python typing decorators

typing_decorator.py

from typing import Any, Callable, TypeVar, cast


F = TypeVar('F', bound=Callable[..., Any])

# A decorator that preserves the signature.
def my_decorator(func: F) -> F:
    def wrapper(*args, **kwargs):
        print("Calling", func)
        return func(*args, **kwargs)
    return cast(F, wrapper)

# A decorator that not preservers the signature.
def dumb_decorator(func):
    def wrapper(*args, **kwargs):
        print("Calling", func)
        return func(*args, **kwargs)
    return wrapper

# A decorated functions
@my_decorator
def foo(a: int) -> str:
    return str(a)

@dumb_decorator
def dumb_foo(a: int) -> str:
    return str(a)

if __name__ == "__main__":
    a = foo(12)
    reveal_type(a)  # str
    foo('x')  # Type check error

    a = dumb_foo(12)
    reveal_type(a)  # unknown
    dumb_foo('x')  # No type error