"Invalid base class" when base class is returned from a function.

[erikwright]

OTT-ErikWright:crucible erikwright$ cat -n ~/mypy_test.py 
     1  import typing
     2
     3  def c() -> type:
     4      class C:
     5          pass
     6      return C
     7
     8  class D(c()):
     9      pass
OTT-ErikWright:crucible erikwright$ mypy ~/mypy_test.py 
/Users/erikwright/mypy_test.py:8: error: Invalid base class

I also tried returning typing.Any, which seemed like it might work based on #2477 (comment) .

[ethanhs]

This is explicitly listed in the wiki as unsupported see https://github.com/python/mypy/wiki/Unsupported-Python-Features. It would be challenging to correctly type your example as of now.

[erikwright]

I don't really expect mypy to be able to do any meaningful type checking, but it would be convenient if it would at least understand that c() returns a type, and just didn't try to do any checking (as it would if c() were replaced with a variable of type typing.Any.

[gvanrossum]

Understood, mypy uses several passes, and currently the return type of c() is not available at the moment it needs the type for the base class of D. Base classes are required to be known in an earlier pass because knowing the bases of a class (actually, its full MRO) is needed to type-check other code.

Your best work-around in this case os to put # type: ignore on the line that causes the error.