Is "if __name__ == '__main__'" required in a __main__.py?

Question

In a project that has a __main__.py, rather than

# __main__.py
# def main...

if __name__ == "__main__":
    main()

...is it OK to just do:

# __main__.py
# def main...

main()

Edit:

@user2357112-supports-Monica's argument made a lot of sense to me, so I went back and tracked down the library that had been giving me issues, causing me to still be adding the if __... line. It's upon calling python -m pytest --doctest-modules.

Maybe that's the only place that makes a mistake in running __main__.py? And maybe that's a bug?

Reproduced by putting the first example in the docs in a __main__.py:

――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――― package/__main__.py ―――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――
package/__main__.py:58: in <module>
    args = parser.parse_args()
/usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/argparse.py:1755: in parse_args
    args, argv = self.parse_known_args(args, namespace)
/usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/argparse.py:1787: in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
/usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/argparse.py:2022: in _parse_known_args
    ', '.join(required_actions))
/usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/argparse.py:2508: in error
    self.exit(2, _('%(prog)s: error: %(message)s\n') % args)
/usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/argparse.py:2495: in exit
    _sys.exit(status)
E   SystemExit: 2
--------------------------------------------------------------------------------------- Captured stderr ---------------------------------------------------------------------------------------
usage: pytest.py [-h] [--sum] N [N ...]
pytest.py: error: the following arguments are required: N

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Results (4.23s):

Answer 1

It's okay to skip the if __name__ == '__main__' guard in most regular scripts, not just __main__.py. The purpose of the guard is to make specific code not run if the file is imported as a module instead of run as the program's entry point, but importing a __main__.py as a module is usually using it wrong anyway.

Even with multiprocessing, you might think you need an if __name__ == '__main__' guard, but in the case of a __main__.py, it wouldn't actually help. It's commonly said that multiprocessing in spawn or forkserver mode imports the __main__ script as a module, but that's a simplification of the real behavior. In particular, one part of the real behavior is that if spawn mode detects the main script was a __main__.py, it just doesn't try to load the original __main__ at all:

# __main__.py files for packages, directories, zip archives, etc, run
# their "main only" code unconditionally, so we don't even try to
# populate anything in __main__, nor do we make any changes to
# __main__ attributes
current_main = sys.modules['__main__']
if mod_name == "__main__" or mod_name.endswith(".__main__"):
    return

forkserver mode also didn't load __main__.py when I tested it, but forkserver goes through a slightly different code path, and I'm not sure where it decided to skip __main__.py.

(This might be different on different Python versions - I only checked 3.8.2.)

---

That said, there's nothing wrong with using an if __name__ == '__main__' guard. Not using it has more weird edge cases than using it, and experienced readers will be more confused by its absence than its presence. Even in a __main__.py, I would probably still use the guard.

If you actually do want to import __main__.py for some reason, perhaps to unit test functions defined there, then you will need the guard. However, it might make more sense to move anything worth importing out of __main__.py and into another file.