[Python] Type hints lost when a decorator is wrapped as a classmethod?

Consider the following code:

from typing import Any, Callable, Coroutine

class Cache[**P, R]:
@classmethod
def decorate(cls, **params):
def decorator(f: Callable[P, Coroutine[Any, Any, R]]) -> Callable[P, Coroutine[Any, Any, R]]:
return f # in the real world, we instantiate a Cache here
return decorator

@Cache.decorate()
async def some_function(i: int) -> int:
return i + 1

cached_function = Cache.decorate()(some_function)


If I ask pyright the type of Cache.decorate before the @classmethod wrapper (inspecting the word decorate in the above code), it returns:

(method) def decorate(
cls: type[Self@Cache[P@Cache, R@Cache]],
**params: Unknown
) -> ((f: ((**P@Cache) -> (Coroutine[Any, Any, R@Cache])) -> ((**P@Cache) -> Coroutine[Any, Any, R@Cache]))


That looks to me like it understands that P (the argument types) and R (the return types) are plumbed through correctly.

However, if I ask it to introspect Cache.decorate in the context where it's being used as a decorator, it returns:

(method) def decorate(**params: Unknown) -> ((f: ((...) -> Coroutine[Any, Any, Unknown])) -> ((...) -> Coroutine[Any, Any, Unknown]))


...which is to say, the relationship between input types and output types has been entirely discarded!

Continue reading...