Think Python - Denison University

142 Chapter 14. Files>>> wc.linecount('wc.py')7So that’show you writemodules inPython.The only problem with this example is that when you import the module it executes the test codeat the bottom. Normally when you import a module, it defines new functions but it doesn’t executethem.Programs that willbe imported asmodules oftenusethe following idiom:if __name__ == '__main__':print linecount('wc.py')__name__ is a built-in variable that is set when the program starts. If the program is running as ascript, __name__ has the value __main__; in that case, the test code is executed. Otherwise, if themodule isbeing imported, thetestcode isskipped.Exercise14.4 Typethisexampleintoafilenamedwc.pyandrunitasascript. ThenrunthePythoninterpreter andimport wc. What isthevalue of__name__when the module isbeing imported?Warning: If you import a module that has already been imported, Python does nothing. It does notre-read thefile, even if ithas changed.Ifyouwanttoreloadamodule,youcanusethebuilt-infunctionreload,butitcanbetricky,sothesafestthing todo isrestarttheinterpreter and then import the module again.14.10 DebuggingWhenyouarereadingandwritingfiles,youmightrunintoproblemswithwhitespace. Theseerrorscan be hard todebug because spaces, tabs and newlines arenormally invisible:>>> s = '1 2\t 3\n 4'>>> print s1 2 34The built-in function repr can help. It takes any object as an argument and returns a string representationof theobject. For strings,itrepresents whitespace characters withbackslash sequences:>>> print repr(s)'1 2\t 3\n 4'This can be helpful fordebugging.Oneotherproblemyoumightrunintoisthatdifferentsystemsusedifferentcharacterstoindicatetheendofaline. Somesystemsuseanewline,represented\n. Othersuseareturncharacter,represented\r. Some use both. If you move files between different systems, these inconsistencies might causeproblems.For most systems, there are applications to convert from one format to another. You can find them(andreadmoreaboutthisissue)atwikipedia.org/wiki/Newline. Or,ofcourse,youcouldwriteone yourself.