Swap-case in Python without using the swapcase built-in method.

swapcase.py

# Challenge: implement swapcase without using the built-in method

def swapcase(string: str) -> str:
    return "".join(
        char.lower() if char.isupper() else char.upper()
        for char in string
    )

assert swapcase("AbCd") == "aBcD"