issue python/mypy#19336

descriptors_and_protocols.py

from collections.abc import Callable
from typing import Concatenate, Generic, ParamSpec, Protocol, TypeVar

P = ParamSpec("P")
Owner = TypeVar("Owner")
Return = TypeVar("Return")


class Runner(Generic[Owner, P, Return]):
    def __init__(self, owner: Owner, func: Callable[Concatenate[Owner, P], Return]):
        self.func = func
        self.owner = owner

    def __call__(self, *args: P.args, **kwargs: P.kwargs) -> Return:
        return self.func(self.owner, *args, **kwargs)


class MyDescriptor(Generic[Owner, P, Return]):
    def __init__(self, func: Callable[Concatenate[Owner, P], Return]):
        self.func = func

    def __get__(self, owner: Owner, type: type[Owner]) -> Runner[Owner, P, Return]:
        return Runner(owner, self.func)


def with_descriptor(func: Callable[Concatenate[Owner, P], Return]) -> MyDescriptor[Owner, P, Return]:
    return MyDescriptor(func)


class MyProtocol(Protocol):
    def my_method(self, i: int) -> str: ...


class MyClass:
    @with_descriptor
    def my_method(self, i: int) -> str:
        return f"hello {i}"


concrete_instance = MyClass()

reveal_type(concrete_instance.my_method) # W: Type of "concrete_instance.my_method" is "Runner[MyClass, (i: int), str]"
as_callable: Callable[[int], str] = concrete_instance.my_method # successful type checking

my_instance: MyProtocol = concrete_instance # type checking fails
reveal_type(my_instance.my_method) # W: Type of "my_instance.my_method" is "(i: int) -> str"


assert my_instance.my_method(8) == "hello 8"