why a main() function?

Ben Finney bignose+hates-spam at benfinney.id.au
Tue Sep 19 02:36:11 CEST 2006

Steve Holden <steve at holdenweb.com> writes:

> beliavsky at aol.com wrote:
> > I think I read a suggestion somewhere to wrap the code where a
> > Python script starts in a main() function
> > [...]
> > What are the advantages of doing this?
> > 
> Guido van Rossum himself can tell you:
>    http://www.artima.com/forums/flat.jsp?forum=106&thread=4829

I read that one a while ago, and now have this in most of my programs:

    def __main__(argv=None):
        """ Perform the main function of this program """
        from sys import argv as sys_argv
        if argv is None:
            argv = sys_argv

        # preparation, e.g. set up environment

        exit_code = None
            do_the_main_thing(argv)  # or whatever the main step is
        except SystemError, e:
            exit_code = e.code

        return exit_code

    if __name__ == '__main__':
        import sys
        exit_code = __main__(sys.argv)

This allows me to import my program as a module, and treat the main
routine as a function (argv as input, exit_code as output), while the
code itself can do sys.exit() without needing to know that it's
wrapped up in a function.

It also encourages me to write code inside do_the_main_thing() that
gets its environment parameterised as input, instead of specifying
sys.argv and the like. This makes the code much easier to unit test.

The name __main__ was chosen because I saw hints some time ago that
Python 3000 might automate some of these semantics for a function with
that name. True or false?

 \                "Crime is contagious ... if the government becomes a |
  `\    lawbreaker, it breeds contempt for the law."  -- Justice Louis |
_o__)                                                         Brandeis |
Ben Finney

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