example. Notice that we are replacing spaces in the comment with underscores . html) module instead of the os. then this is attached to the name of the zip archive just before the . Can you try writing the fifth version that uses the zipfile (http:/ / docs. if a comment was supplied. If the user has just pressed enter without entering anything (maybe it was just a routine backup or no special changes were made). then we proceed as we have done before.zip $ python backup_ver4. I have been using the os.zip extension.this is because managing filenames without spaces are much easier. 79 More Refinements The fourth version is a satisfactorily working script for most users.system way of creating a backup in the above examples purely for pedagogical purposes. However. The most important refinement would be to not use the os.Python en:Problem Solving $ python backup_ver4. you can include a verbosity level for the program where you can specify a -v option to make your program become more talkative. python.argv list and we can add them to our source list using the extend method provided by the list class.zip How It Works: This program now works! Let us go through the actual enhancements that we had made in version 3. They are part of the standard library and available already for you to use without external dependencies on the zip program to be available on your computer. Another possible enhancement would be to allow extra files and directories to be passed to the script at the command line.py Enter a comment --> Successful backup to E:\Backup\20080702\202839. We take in the user's comments using the input function and then check if the user actually entered something by finding out the length of the input using the len function. However.
  Use it to ensure that it works as expected. Test and debug it. com/ mediawiki/ index. Now.Python en:Problem Solving 80 The Software Development Process We have now gone through the various phases in the process of writing a software. php? oldid=1395 Contributors: Swaroop. Remember. add any features that you want and continue to repeat the Do It-Test-Use cycle as many times as required. not built. 3. 2. swaroopch. 5. 6. Summary We have seen how to create our own Python programs/scripts and the various stages involved in writing such programs. we will discuss object-oriented programming. Start implementing with a simple version. Next. 4. You may find it useful to create your own program just like we did in this chapter so that you become comfortable with Python as well as problem-solving. What (Analysis) How (Design) Do It (Implementation) Test (Testing and Debugging) Use (Operation or Deployment) Maintain (Refinement) A recommended way of writing programs is the procedure we have followed in creating the backup script: Do the analysis and design. These phases can be summarised as follows: 1. Software is grown. Previous Next Source: http:/ / www. 3 anonymous edits .
  This terminology is important because it helps us to differentiate between functions and variables which are independent and those which belong to a class or object. Note for Static Language Programmers Note that even integers are treated as objects (of the int class).any other name is definitely frowned upon. This is called the object oriented programming paradigm. we have designed our program around functions i. They are called instance variables and class variables respectively. Collectively. C# and Java 1. Although. An analogy is that you can have variables of type int which translates to saying that variables that store integers are variables which are instances (objects) of the int class. Such functions are called methods of the class. the fields and methods can be referred to as the attributes of that class. Objects can also have functionality by using functions that belong to a class. Python will provide it. This particular variable refers to the object itself. This is called the procedure-oriented way of programming. but you do not give a value for this parameter when you call the method. The fields and methods of the class are listed in an indented block. you can give any name for this parameter. There are many advantages to using a standard name . but when writing large programs or have a problem that is better suited to this method. you can use object oriented programming techniques.e. Fields are of two types . There is another way of organizing your program which is to combine data and functionality and wrap it inside something called an object. Objects can store data using ordinary variables that belong to the object. A class is created using the class keyword. Variables that belong to an object or class are referred to as fields.they can belong to each instance/object of the class or they can belong to the class itself. . it is strongly recommended that you use the name self . A class creates a new type where objects are instances of the class.5) where integers are primitive native types. and by convention. The self Class methods have only one specific difference from ordinary functions . Most of the time you can use procedural programming. See help(int) for more details on the class. Classes and objects are the two main aspects of object oriented programming.any reader of your program will immediately recognize it and even specialized IDEs (Integrated Development Environments) can help you if you use self. blocks of statements which manipulate data. it is given the name self.they must have an extra first name that has to be added to the beginning of the parameter list.5 programmers will find this similar to the boxing and unboxing concept. This is unlike C++ and Java (before version 1.Python en:Object Oriented Programming 81 Python en:Object Oriented Programming Introduction In all the programs we wrote till now.
  we confirm the type of the variable by simply printing it. . 82 Classes The simplest class possible is shown in the following example.py <__main__.Person object at 0x019F85F0> How It Works: We create a new class using the class statement and the name of the class. For our verification. Say you have a class called MyClass and an instance of this class called myobject. we have an empty block which is indicated using the pass statement. An example will make this clear. In this case. arg2). #!/usr/bin/python # Filename: simplestclass.py class Person: pass # An empty block p = Person() print(p) Output: $ python simplestclass. (We will learn more about instantiation in the next section). This also means that if you have a method which takes no arguments. this is automatically converted by Python into MyClass.Python en:Object Oriented Programming Note for C++/Java/C# Programmers The self in Python is equivalent to the this pointer in C++ and the this reference in Java and C#. we create an object/instance of this class using the name of the class followed by a pair of parentheses. The address will have a different value on your computer since Python can store the object wherever it finds space.method(arg1.this is all the special self is about. Next. When you call a method of this object as myobject.the self.method(myobject. arg2) . It tells us that we have an instance of the Person class in the __main__ module. This is followed by an indented block of statements which form the body of the class. Notice that the address of the computer memory where your object is stored is also printed. arg1. then you still have to have one argument . You must be wondering how Python gives the value for self and why you don't need to give a value for it.
  how are you?') p = Person() p.sayHi() # This short example can also be written as Person('Swaroop'). how are you? How It Works: Here we see the self in action.name) p = Person('Swaroop') p.name = name def sayHi(self): print('Hello. Notice the double underscores both at the beginning and at the end of the name. self. The __init__method There are many method names which have special significance in Python classes. name): self. Notice that the sayHi method takes no parameters but still has the self in the function definition.Python en:Object Oriented Programming 83 Object Methods We have already discussed that classes/objects can have methods just like functions except that we have an extra self variable. The method is useful to do any initialization you want to do with your object. We will now see an example. Example: #!/usr/bin/python # Filename: class_init.py class Person: def sayHi(self): print('Hello.sayHi() # This short example can also be written as Person(). #!/usr/bin/python # Filename: method.sayHi() Output: .py class Person: def __init__(self.sayHi() Output: $ python method. We will see the significance of the __init__ method now. The __init__ method is run as soon as an object of a class is instantiated. my name is'.py Hello.
  There is only one copy of the class variable and when any one object makes a change to a class variable. they are not shared and are not related in any way to the field by the same name in a different instance. that change will be seen by all the other instances. Here.py Hello. i. counting the number of robots population = 0 def __init__(self.''' self. notice that we do not explicitly call the __init__ method but pass the arguments in the parentheses following the class name when creating a new instance of the class. 84 Class And Object Variables We have already discussed the functionality part of classes and objects (i. we are able to use the self. An example will make this easy to understand: #!/usr/bin/python # Filename: objvar.name field in our methods which is demonstrated in the sayHi method. methods). Object variables are owned by each individual object/instance of the class.Python en:Object Oriented Programming $ python class_init. This is the special significance of this method. name): '''Initializes the data. are nothing but ordinary variables that are bound to the namespaces of the classes and objects. The dotted notation allows us to differentiate between them. the robot # adds to the population Robot.name)) # When this person is created.class variables and object variables which are classified depending on whether the class or the object owns the variables respectively. Class variables are shared . This means that these names are valid within the context of these classes and objects only. we define the __init__ method as taking a parameter name (along with the usual self). Notice these are two different variables even though they are both called 'name'. Most importantly.format(self.e. fields. my name is Swaroop How It Works: Here.py class Robot: '''Represents a robot.e.''' # A class variable. The data part. we just create a new field also called name. In this case.population += 1 . with a name.they can be accessed by all instances of that class. now let us learn about the data part.e. There are two types of fields . each object has its own copy of the field i. That's why they are called name spaces. Now.name = name print('(Initializing {0})'.
  format(self. they can do that.howMany() print("\nRobots can do some work here.'.format(self.population -= 1 if Robot.'.population)) howMany = staticmethod(howMany) droid1 = Robot('R2-D2') droid1.name)) Robot.'.''' print('{0} is being destroyed!'.\n") print("Robots have finished their work. my masters call me C-3PO. So let's destroy them.'. Yeah.Python en:Object Oriented Programming 85 def __del__(self): '''I am dying. .format(self. We have 1 robots. We have 2 robots. my masters call me {0}.format(Robot.name)) def howMany(): '''Prints the current population.name)) else: print('There are still {0:d} robots working.howMany() droid2 = Robot('C-3PO') droid2.sayHi() Robot.''' print('Greetings. (Initializing C-3PO) Greetings.format(Robot.population == 0: print('{0} was the last one.population)) def sayHi(self): '''Greeting by the robot.''' print('We have {0:d} robots. my masters call me R2-D2.howMany() Output: (Initializing R2-D2) Greetings.sayHi() Robot.") del droid1 del droid2 Robot.
  population)) Decorators can be imagined to be a shortcut to calling an explicit statement. ibm.format(Robot. Remember. We refer to the object variable name using self. How It Works: This is a long example but helps demonstrate the nature of class and object variables. Robots have finished their work.e. We could have also achieved the same using decorators (http:/ / www. In this method. html): @staticmethod def howMany(): '''Prints the current population.''' print('We have {0:d} robots.name notation in the methods of that object. . there is another special method __del__ which is called when an object is going to die i. Remember this simple difference between class and object variables. we refer to the population class variable as Robot. This means we can define it as either a classmethod or a staticmethod depending on whether we need to know which class we are part of.population count by 1.sayHi. com/ developerworks/ linux/ library/ l-cpdecor. Observe that the __init__ method is used to initialize the Robot instance with a name. we will go for staticmethod.name is specific to each object which indicates the nature of object variables.Python en:Object Oriented Programming 86 Robots can do some work here. This is called an attribute reference.'.population and not as self. as we have seen in this example. we also see the use of docstrings for classes as well as methods. We have 0 robots. R2-D2 is being destroyed! There are still 1 robots working. Thus. We can access the class docstring at runtime using Robot. Here. we simply decrease the Robot. The name variable belongs to the object (it is assigned using self) and hence is an object variable. In this program.. In this method.population. it is no longer being used and is being returned to the computer system for reusing that piece of memory. So let's destroy them. we increase the population count by 1 since we have one more robot being added.__doc__ and the method docstring as Robot. C-3PO is being destroyed! C-3PO was the last one. population belongs to the Robot class and hence is a class variable.__doc__ Just like the __init__ method. Since we don't need such information. Also observe that the values of self.
  There are many advantages to this approach. If we add/change any functionality in SchoolMember. #!/usr/bin/python # Filename: inherit. One exception: If you use data members with names using the double underscore prefix such as __privatevar. changes in the subtypes do not affect other subtypes. Inheritance can be best imagined as implementing a type and subtype relationship between classes. We will now see this example as a program. age and address. courses and leaves for teachers and. The SchoolMember class in this situation is known as the base class or the superclass. you can add a new ID card field for both teachers and students by simply adding it to the SchoolMember class. we have to use the del statement which is what we have done here. 87 Inheritance One of the major benefits of object oriented programming is reuse of code and one of the ways this is achieved is through the inheritance mechanism. A better way would be to create a common class called SchoolMember and then have the teacher and student classes inherit from this class i. they will become sub-types of this type (class) and then we can add specific characteristics to these sub-types.Python en:Object Oriented Programming The __del__ method is run when the object is no longer in use and there is no guarantee when that method will be run. this is automatically reflected in the subtypes as well.py .e. the object can be treated as an instance of the parent class. Thus. You can create two independent classes for each type and process them but adding a new common characteristic would mean adding to both of these independent classes.e. Python uses name-mangling to effectively make it a private variable. Also observe that we reuse the code of the parent class and we do not need to repeat it in the different classes as we would have had to in case we had used independent classes. They also have specific characteristics such as salary. marks and fees for students. the convention followed is that any variable that is to be used only within the class or object should begin with an underscore and all other names are public and can be used by other classes/objects. For example. This is called polymorphism where a sub-type can be substituted in any situation where a parent type is expected i. They have some common characteristics such as name. Another advantage is that if you can refer to a teacher or student object as a SchoolMember object which could be useful in some situations such as counting of the number of school members. The Teacher and Student classes are called the derived classes or subclasses. Suppose you want to write a program which has to keep track of the teachers and students in a college. If you want to explicitly see it in action. Remember that this is only a convention and is not enforced by Python (except for the double underscore prefix). This quickly becomes unwieldy. Note for C++/Java/C# Programmers All class members (including the data members) are public and all the methods are virtual in Python. However.
  name.salary = salary print('(Initialized Teacher: {0})'. age) self. 30000) s = Student('Swaroop'.format(self.age). 75) print() # prints a blank line members = [t.name = name self. 40.name)) def tell(self): SchoolMember.''' def __init__(self. name. age.__init__(self.''' def __init__(self. name.''' def __init__(self.marks = marks print('(Initialized Student: {0})'.''' print('Name:"{0}" Age:"{1}"'.age = age print('(Initialized SchoolMember: {0})'.tell(self) print('Marks: "{0:d}"'.format(self. salary): SchoolMember.name)) def tell(self): '''Tell my details. Shrividya) . name.name)) def tell(self): SchoolMember.tell() # works for both Teachers and Students Output: $ python inherit.name. 25.format(self. Shrividya'. self.__init__(self.salary)) class Student(SchoolMember): '''Represents a student. age. age): self. name. s] for member in members: member.marks)) t = Teacher('Mrs. age) self.format(self. end=" ") class Teacher(SchoolMember): '''Represents a teacher.format(self. marks): SchoolMember.py (Initialized SchoolMember: Mrs.format(self.tell(self) print('Salary: "{0:d}"'.Python en:Object Oriented Programming 88 class SchoolMember: '''Represents any school member.
  then it is called multiple inheritance. swaroopch. A note on terminology . you have to explicitly call it yourself. we observe that the __init__ method of the base class is explicitly called using the self variable so that we can initialize the base class part of the object. This is very important to remember . which in this case it does. One way to understand this is that Python always starts looking for methods in the actual type. Notice that we can treat instances of Teacher or Student as just instances of the SchoolMember when we use the tell method of the SchoolMember class. Swaroop. Shrividya) (Initialized SchoolMember: Swaroop) (Initialized Student: Swaroop) Name:"Mrs. it starts looking at the methods belonging to its base classes one by one in the order they are specified in the tuple in the class definition. We also observe that we can call methods of the base class by prefixing the class name to the method call and then pass in the self variable along with any arguments. 89 Summary We have now explored the various aspects of classes and objects as well as the various terminologies associated with it. com/ mediawiki/ index. We have also seen the benefits and pitfalls of object-oriented programming. php? oldid=2359 Contributors: Horstjens.Python does not automatically call the constructor of the base class.Python en:Object Oriented Programming (Initialized Teacher: Mrs. If it could not find the method. Shrividya" Age:"40" Salary: "30000" Name:"Swaroop" Age:"25" Marks: "75" How It Works: To use inheritance. Next. 6 anonymous edits . Previous Next Source: http:/ / more than one class is listed in the inheritance tuple. Also. observe that the tell method of the subtype is called and not the tell method of the SchoolMember class. Next. Python is highly object-oriented and understanding these concepts carefully will help you a lot in the long run. we will learn how to deal with input/output and how to access files in Python. we specify the base class names in a tuple following the class name in the class definition.
  it is a palindrome $ python user_input. We can achieve this using the input() and print() functions respectively. it is a palindrome") else: print("No.py Enter text: racecar Yes. Another common type of input/output is dealing with files. We can also provide a third argument that determines the step by which the slicing is done.py Enter text: madam Yes. For example. See help(str) for more details. The ability to create. We've already seen how we can make slices from sequences using the seq[a:b] code starting from position a to position b.Python en:Input Output 90 Python en:Input Output Introduction There will be situations where your program has to interact with the user. we can also use the various methods of the str (string) class. For output.py def reverse(text): return text[::-1] def is_palindrome(text): return text == reverse(text) something = input('Enter text: ') if (is_palindrome(something)): print("Yes. you would want to take input from the user and then print some results back. The . it is a palindrome How It Works: We use the slicing feature to reverse the text. you can use the rjust method to get a string which is right justified to a specified width. read and write files is essential to many programs and we will explore this aspect in this chapter. Input from user #!/usr/bin/python # user_input. it is not a palindrome") Output: $ python user_input. it is not a palindrome $ python user_input.py Enter text: sir No. For example.
  Giving a negative step.close() # close the file Output: $ python using_file. Example: #!/usr/bin/python # Filename: using_file.txt') # if no mode is specified. -1 will return the text in reverse. Then finally. For example. readline or write methods appropriately to read from or write to the file. org/ wiki/ palindrome). We take that text and reverse it.txt'. spaces and case. end='') f. then the text is a palindrome (http:/ / en. wiktionary.. 'r'ead mode is assumed by default while True: line = f.py Programming is fun . If the original text and reversed text are equal. Homework exercise: Checking whether a text is a palindrome should also ignore punctuation. you call the close method to tell Python that we are done using the file. the input() function will then return that text. The ability to read or write to the file depends on the mode you have specified for the file opening.readline() if len(line) == 0: # Zero length indicates EOF break print(line." is also a palindrome but our current program doesn't say it is.write(poem) # write text to file f. 'w') # open for 'w'riting f. The input() function takes a string as argument and displays it to the user. i.e. Once the user has entered.Python en:Input Output default step is 1 because of which it returns a continuous part of the text. "Rise to vote. sir. when you are finished with the file. Then it waits for the user to type something and press the return key.close() # close the file f = open('poem.
  'mango'. Then. By default. 'carrot'] # Write to the file f = open(shoplistfile. We read in each line of the file using the readline method in a loop. open a file by using the built-in open function and specifying the name of the file and the mode in which we want to open the file. Next. we first open the file in write text mode and use the write method of the file object to write to the file and then we finally close the file. we finally close the file. When an empty string is returned. The mode can be a read mode ('r').py import pickle # the name of the file where we will store the object shoplistfile = 'shoplist. 92 Pickle Python provides a standard module called pickle using which you can store any Python object in a file and then get it back later.data' # the list of things to buy shoplist = ['apple'.dump(shoplist.close() del shoplist # destroy the shoplist variable # Read back from the storage . We don't need to specify a mode because 'read text file' is the default mode. 'wb') pickle. write mode ('w') or append mode ('a'). it means that we have reached the end of the file and we 'break' out of the loop. We can also by dealing with a text file ('t') or a binary file ('b'). In our example. the print() function prints the text as well as an automatic newline to the screen.txt file to confirm that the program has indeed written and read from that file. f) # dump the object to a file f. This method returns a complete line including the newline character at the end of the line. open() considers the file to be a 't'ext file and opens it in 'r'ead mode.Python en:Input Output When the work is done if you wanna make your work also fun: use Python! How It Works: First. By deafult. We are suppressing the newline by specifying end='' because the line that is read from the file already ends with a newline character. There are actually many more modes available and help(open) will give you more details about them. Now. we open the same file again for reading. check the contents of the poem. Example: #!/usr/bin/python # Filename: pickling. This is called storing the object persistently.
  Next. we will explore the concept of exceptions. 'carrot'] How It Works: To store an object in a file. what if your program had some invalid statements? This is handled by Python which raises its hands and tells you there is an error. For example. Similarly.py ['apple'. In this case.Python en:Input Output f = open(shoplistfile. What if we misspelt print as Print? Note the capitalization. line 1. 1 anonymous edits Python en:Exceptions Introduction Exceptions occur when certain exceptional situations occur in your program. we have to first open the file in 'w'rite 'b'inary mode and then call the dump function of the pickle module. 93 Summary We have discussed various types of input/output and also file handling and using the pickle module. php? oldid=1583 Contributors: Horstjens. This process is called pickling. swaroopch.load(f) # load the object from the file print(storedlist) Output: $ python pickling. we retrieve the object using the load function of the pickle module which returns the object. 'rb') storedlist = pickle. Previous Next Source: http:/ / www. Swaroop. what if you are going to read a file and the file does not exist? Or what if you accidentally deleted it when the program was running? Such situations are handled using exceptions. This process is called unpickling. 'mango'. com/ mediawiki/ index. Python raises a syntax error. Next. Errors Consider a simple print function call. in <module> Print('Hello World') NameError: name 'Print' is not defined >>> print('Hello World') Hello World . >>> Print('Hello World') Traceback (most recent call last): File "<pyshell#0>".
  py Enter something --> no exceptions You entered no exceptions How It Works: We put all the statements that might raise exceptions/errors inside the try block and then put handlers for the appropriate errors/exceptions in the except clause/block.') else: print('You entered {0}'.py Enter something --> # Press ctrl-d Why did you do an EOF on me? $ python try_except. line 1. >>> s = input('Enter something --> ') Enter something --> Traceback (most recent call last): File "<pyshell#2>". The except . #!/usr/bin/python # Filename: try_except. Press ctrl-d and see what happens..py try: text = input('Enter something --> ') except EOFError: print('Why did you do an EOF on me?') except KeyboardInterrupt: print('You cancelled the operation. $ python try_except.format(text)) Output: $. We basically put our usual statements within the try-block and put all our error handlers in the except-block.except statement. This is what an error handler for this error does.py Enter something --> # Press ctrl-c You cancelled the operation.Python en:Exceptions Observe that a NameError is raised and also the location where the error was detected is printed. 94 Exceptions We will try to read input from the user.
  Python en:Exceptions clause can handle a single specified error or exception. atleast): Exception. Note that there has to be at least one except clause associated with every try clause. then the default Python handler is called which just stops the execution of the program and prints an error message.py Enter something --> a . ex. 95 Raising Exceptions You can raise exceptions using the raise statement by providing the name of the error/exception and the exception object that is to be thrown.__init__(self) self. In the next example.''' def __init__(self.py class ShortInputException(Exception): '''A user-defined exception class.except block.') Output: $ python raising. You can also have an else clause associated with a try.. If no names of errors or exceptions are supplied. we will also see how to get the exception object so that we can retrieve additional information. length. 3) # Other work can continue as usual here except EOFError: print('Why did you do an EOF on me?') except ShortInputException as ex: print('ShortInputException: The input was {0} long.atleast)) else: print('No exception was raised.format(ex. #!/usr/bin/python # Filename: raising. or a parenthesized list of errors/exceptions.atleast = atleast try: text = input('Enter something --> ') if len(text) < 3: raise ShortInputException(len(text). Otherwise. what's the point of having a try block? If any error or exception is not handled. The error or exception that you can arise should be class which directly or indirectly must be a derived class of the Exception class.length.length = length self. it will handle all errors and exceptions. We have already seen this in action above. The else clause is executed if no exception occurs. expected at least {1}'\ .
  Finally Suppose you are reading a file in your program.') finally: f. we mention the class of error which will be stored as the variable name to hold the corresponding error/exception object.py import time try: f = open('poem..txt') while True: # our usual file-reading idiom line = f. How It Works: Here. This is analogous to parameters and arguments in a function call.close() print('(Cleaning up: Closed the file)') Output: $ python finally.. we are creating our own exception type. . we use the length and atleast fields of the exception object to print an appropriate message to the user. This new exception type is called ShortInputException. It has two fields . 96 Try .sleep(2) # To make sure it runs for a while except KeyboardInterrupt: print('!! You cancelled the reading from the file.py Enter something --> abc No exception was raised. In the except clause. #!/usr/bin/python # Filename: finally. and atleast which is the minimum length that the program was expecting. end='') time.readline() if len(line) == 0: break print(line. Note that you can use an except clause along with a finally block for the same corresponding try block.Python en:Exceptions ShortInputException: The input was 1 long.length which is the length of the given input. You will have to embed one within another if you want to use both. Within this particular except clause. expected at least 3 $ python raising.
  com/ mediawiki/ index.txt") as f: for line in f: print(line... We have seen how to create our own exception types and how to raise exceptions as well. This is what helps us to avoid having to use explicit try. The difference here is that we are using the open function with the with statement . php? oldid=1470 Contributors: Swaroop. end='') How It Works: The output should be same as the previous example. Hence. swaroopch. but we have arbitrarily introduced sleeping for 2 seconds after printing each line using the time.Python en:Exceptions (Cleaning up: Closed the file) How It Works: We do the usual file-reading stuff. before the program exits. the finally clause is executed and the file object is always closed.except and try. It fetches the object returned by the open statement. Next. we will explore the Python Standard Library. It always calls the thefile. However. let's call it "thefile" in this case. When the program is still running. Previous Next Source: http:/ / leave the closing of the file to be done automatically by with open. What happens behind the scenes is that there is a protocol used by the with statement. 2 anonymous edits . there is also a with statement that enables this to be done in a clean manner: #!/usr/bin/python # Filename: using_with. 97 The with statement Acquiring a resource in the try block and subsequently releasing the resource in the finally block is a common pattern.finally statements.sleep function so that the program runs slowly (Python is very fast by nature).__exit__ after finishing the block of code..finally statements repeatedly. org/ dev/ peps/ pep-0343/ ) for comprehensive explanation. Summary We have discussed the usage of the try.py with open("poem. python. More discussion on this topic is beyond scope of this book. press ctrl-c to interrupt/cancel the program. so please refer PEP 343 (http:/ / function before starting the block of code under it and always calls thefile. So the code that we would have written in a finally block is should be taken care of automatically by the __exit__ method. Observe that the KeyboardInterrupt exception is thrown and the program quits.
  We can check this to. 0. ensure the program runs only under Python 3.version_info (3. for example. python. Note If you find the topics in this chapter too advanced. It is important to become familiar with the Python Standard Library since many problems can be solved quickly if you are familiar with the range of things that these libraries can do. >>> import sys >>> sys.warn("Need Python 3. say. sys module The sys module contains system-specific functionality. RuntimeWarning) else: print('Proceed as normal') Output: . Let us explore a few useful modules. 0/ library/ ) of the documentation that comes with your Python installation. 'beta'. Suppose we want to check the version of the Python command being used so that. The first entry is the major version. We have already seen that the sys.py import sys. You can find complete details for all of the modules in the Python Standard Library in the 'Library Reference' section (http:/ / docs.Python en:Standard Library 98 Python en:Standard Library Introduction The Python Standard Library contains a huge number of useful modules and is part of every standard Python installation. you may skip this chapter.argv list contains the command-line arguments. The sys module gives us such functionality. 0.version_info[0] < 3: warnings.0: #!/usr/bin/python # Filename: versioncheck. I highly recommend coming back to this chapter when you are more comfortable with programming using Python.0 for this program to run". we want to ensure that we are using at least version 3. However. org/ dev/ 3. 2) >>> sys.version_info[0] >= 3 True How It Works: The sys module has a version_info tuple that gives us the version information. We will explore some of the commonly used modules in this library. warnings if sys.
  join(os.233 : WARNING : Dying now . If the Python version number is not at least 3. we display a corresponding warning.log') logging.join(os.log If we check the contents of test.Python en:Standard Library $ python2.0 for this program to run RuntimeWarning) $ python3 versioncheck.basicConfig( level=logging. filename = logging_file.233 : DEBUG : Start of the program 2008-09-03 13:18:16. 'test.path..py Proceed as normal How It Works: We use another module from the standard library called warnings that is used to display warnings to the end-user.getenv('HOMEPATH').path.DEBUG. 'test.getenv('HOMEDRIVE').warning("Dying now") Output: $python use_logging.log.233 : INFO : Doing something 2008-09-03 13:18:16.log') else: logging_file = os. format='%(asctime)s : %(levelname)s : %(message)s'. #!/usr/bin/python # Filename: use_logging. ) logging.5 versioncheck. logging if platform.getenv('HOME').py:6: RuntimeWarning: Need Python 3. filemode = 'w'.startswith('Windows'): logging_file = os.platform().debug("Start of the program") logging.info("Doing something") logging.py Logging to C:\Users\swaroop\test.py versioncheck. os. platform.py import os. it will look something like this: 2008-09-03 13:18:16.
  response # Get your own APP ID at. Once the program has run. This can be achieved using a few modules.e.parse. The reason to use a special function rather than just adding the strings together is because this function will ensure the full location matches the format expected by the operating system.version_info[0] != 3: sys.com/WebSearchService/V1/webSearch' .0 beta 2 (http:/ / bugs. 100 urllib and json modules How much fun would it be if we could write our own program that will get search results from the web? Let us explore that now. org/ issue3763). we need to know just the home folder of the user and we get the full location of the file. see import platform. For other platforms.exit('This program needs Python 3. urllib.py import sys if sys.lBSfPhwr' SEARCH_BASE = '. urllib. the operating system and the logging module to log information.path. If it is Windows.0') import json import urllib. even though no information was displayed to the user running the program.yahoo. we can check this file and we will know what happened in the program. We will make use of Yahoo! Search to get the search results and luckily they can give us the results in a format called JSON which is easy for us to parse because of the built-in json module in the standard library. First. Putting these three parts together.platform() (for more information. the platform module for information about the platform i. we get the full location of the file. First is the urllib module that we can use to fetch any webpage from the internet. information. TODO This program doesn't work yet which seems to be a bug in Python 3. We use the os.join() function to put these three parts of the location together. Finally.Python en:Standard Library How It Works: We use three modules from the standard library .yahooapis. the home folder and the filename where we want to store the information. We configure the logging module to write all the messages in a particular format to the file we have specified. we can put messages that are either meant for debugging. we figure out the home drive. python. warning or even critical messages. #!/usr/bin/python # Filename: yahoo_search.request. we check which operating system we are using by checking the string returned by platform.the os module for interacting with the operating system. help(platform)).
  load() which will read the content and simultaneously convert it to a Python object.parse. start=1.urlencode(kwargs) result = json. **kwargs): kwargs.request.urlopen() function and pass that file handle to json. We then loop through these results and display it to the end-user.yahoo.update({ 'appid': YAHOO_APP_ID. 'query': query.parse. . results=20.html def search(query. starting from the first result.format(result['Title'].com/python/python-json. 'output': 'json' }) url = SEARCH_BASE + '?' + urllib. open this link in your web browser (http:/ / search. lBSfPhwr& results=20& start=1& output=json) and you will see 20 results.. We make a connection to this URL using the urllib.load(urllib.urlopen(url)) if 'Error' in result: raise YahooSearchError(result['Error']) return result['ResultSet'] query = input('What do you want to search for? ') for result in search(query)['Result']: print("{0} : {1}". 'start': start.Python en:Standard Library 101 class YahooSearchError(Exception): pass # Taken from. We have to specify many options which we combine using key1=value1&key2=value2 format which is handled by the urllib.request. result['Url'])) Output: TODO How It Works: We can get the search results from a particular website by giving the text we are searching for in a particular format.urlencode() function. and we are asking for the output in JSON format. 'results': results. for the words "byte of python". So for example. yahooapis.
  <everything else>). html) and so on. python. com/ projects/ PyMOTW/ ) series. diveintopython. errstr = get_error_details() >>> errnum 2 >>> errstr 'second error details' Notice that the usage of a. Passing tuples around Ever wished you could return two different values from a function? You can. html). 'second error details') . Next. org/ dev/ 3. handling command line options (http:/ / docs. 0/ library/ ) to get an idea of all the modules that are available. Previous Next Source: http:/ / www. In this chapter. org/ regular_expressions/ index. doughellmann. org/ dev/ library/ pdb. html).. Summary We have explored some of the functionality of many modules in the Python Standard Library. then you just need to star it just like you would in function parameters: .. com/ mediawiki/ index. swaroopch. All you have to do is use a tuple..Python en:Standard Library 102 Module of the Week Series There is much more to be explored in the standard library such as debugging (http:/ / docs. >>> errnum. php? oldid=1156 Contributors: Swaroop.. python. return (2. If you want to interpret the results as (a. we will cover various aspects of Python that will make our tour of Python more complete. It is highly recommended to browse through the Python Standard Library documentation (http:/ / docs. b = <some expression> interprets the result of the expression as a tuple with two values. python. 0/ library/ getopt. The best way to further explore the standard library is to read Doug Hellmann's excellent Python Module of the Week (http:/ / www. org/ dev/ 3. 2 anonymous edits Python en:More Introduction So far we have covered majority of the various aspects of Python that you will use. regular expressions (http:/ / www. >>> def get_error_details(): . we will cover some more aspects that will make our knowledge of Python more well-rounded.
  b = b. For example. a = 5. 3. org/ dev/ 3. Although. Special methods are used to mimic certain behaviors of built-in types. other) Explanation This method is called just before the newly created object is returned for usage. there is one caveat.Python en:More >>> >>> 1 >>> [2. say. see the manual (http:/ / docs. python.. then you can specify it on the same line of. 4] 103 This also means the fastest way to swap two variables in Python is: >>> >>> >>> (8. html#special-method-names). Similarly. 2. you can use this for making your program smaller. The following example should make this clear: >>> flag = True >>> if flag: print 'Yes' . If your block of statements contains only one single statement. then all you have to do is implement the __getitem__() method and your job is done. >.. Yes Notice that the single statement is used in-place and not as a separate block. Name __init__(self. etc... 0/ reference/ datamodel. a a. __getitem__(self. If you want to know about all the special methods. Called when the built-in len() function is used for the sequence object. Well. this is what Python does for the list class itself! Some useful special methods are listed in the following table. a. there are special methods for all the operators (+. Called when the less than operator (<) is used. a conditional statement or looping statement. b 5) Special Methods There are certain methods such as the __init__ and __del__ methods which have special significance in classes. mainly because it will be much easier to add an extra .) __del__(self) __str__(self) __lt__(self. if you want to use the x[key] indexing operation for your class (just like you use it for lists and tuples). except for error checking. I strongly recommend avoiding this short-cut method. . key) __len__(self) Single Statement Blocks We have seen that each block of statements is set apart from the rest by its own indentation level. b = 8 a. Called just before the object is destroyed Called when we use the print function or when str() is used. 4] a b 3. *b = [1. If you think about it.) Called when x[key] indexing operation is used.
  Suppose you have a list of numbers and you want to get a corresponding list with all the numbers multiplied by 2 only when the number itself is greater than 2. A lambda statement is used to create the function object.py def make_repeater(n): return lambda s: s * n twice = make_repeater(2) print(twice('word')) print(twice(5)) Output: $ python lambda.sort() by providing a compare function created using lambda? points = [ { 'x' : 2.sort(lambda a. b : cmp(a['x']. 3. { 'x' : 4. 104 Lambda Forms A lambda statement is used to create new function objects and then return them at runtime. Essentially. #!/usr/bin/python # Filename: lambda.py listone = [2. TODO Can we do a list. Note that even a print statement cannot be used inside a lambda form. #!/usr/bin/python # Filename: list_comprehension. 'y' : 3 }.Python en:More statement if you are using proper indentation. only expressions. 4] listtwo = [2*i for i in listone if i > 2] print(listtwo) Output: . List comprehensions are ideal for such situations. 'y' : 1 } ] # points. we use a function make_repeater to create new function objects at runtime and return it. b['x'])) List Comprehension List comprehensions are used to derive a new list from an existing list. the lambda takes a parameter followed by a single expression only which becomes the body of the function and the value of this expression is returned by the new function.py wordword 10 How It Works: Here.
  Python en:More $ python list_comprehension. total = 0 .. >>> powersum(2.. Note that the original list remains unmodified.. >>> eval('2*3') 6 . total += pow(i. >>> def powersum(power. we can generate a string containing Python code at runtime and then execute these statements using the exec statement: >>> exec('print("Hello World")') Hello World Similarly. power) ...... the eval function is used to evaluate valid Python expressions which are stored in a string. '''Return the sum of each argument raised to specified power. all extra arguments passed to the function are stored in args as a tuple.''' ..py [6. 4) 25 >>> powersum(2. 10) 100 Because we have a * prefix on the args variable. The advantage of using list comprehensions is that it reduces the amount of boilerplate code required when we use loops to process each element of a list and store it in a new list. For example. 3.. we derive a new list by specifying the manipulation to be done (2*i) when some condition is satisfied (if i > 2)... as opposed to written in the program itself.. 8] How It Works: Here. If a ** prefix had been used instead. A simple example is shown below. for i in args: . *args): . This is useful when taking variable number of arguments in the function. the extra parameters would be considered to be key/value pairs of a dictionary. return total .
  at this stage. When the assert statement fails. >>> mylist = ['item'] >>> assert len(mylist) >>> mylist. the repr function is used to obtain a printable representation of the object. Most of the time.Python en:More 106 The assert statement The assert statement is used to assert that something is true. Summary We have covered some more features of Python in this chapter and yet we haven't covered all the features of Python.append('item') >>> repr(i) "['item']" >>> eval(repr(i)) ['item'] >>> eval(repr(i)) == i True Basically. >>> i = [] >>> i. This is sufficient for you to get started with whatever programs you are going to create.pop() 'item' >>> mylist [] >>> assert len(mylist) Traceback (most recent File "<stdin>". The repr function The repr function is used to obtain a canonical string representation of the object. line AssertionError >= 1 >= 1 call last): 1. if you are very sure that you will have at least one element in a list you are using and want to check this. The interesting part is that you will have eval(repr(object)) == object most of the time. and raise an error if it is not true. then assert statement is ideal in this situation. in <module> The assert statement should be used judiciously. either handle the problem or display an error message to the user and then quit. . For example. we have covered most of what you are ever going to use in practice. Next. it is better to catch exceptions. However. You can control what your classes return for the repr function by defining the __repr__ method in your class. we will discuss how to explore Python further. an AssertionError is raised.
  The replace command can be as simple or as sophisticated as you wish. If you found that program easy. Details must be stored for later retrieval. you should. here's another one: Implement the replace command (http:/ / unixhelp. swaroopch. then you must have become comfortable and familiar with Python. Also. 1 anonymous edits Python en:What Next If you have read this book thoroughly till now and practiced writing a lot of programs. from simple string substitution to looking for patterns (regular expressions). ed. com/ contact/ ) thanking me for this great book . Now. contributing improvements or volunteering translations to support the continued development of this book. swaroopch. please consider making a donation. If you still want directions on how to proceed. Use the dictionary built-in methods to add. Once you are able to do this. ac. If you have not done it already. delete or search for your contacts such as friends. The question now is 'What Next?'. com/ mediawiki/ index.Python en:More 107 Previous Next Source: http:/ / www. This step is optional but recommended. php? oldid=1463 Contributors: Swaroop. Use a dictionary to store person objects with their name as the key. here are some ways to continue your journey with Python: . You have probably created some Python programs to try out stuff and to exercise your Python skills as well. uk/ CGI/ man-cgi?replace). After that. Hint (Don't read) Create a class to represent the person's information. Use the pickle module to store the objects persistently on your hard disk. family and colleagues and their information such as email address and/or phone number. I would suggest that you tackle this problem: Create your own command-line address-book program using which you can browse. add. This command will replace one string with another in the list of files provided. then here's a hint.-) . immediately send me a mail (http:/ / www. This is fairly easy if you think about it in terms of all the various stuff that we have come across till now. delete and modify the persons. modify. you can claim to be a Python programmer.
  html) is an excellent series of Python-related articles by David Mertz. html) • StackOverflow questions tagged with python (http:/ / beta. org/ dev/ howto/ doanddont. This is a must-read for every Python user. com/ results?search_query=googletechtalks+ python) • Awaretek's comprehensive list of Python tutorials (http:/ / www. idyll. The Dive Into Python book explores topics such as regular expressions. com/ tutorials. sourceforge. org/ doc/ faq/ general/ ) • Norvig's list of Infrequently Asked Questions (http:/ / norvig. rosettacode. com/ Code/ Python/ CatalogPython. html) • Official Python FAQ (http:/ / www. org) book which you can read fully online as well. com/ recipes/ langs/ python/ ) is an extremely valuable collection of recipes or tips on how to solve certain kinds of problems using Python. announce/ t/ 37de95ef0326293d) • Python Papers (http:/ / pythonpapers.Python en:What Next 108 Example Code The best way to learn a programming language is to write a lot of code and read a lot of code: • The PLEAC project (http:/ / pleac. com/ group/ comp. stackoverflow. unit testing. web services. siafoo. XML processing. org/ zone/ ) • Links at the end of every Python-URL! email (http:/ / groups. google. fyicenter. Videos The logical next step after this book is to read Mark Pilgrim's awesome Dive Into Python (http:/ / www. net/ article/ 52) • Advanced Software Carpentry using Python (http:/ / ivory. org) . net/ pleac_python/ index. python. com/ python-iaq. Questions and Answers • Official Python Dos and Don'ts (http:/ / docs. Tutorials. html) • Python Interview Q & A (http:/ / dev. awaretek. in detail. htm) • Python Cookbook (http:/ / code. Books. html) • The Effbot's Python Zone (http:/ / effbot. etc. com/ videos/ python) • GoogleTechTalks videos on Python (http:/ / youtube. lang. html) • Rosetta code repository (http:/ / www. python. java2s. diveintopython. Papers. com/ questions/ tagged/ python) Tips and Tricks • Python Tips & Tricks (http:/ / www. org/ articles/ advanced-swc/ ) • Charming Python (http:/ / gnosis. org/ wiki/ Category:Python) • Python examples at java2s (http:/ / www. com/ Interview-Questions/ Python/ index. activestate. Other useful resources are: • ShowMeDo videos for Python (http:/ / showmedo. cx/ publish/ tech_index_cp. python.
  google. wxPython This is the Python bindings for the wxWidgets toolkit. planetpython. you can create GUI apps fast. then the comp. com/ DevCenter/ EasyInstall#using-easy-install). Installing libraries There are a huge number of open source libraries at the Python Package Index (http:/ / pypi. html). read the PyGTK tutorial (http:/ / www. Starting with Qt 4. There are many IDEs available for wxPython which include GUI designers as well such as SPE (Stani's Python Editor) (http:/ / spe. com/ group/ comp.lang. lang. it is very portable and runs on Linux.python discussion group (http:/ / groups. pygtk. and don't know whom to ask. The Glade graphical interface designer is indispensable. qtrac. net/ ) and . Bindings are what allow you to write programs in Python and use the libraries which are themselves written in C or C++ or other languages.Python en:What Next 109 Discussion If you are stuck with a Python problem. To get started. News If you want to learn what is the latest in the world of Python. Graphical Software Suppose you want to create your own graphical programs using Python. Qt is extremely easy to use and very powerful especially due to the Qt Designer and the amazing Qt documentation. telecommunity. pycs. However. PyGTK This is the Python binding for the GTK+ toolkit which is the foundation upon which GNOME is built. This can be done using a GUI (Graphical User Interface) library with their Python bindings. PyQt is free if you want to create open source (GPL'ed) software and you need to buy it if you want to create proprietary closed source software. There are lots of choices for GUI using Python: PyQt This is the Python binding for the Qt toolkit which is the foundation upon which the KDE is built. GTK+ works well on Linux but its port to Windows is incomplete. Make sure you do your homework and have tried solving the problem yourself first. org) and/or the Unofficial Python Planet (http:/ / www. To install and use these libraries. html). To get started.5 you can use it to create non-GPL software as well. python/ topics) is the best place to ask your question. python. com/ tutorials/ pyqt4/ ) or the PyQt book (http:/ / www. GTK+ has many quirks in usage but once you become comfortable. Windows. org/ pypi) which you can use in your own programs. you can use Philip J. org). then follow the Official Python Planet (http:/ / planet. Mac and even embedded platforms. wxPython has a learning curve associated with it. org/ tutorial. Eby's excellent EasyInstall tool (http:/ / peak. eu/ pyqtbook. The documentation is yet to improve. You can create both free as well as proprietary software using GTK+. python. read the PyQt tutorial (http:/ / zetcode.
  The software is what actually runs our programs. The third factor. Importantly. TkInter is portable and works on both Linux/Unix as well as Windows. org/ cgi-bin/ moinmoin/ GuiProgramming). net/ pypy/ dist/ pypy/ doc/ home. pythonware. com) A Python implementation that is specialized for thread-based performance. jython. TkInter is part of the standard Python distribution. Issue 1 (http:/ / archive. org) A Python implementation that runs on the Java platform. Java or C# in the above three implementations) Stackless Python (http:/ / www. com/ library/ tkinter/ introduction/ ). html) A Python implementation written in Python! This is a research project to make it fast and easy to improve the interpreter since the interpreter itself is written in a dynamic language (as opposed to static languages such as C. IronPython (http:/ / www. TkInter This is one of the oldest GUI toolkits in existence. you have seen a TkInter program at work. There are also other software that can run your Python programs: Jython (http:/ / www. You can create free as well as proprietary software using wxPython. Volume 3. The first factor is whether you are willing to pay to use any of the GUI tools. pythonpapers. It doesn't have one of the best look & feel because it has an old-school look to it. 110 Summary of GUI Tools Unfortunately. A language is how you write something. A Python implementation that runs on the . stackless. if Linux is a chosen platform. sourceforge. read the Tkinter tutorial (http:/ / language and the software. I suggest that you choose one of the above tools depending on your situation.NET platform. For more choices. python. is whether you are a KDE or GNOME user on Linux. To get started. This means you can use . com/ Wiki/ View. see Page 26 of the The Python Papers. To get started. see the GuiProgramming wiki page at the official python website (http:/ / www. aspx?ProjectName=IronPython) codeplex. com/ wxpython/ ). It is referred to as CPython because it is written in the C language and is the Classical Python interpreter. there is no one standard GUI tool for Python.NET libraries and classes from within Python language and vice-versa. If you have used IDLE. This means you can use Java libraries and classes from within Python language and vice-versa. Various Implementations There are usually two parts a programming language . org/ ThePythonPapersVolume3Issue1. The second factor is whether you want the program to run only on Windows or on Mac and Linux or all of them. net/ ) GUI builder. PyPy (http:/ / codespeak. We have been using the CPython software to run our programs. pdf).Python en:What Next the wxGlade (http:/ / wxglade. For a more detailed and comprehensive analysis. read the wxPython tutorial (http:/ / zetcode. .
  org) ] • Ubuntu. [ OpenOffice (http:/ / www. Each of these implementations have their specialized areas where they are useful. com) ] • OpenOffice. If you have already read this book. It allows you to install a plethora of FLOSS available and all this in an easy-to-use and easy-to-install manner. spreadsheet and drawing components among other things. This is a community-driven distribution. which itself is based on the concept of sharing. net/ project/ clpython/ ) . Best of all. You can start automating your computer to do all kinds of previously unimaginable things or write your own games and much much more. com/ mediawiki/ index.Python en:What Next There are also others such as CLPython (http:/ / common-lisp. This is an excellent office suite with a writer. then you are already familiar with FLOSS since you have been using Python all along and Python is an open source software! Here are some examples of FLOSS to give an idea of the kind of things that community sharing and building can create: • Linux. 111 Summary We have now come to the end of this book but. . The extensions concept allows any kind of plugins to be used. kernel. as they say. this is the the beginning of the end!. org) ] • Mozilla Firefox. it is giving competition to Microsoft Windows. org/ Tamarin:IronMonkey) which is a port of IronPython to work on top of a JavaScript interpreter which could mean that you can use Python (instead of JavaScript) to write your web-browser ("Ajax") programs. You are now an avid Python user and you are no doubt ready to solve many problems using Python. [ Linux Kernel (http:/ / www. and particularly the sharing of knowledge. This is the next generation web browser which is giving great competition to Internet Explorer. mozilla. org/ wiki/ FLOSS) is based on the concept of a community. sponsored by Canonical and it is the most popular Linux distribution today. 2 anonymous edits Python en:Appendix FLOSS Free/Libre and Open Source Software (FLOSS) FLOSS (http:/ / en. php? oldid=1845 Contributors: Swaroop. ubuntu. wikipedia. FLOSS are free for usage. It is blazingly fast and has gained critical acclaim for its sensible and impressive features.org.a Python implementation written in Common Lisp and IronMonkey (http:/ / wiki. It runs on almost all platforms. modification and redistribution. So. you can just reboot your computer and run Linux off the CD! This allows you to completely try out the new OS before installing it on your computer. Now. openoffice. swaroopch. get started! Previous Next Source: http:/ / www. presentation. [ Ubuntu Linux (http:/ / www. It can even open and edit MS Word and MS PowerPoint files with ease. This is a FLOSS operating system that the whole world is slowly embracing! It was started by Linus Torvalds as a student.
  org/ vlc/ ) ] • GeexBox is a Linux distribution that is designed to play movies as soon as you boot up from the CD! [ GeexBox (http:/ / geexbox. com) NewsForge (http:/ / (http:/ / www. mozilla.Apache handles more websites than all the competition (including Microsoft IIS) combined. distrowatch.. go ahead and explore the vast. 1 anonymous edits . org). it is the most popular web server on the planet! It runs nearly more than half of the websites out there. KDevelop IDE.-) [ VLC media player (http:/ / www. Mac OS and many other platforms as well. linux. linuxtoday. [ Apache (http:/ / httpd. PHP language.there are many more excellent FLOSS out there.com (http:/ / www. check out the following websites: • • • • linux. that's right . php? oldid=1580 Contributors: Swaroop. microsoft. net) So. com) DistroWatch (http:/ / en:Appendix FLOSS • Its companion product Thunderbird is an excellent email client that makes reading email a snap. This is the popular open source web server. com/ mediawiki/ index. newsforge.. mozilla. . swaroopch. net) • FreshMeat (http:/ / www. mono-project. org/ en/ start. com) 112 Visit the following websites for more information on FLOSS: • SourceForge (http:/ / www. mysql. Yes. Mozilla Thunderbird (http:/ / www. VIM editor. who says open source ain't fun? . [ Mozilla Firefox (http:/ / www. com) ] • VLC Player. It is the M in the famous LAMP stack which runs most of the websites on the internet.the movie player. com).NET platform. PostgreSQL database server. ECMA (http:/ / www. Xine . com) LinuxToday (http:/ / www. [ Mono (http:/ / www. It allows . html) ] This list is just intended to give you a brief idea . [ MySQL (http:/ / www. It is most famous for it's blazing speed. videolan. org/ products/ firefox). Drupal content management system for websites. FreeBSD. This is an open source implementation of the Microsoft . Quanta+ editor. Microsoft . apache. GIMP image editing program. In fact. ecma-international. such as the Perl language. Windows. Banshee audio player. com/ net) ] • Apache web server.. To get the latest buzz in the FLOSS world. This is an extremely popular open source database server. This list could go on forever. This is a video player that can play anything from DivX to MP3 to Ogg to VCDs and DVDs to . TORCS racing game. freshmeat. sourceforge. org) ] • MySQL.NET applications to be created and run on Linux. free and open world of FLOSS! Previous Next Source: http:/ / www. org/ products/ thunderbird) ] • Mono..
  I switched to OpenOffice which was just excellent with the level of control it provided for formatting as well as the PDF generation. com/ notes/ ). The standard XSL stylesheets that came with Fedora Core 3 Linux were being used. org/ en-US/ firefox/ addon/ 394) that integrates with Vim. Initially. The standard default fonts are used as well. but it produced very sloppy HTML from the document. mozilla. Now I edit everything online and the readers can directly read/edit/discuss within the wiki website. I discovered XEmacs and I rewrote the book from scratch in DocBook XML (again) after I decided that this format was the long term solution. . I'm using MediaWiki (http:/ / www. The standard fonts are used as well. swaroopch. Now For this seventh draft. Teenage Years Later.Python en:Appendix About 113 Python en:Appendix About Colophon Almost all of the software that I have used in the creation of this book are free and open source software. So. I had used Red Hat 9. In the sixth draft. in Python of course. org) as the basis of my setup (http:/ / www. mediawiki. Finally. I had also written a crude lexical analyzer. I used Fedora Core 3 Linux as the basis of my setup. Birth of the Book In the first draft of this book. However. which automatically provides syntax highlighting to all the program listings. I switched to DocBook XML using Kate but I found it too tedious. I was using KWord to write the book (as explained in the History Lesson in the preface). I had written a CSS document to give color and style to the HTML pages.0 Linux as the foundation of my setup and in the sixth draft. I decided to use Quanta+ to do all the editing. I still use Vim for editing thanks to the ViewSourceWith extension for Firefox (https:/ / addons.
  Many new examples. com/ about/ Previous Next Source: http:/ / is more coherent and readable. • 0. swaroopch. I have made significant revisions to the content along with typo corrections.98 • 16/02/2004 • Wrote a Python script and CSS stylesheet to improve XHTML output. com/ mediawiki/ index. Added details about variable number of arguments in functions. . • 1.0 • Rewrite using MediaWiki (again) • 1. Book has improved a lot . • 0. in DocBook XML (again).99 • 22/02/2004 • Added a new chapter on modules. swaroopch.15 • 28/03/2004 • Minor revisions • 1.20 • 13/01/2005 • Complete rewrite using Quanta+ on FC3 with lot of corrections and updates.12 • 16/03/2004 • Additions and corrections. • 0. thanks to many enthusiastic and helpful readers. php? oldid=236 Contributors: Swaroop Python en:Appendix Revision History • 1. Rewrote my DocBook setup from scratch.00 • 08/03/2004 • After tremendous feedback and suggestions from readers. including a crude-yet-functional lexical analyzer for automatic VIM-like syntax highlighting of the program listings.Python en:Appendix About 114 About The Author http:/ / years! • Updating to Python 3.97 • 13/02/2004 • Another completely rewritten draft.10 • 09/03/2004 • More typo corrections. • 1.90 • 04/09/2008 and still in progress • Revival after a gap of 3. • 1.
  10 • 14/11/2003 • Initial draft using KWord.20 • 20/11/2003 • Corrected some typos and errors.90 • 18/12/2003 • Added 2 more chapters. com/ mediawiki/ index. php? oldid=240 Contributors: Swaroop 115 .60 • 21/11/2003 • Fully rewritten and expanded.92 • 05/01/2004 • Changes to few examples. • 0.15 • 20/11/2003 • Converted to DocBook XML. • 0.91 • 30/12/2003 • Corrected typos. • 0. swaroopch.Python en:Appendix Revision History • 0. → Previous → Back to Table of Contents Source: http:/ / www. • 0. • 0. OpenOffice format with revisions. Improvised many topics.93 • 25/01/2004 • Added IDLE talk and more Windows-specific stuff • 0. • 0.
  unicode only • http:/ / docs. net/ packages/ zip.py. org/ dev/ peps/ pep-3127/ • nonlocal statement • http:/ / www. org/ dev/ 3. python. org/ dev/ peps/ pep-3111/ • Integer Literal Support and Syntax • http:/ / www. sourceforge.format() instead of % operator • http:/ / www. python. com/ 2008/ 05/ 09/ emacs-as-a-powerful-python-ide/ • String . org/ dev/ library/ string. in data structures chapter • Problem Solving • Use http:/ / gnuwin32. python. htm on Windows • Classes • http:/ / docs.Python en:Appendix Changes for Python 3000 116 Python en:Appendix Changes for Python 3000 • Vim and Emacs editors • http:/ / henry. org/ dev/ peps/ pep-3116/ • Exception handling . python. python. precheur. html#about-unicode • Non-ASCII identifiers allowed • http:/ / www. enigmacurry. python. 0/ tutorial/ introduction. python. python. org/ dev/ peps/ pep-3106/ • Built-in set class. packages and their organization (including __init__.) • http:/ / ivory. org/ dev/ peps/ pep-3115/ • Abstract Base Classes • http:/ / www. html#formatstrings • Dict method changes • http:/ / www. html • http:/ / www. python. org/ dev/ peps/ pep-3107/ • Better explanation of modules. html • Metaclasses • http:/ / www. python. idyll. 0/ reference/ datamodel. python. org/ dev/ peps/ pep-3101/ • http:/ / docs. python. org/ dev/ peps/ pep-3104/ • Functions can take * argument (varargs) for lists and keyword-only arguments • http:/ / www. org/ dev/ 3. python. org/ dev/ peps/ pep-3119/ • Not sure if any changes required for New I/O • http:/ / www. org/ 2008/ 4/ 18/ Indenting%20Python%20with%20VIM. org/ dev/ peps/ pep-3131/ • print() function • http:/ / www. org/ articles/ advanced-swc/ #packages • String . etc. python. org/ dev/ peps/ pep-3105/ • raw_input() becomes input() • http:/ / www. python. org/ dev/ peps/ pep-3102/ • Functions can have annotations (make a passing note?) • http:/ / www.
  org/ dev/ library/ pdb. org/ dev/ peps/ pep-3132/ • with statement • http:/ / www. java2s. html • eval. fyicenter. python. python. html http:/ / docs. announce/ t/ 37de95ef0326293d • Examples • http:/ / www. mobilepythonbook. html http:/ / docs. google. ac. lang. org/ dev/ peps/ pep-3108/ http:/ / docs.interesting additions • • • • • • Reorganization : http:/ / www. org http:/ / www. htm • Tips & Tricks • http:/ / www. org/ dev/ peps/ pep-3109/ • http:/ / www. html • http:/ / docs. org • http:/ / dev. org/ dev/ howto/ doanddont. html Debugging 117 • http:/ / docs. org/ dev/ library/ json. html • More • Unpacking can take * argument • http:/ / www. org/ dev/ library/ warnings. com/ Interview-Questions/ Python/ index. python. net/ article/ 52 Source: http:/ / www. org/ doc/ faq/ general/ • http:/ / norvig. swaroopch. python. org http:/ / effbot. repr/ascii functions • getopt/optparse . python. org/ dev/ peps/ pep-0352/ • http:/ / www. com/ mediawiki/ index. html • http:/ / www. php? oldid=242 . org/ dev/ peps/ pep-3110/ • Standard Library .Python en:Appendix Changes for Python 3000 • http:/ / www. python. python. org/ dev/ 3. org/ zone/ Links at the end of every Python-URL! email • http:/ / groups. python. com/ Code/ Python/ CatalogPython.how to write a standard command-line program using python? • something like replace? • http:/ / hpux. 0/ library/ trace. coderholic. html • Books & Resources • • • • • http:/ / www. com/ python-iaq. python. python. uk/ hppd/ hpux/ Users/ replace-2. ed. com/ group/ comp. org/ dev/ peps/ pep-0343/ • What Next? • Implement 'replace' • http:/ / unixhelp. rosettacode. org/ dev/ library/ logging. html • http:/ / www. python. python. com/ free-python-programming-books/ http:/ / pythonpapers. python. connect. uk/ CGI/ man-cgi?replace • Mention use of PyPI • Q&A • http:/ / docs. org/ dev/ library/ urllib. siafoo. 24/ man. python. org. html (important) http:/ / docs. python.
  0 Unported http:/ / creativecommons. 0/ .Python en:Appendix Changes for Python 3000 Contributors: Swaroop 118 License Creative Commons Attribution-Share Alike 3. org/ licenses/ by-sa/ 3.