Decorator taking a (covariant?) callable and returning it again

[Bluenix2]

Hello I have code such as the following:

from typing import Callable, TypeVar

C = TypeVar('C', bound='Callable[[str], None]')


def do_nothing(func: C) -> C:
    return func


class StrSubclass(str):
    pass


@do_nothing
def test(arg: StrSubclass) -> None:
    pass

But this causes the following error:

Argument of type "(arg: StrSubclass) -> None" cannot be assigned to parameter "func" of type "C@do_nothing" in function "do_nothing"
  Type "(str) -> None" cannot be assigned to type "(arg: StrSubclass) -> None"

How would I type this code?

I've tried both covariant and contravariant but neither of them allow being used as a return type and parameter type.

I could reproduce this with MyPy here so it doesn't seem like a bug with Pyright.

[erictraut]

Parameters within callables are implicitly contravariant, which is why you're seeing the error.

Think about it this way. The function test requires that the caller pass a StrSubclass instance as the first argument. But C is bound to a callback that accepts a str. If the decorator do_nothing were to call this callback with a str argument, it wouldn't satisfy the type requirements of test.

The workaround is to either change the bound type in C to accept a StrSubclass or change test to accept a str argument.

[Bluenix2]

Okay I think I understand why it is contravariant but I think I would need to make it covariant in some way..

During runtime I actually do some introspection and instantiate the specific type of the annotation. I am mostly looking to statically enforce that this is an event subclass.

That said I have since realized that I would like to allow the user to pass a callable with more arguments (but they would need to have a default), so I will just rely on my runtime checks.
Thanks.