from typing import TypeVar, List, Set

T = TypeVar("T", List[str], Set[str], str)

def foo(i: T) -> T:
    args: List[str] = []
    if isinstance(i, str):
        args = [i]
    else:
        args = list(i)

    if isinstance(i, str):
        return args[0]
    elif isinstance(i, set):
        return set(args)
    else:
        return args