rapid introduction to procedural programming tems ( linux and mac os xsome python applications have no extensionand python gui (graphical user interfaceprograms usually have an extension of pywparticularly on windows and mac os in this book we always use an extension of py for python console programs and python modulesand pyw for gui programs all the examples presented in this book run unchanged on all platforms that have python available just to make sure that everything is set up correctlyand to show the classical first examplecreate file called hello py in plain text editor (windows notepad is fine--we'll use better editor shortly)with the following contents#!/usr/bin/env python print("hello""world!"the first line is comment in pythoncomments begin with and continue to the end of the line (we will explain the rather cryptic comment in moment the second line is blank--outside quoted stringspython ignores blank linesbut they are often useful to humans to break up large blocks of code to make them easier to read the third line is python code herethe print(function is called with two argumentseach of type str (stringi sequence of characterseach statement encountered in py file is executed in turnstarting with the first one and progressing line by line this is different from some other languagesfor examplec+and javawhich have particular function or method with special name where they start from the flow of control can of course be diverted as we will see when we discuss python' control structures in the next section we will assume that windows users keep their python code in the :\py eg directory and that unix ( unixlinuxand mac os xusers keep their code in the $home/py eg directory save hello py into the py eg directory and close the text editor now that we have programwe can run it python programs are executed by the python interpreterand normally this is done inside console window on windows the console is called "console"or "dos prompt"or "ms-dos prompt"or something similarand is usually available from start-all programs-accessories on mac os the console is provided by the terminal app program (located in applications/utilities by default)available using finderand on other unixeswe can use an xterm or the console provided by the windowing environmentfor examplekonsole or gnome-terminal start up consoleand on windows enter the following commands (which assume that python is installed in the default location)--the console' output is shown in lightfacewhat you type is shown in bold