How to use __subclasshook__ with Mypy?

[geryogam]

How come that under Mypy, __subclasshook__ works for one-trick ponies from collections.abc, but not for user-defined classes?

For instance, this program

from collections.abc import Hashable

class A:
    def __hash__(self) -> int:
        return 0

a: Hashable = A()

outputs

$ mypy demo.py --strict
Success: no issues found in 1 source file

But this equivalent program

from abc import ABCMeta, abstractmethod

def _check_methods(C: type, *methods: str) -> bool:
    mro = C.__mro__
    for method in methods:
        for B in mro:
            if method in B.__dict__:
                if B.__dict__[method] is None:
                    return NotImplemented
                break
        else:
            return NotImplemented
    return True

class Hashable(metaclass=ABCMeta):
    __slots__ = ()

    @abstractmethod
    def __hash__(self) -> int:
        return 0

    @classmethod
    def __subclasshook__(cls, C: type) -> bool:
        if cls is Hashable:
            return _check_methods(C, "__hash__")
        return NotImplemented

class A:
    def __hash__(self) -> int:
        return 0

a: Hashable = A()

outputs

$ mypy demo.py --strict
demo.py:32: error: Incompatible types in assignment (expression has type "A", variable has type "Hashable")
Found 1 error in 1 file (checked 1 source file)

Does Mypy handle one-trick ponies in a special way?

[erictraut]

collections.abc.Hashable is a Protocol class, conformant with PEP 544. If you make your custom Hashable class a Protocol class, it will work with static type checkers. The __subclasshook__ and register methods are dynamic mechanisms that won't work with static type checking.

[geryogam]

Thanks for the reply @erictraut.

collections.abc.Hashable is a Protocol class

Yet that is not what I see here (https://github.com/python/cpython/blob/v3.10.4/Lib/_collections_abc.py#L90):

class Hashable(metaclass=ABCMeta):

[JelleZijlstra]

Type checkers don't look at the runtime implementation. What matters to mypy is the definition in typeshed: https://github.com/python/typeshed/blob/master/stdlib/_collections_abc.pyi#L16. That points to https://github.com/python/typeshed/blob/40553277ad7a7beccaa8698d72f3ecae9b1a81b8/stdlib/typing.pyi#L666 which is indeed a Protocol.

[geryogam]

Thanks @JelleZijlstra!

[AlexWaygood]

It's important to recognise that mypy has no knowledge of the runtime implementations of anything in the standard library (this is because most of the standard library doesn't have type annotations). The same goes for most other static type checkers.

For the standard library, all mypy knows about is the stubs that are provided for these classes in typeshed. You'll find the definition for Hashable in typeshed here, and you'll find that it's completely different to the implementation at runtime. This is why simply copying the runtime implementation produces code that mypy cannot understand — if you copy the implementation in typeshed instead, you should find that mypy is perfectly able to understand your code.

[geryogam]

Thanks!

[AlexWaygood]

No problem!