[Python] How to write type hints when you change type of a variable?

Say that I have

def foo(x: Union[str, list[str]], y: Union[str, list[str]]) -> tuple[list[str], list[str]]:

x = cast(Union[list[str], tuple[list[str], ...]], str2list(x))
y = cast(Union[list[str], tuple[list[str], ...]], str2list(y))

return x,y

def str2list(*args: Union[str, list[str]]) -> Union[list[str], tuple[list[str], ...]]:
vals = []
for x in args:
if not isinstance(x, list):
x = [x]
vals.append(x)
if len(args) == 1:
return vals[0]
else:
return tuple(vals)


x = 'My name is x'
y = ['First element', 'Second element']

z = str2list(x,y)


As you see, there are two cast calls in foo, but still mypy complains that

error: Incompatible types in assignment (expression has type "Union[List[str], Tuple[List[str], ...]]", variable has type "Union[str, List[str]]") [assignment]


A workaround could be to define new variables and adjust the return type of foo as it follows

def foo(
x: Union[str, list[str]], y: Union[str, list[str]]
) -> tuple[
Union[list[str], tuple[list[str], ...]],
Union[list[str], tuple[list[str], ...]],
]:

xl = cast(Union[list[str], tuple[list[str], ...]], str2list(x))
yl = cast(Union[list[str], tuple[list[str], ...]], str2list(y))

return xl, yl


But I would prefer to overwrite x and y rather than defining two new variables xl and yl.

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