Dynamic base classes

[JukkaL]

Currently mypy isn't happy about dynamic base classes, even though these are not uncommon. Maybe mypy could do a better job.

Currently this works:

base: Any = create_class()

class A(base): ...

However, these don't work:

base2 = create_class()  # no annotation for base2 -> doesn't work
class B(base2): ...

class C(create_class()): ...  # function call as base class

base3: Type[A] = ...
class D(base3): ...

A plugin architecture (#1240) would help with commonly used libraries, but it wouldn't help with more ad-hoc uses of dynamic base classes.

[danmou]

Another use-case for dynamic base classes is in class decorators. For example:

from typing import ContextManager, Type

def decorator(cls: Type[ContextManager]) -> Type[ContextManager]:
    class Decorated(cls):
        pass
    return Decorated

@decorator
class TestClass(ContextManager):
    pass

This fails with:

test.py:4: error: Invalid type "cls"
test.py:4: error: Invalid base class

Even using a more generic annotation such as Type or Any doesn't work. The only workaround I've found to get rid of the error is:

def decorator(cls: Type[ContextManager]) -> Type[ContextManager]:
    cls_: Any = cls
    class Decorated(cls_):
        pass
    return Decorated

[msullivan]

We've got a plugin system that supports this now and I doubt we'll do better