Python program start

Question

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.

Answer 1

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

Answer 2

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.

Answer 3

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.

Answer 4

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.