Shared via mypy Playground

main.py

from typing import Callable, Protocol

### Example 1: Works as intended. The method is decorated and is
### callable as usual.

def decorate_method[T](method: Callable[[T], None]) -> Callable[[T], None]:
    def _inner(_self: T) -> None:
        print('Decorated!')
        method(_self)
    return _inner

class A:
    @decorate_method
    def my_method(self) -> None:
        print('my_method')

# Explicit call syntax. Fine.
print(A.my_method(A()))
# Instance call syntax. Fine.
print(A().my_method())

### Example 2: Instead of using Callable[[T], None], an equivalent
### protocol is declared. This is equivalent at runtime but mypy
### rejects it.

class MyCallable[T](Protocol):
    def __call__(self, _self: T, /) -> None:
        ...

def decorate_method_1[T](method: MyCallable[T]) -> MyCallable[T]:
    def _inner(_self: T) -> None:
        print('Decorated!')
        method(_self)
    return _inner

class B:
    @decorate_method_1
    def my_method(self) -> None:
        print('my_method')

# Explicit call syntax. Fine.
print(B.my_method(B()))
# Instance call syntax. Rejected by mypy (Too few arguments to
# __call__) but works at runtime.
print(B().my_method())