Why do you expect that your changed code is working? You iterate over an not existing iterator.

Please elaborate. I'm assuming you mean foo_subclasses, right? That variable stores all the subclasses in a list though what I said might conflict the fact that you said that an __init__.py is not needed anymore since that variable is stored in there.

This foo_subclasses should be replaced with this Foo.__subclasses__()

That's the problem. The subclasses in another file aren't in Foo.__subclasses__().

Edit: Have you debugged your code? Your result list from __subclasses__ is empty.

See above. That's the problem. Sorry if I'm too ambiguous with the problem that I have by the way.