Python program start

Should I start a Python program with:

if__name__ == '__main__':
some code...

And if so, why? I saw it many times but don't have a clue about it.

From stackoverflow

• If your program is usable as a library but you also have a main program (e.g. to test the library), that construct lets others import the file as a library and not run your main program. If your program is named foo.py and you do "import foo" from another python file, __name__ evaluates to 'foo', but if you run "python foo.py" from the command line, __name__ evaluates to '__main__'.

  Note that you do need to insert a space between if and _, and indent the main program:

  if __name__ == '__main__':
      main program here
  

  S.Lott : +1: Reuse is important. A module can have 2 lives. As main program. As component in some other main program. As main program is DOES things. As component, it merely defines things.

  Jeremy Cantrell : This may go without saying, but be sure that this conditional goes at the end of your file.

• This is good practice. First, it clearly marks your module entry point (assuming you don't have any other executable code at toplevel - yuck). Second, it makes your module importable by other modules without executing, which some tools like code checkers, packagers etc. need to do.

• A better pattern is this:

  def main():
     ...
  
  if __name__ == '__main__':
     main()
  

  This allows your code to be invoked by someone who imported it, while also making programs such as pychecker and pylint work.

  J.J. : thank you! i was worried that the whole program had to be indented from the get go. I didin't like that.

• Guido Van Rossum suggests:

  def main(argv=None):
    if argv is None:
      argv = sys.argv
    ...
  
  if __name__ == "__main__":
      sys.exit(main())
  

  This way you can run main() from somewhere else (supplying the arguments), and if you want to exit with an error code just return 1 from main(), and it won't make an interactive interpreter exit by mistake.

  Ali A : Perhaps ensure the example has a return value as outlined in the text?

  Jeremy Cantrell : All functions have an implicit return value of None, which translates to an exit code of 0.