collections.abc.Callable incorrectly considered a _SpecialForm

[randolf-scholz]

Typeshed currently treats collections.abc.Callable as equivalent to typing.Callable, but they aren't

import typing
from collections import abc
assert isinstance(abc.Callable, type)  # ✅
assert isinstance(typing.Callable, type)  # ❌

This causes a very annoying bug: python/mypy#14014

collections.abc.Callable is a subclass of ABCMeta:, a real type that can be used in match-case and isinstance/issubclass.

class Callable(metaclass=ABCMeta):


    __slots__ = ()


    @abstractmethod
    def __call__(self, *args, **kwds):
        return False


    @classmethod
    def __subclasshook__(cls, C):
        if cls is Callable:
            return _check_methods(C, "__call__")
        return NotImplemented


    __class_getitem__ = classmethod(_CallableGenericAlias)

[AlexWaygood]

Possibly a duplicate of #6257?

[JelleZijlstra]

Callable is indeed a special form in the type system (https://typing.readthedocs.io/en/latest/spec/annotations.html#expression-grammar). As such, type checkers need to special case it. Mypy here may not be special casing it correctly, but that's a bug in mypy, not typeshed.

If you have a concrete proposal for a change to typeshed that would help mypy here, we can consider it, but so far I don't see any.