How can I write a python decorator for a input function with type hinting?

I'm trying create the following python decorator for a function with type hint but it doesn't seem to work. It returned None instead of True. What did I do wrong?

    def do_nothing(func):
        def Inner_Function(*args, **kwargs):
            func(*args, **kwargs)
        return Inner_Function
    @do_nothing
    def is_cold(name: str) -> bool:
        return True
    
    g = is_cold("hi")
    print(g)
>> None

To fix the actual runtime behavior of your decorator, the inner function needs to return the result of calling the original function:

def do_nothing(func):
    def Inner_Function(*args, **kwargs):
        return func(*args, **kwargs)
    return Inner_Function

@do_nothing
def is_cold(name: str) -> bool:
    return True

g = is_cold("hi")
print(g)  # True

If you're using typechecking, you'll notice that this decorator breaks the typing of the decorated function. If we check the type of g in mypy we'll see that it's Any:

g = is_cold("hi")
reveal_type(g)  # Revealed type is "Any"

To fix this, you need to add typing to your decorator to indicate that it takes a function as its argument and returns the same type of function:

from typing import Callable, TypeVar

_Func = TypeVar("_Func", bound=Callable)

def do_nothing(func: _Func) -> _Func:
    def Inner_Function(*args, **kwargs):
        return func(*args, **kwargs)
    return Inner_Function  # type: ignore

(You can do more complex typing for Inner_Function to eliminate the need for the # type: ignore, but as far as the users of the @do_nothing decorator are concerned, what happens inside doesn't matter, only the signature of do_nothing itself.)

Now is_cold has its type preserved by the decorator:

g = is_cold("hi")
reveal_type(g)  # Revealed type is "builtins.bool"

InnerFunction calls func but doesn't return anything. is_cold will return True, but then InnerFunction tosses that away and, by default, returns None.