Python Decorator Optional Argument

decorator_optional_arg.py

#!/bin/python3

import functools
from typing import Callable


def wrapped(fn: Callable[..., str] = None, /, chars: str = '"') -> Callable[..., str] or str:
    """
    A decorator to wrap a method response in quotes

    :param fn: (pos only) function to decorate
    :param chars: the characters to surround before and after fn() response
    :return: fn() response wrapped in chars
    """

    def wrapped_decorator(func):

        @functools.wraps(func)
        def wrapper(*args, **kwargs):
            return f"{chars}{func(*args, **kwargs)}{chars}"

        return wrapper

    if fn:
        return wrapped_decorator(fn)

    return wrapped_decorator


@wrapped
def string() -> str:
    return 'ABC'


assert string() == '"ABC"'


@wrapped(chars='_')
def string() -> str:
    return 'ABC'


assert string() == '_ABC_'