Impossible to specify a metaclass for a Generic class

[rowillia]

Below is a contrived example. In the real world I might not actually control the metaclass for Foo or I may be inheriting from some other class that also has a metaclass.

from typing import Generic, TypeVar

class FooMeta(type):
    pass

_T = TypeVar('_T')

class Foo(Generic[_T], metaclass=FooMeta):
    pass

This passes MyPy just fine, but fails at runtime with:

$ python3.6 test_generics.py
Traceback (most recent call last):
  File "test_generics.py", line 8, in <module>
    class Foo(Generic[_T], metaclass=FooMeta):
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

Moved from python/mypy#3720

[ilevkivskyi]

As a workaround in cases where possible, one can just do:

from typing import Generic, TypeVar, GenericMeta

class FooMeta(type):
    pass

class GenericFooMeta(GenericMeta, FooMeta):
    pass

T = TypeVar('T')

class Foo(Generic[T], metaclass=GenericFooMeta):
    pass

[bwo]

We recently encountered this in a case where a superclass we don't control used ABCMeta as its metaclass. The answer seems to be to use __extra__, but this doesn't appear to be documented anywhere. Should it be treated as an implementation detail not to be relied on?

[ilevkivskyi]

Should it be treated as an implementation detail not to be relied on?

Yes. Moreover GenericMeta is completely removed in Python 3.7. Generic will no more have a non-trivial metaclass, type(Generic) is now type.

[ilevkivskyi]

This is fixed in Python 3.7+ and we can't really do anything for older versions, so I think this can be closed.