Typing Function When Decorator Change Return Type

how to correctly write types for the function whose return type is modified by decorator ? Simple example: def example_decorator(fn): def wrapper(data): res = fn(data)

Solution 1:

May be a typechecker problem. Solution below. With mypy its ok but pycharm and pyre-check is complaining.

from typing import *

def example_decorator(
    fn: Callable[[List[str]], List[str]]
) -> Callable[[List[str]], str]:
    def wrapper(data: List[str]) -> str:
        res = fn(data)
        return ', '.join(res)

    return wrapper


@example_decorator
def func(data: List[str]) -> List[str]:  
    data.append('XYZ')
    return data


def test() -> str:
    result = func(['ABC', 'EFG'])
    print(type(result))  # <class 'str'>
    return result


test()

Solution 2:

The function returns a list, so like this:

@example_decorator
def func(data: list) -> list:
    new_data = data
    new_data.append('XYZ')
    return new_data