It's possible to work around without forcing wrong type on parameter def f(l: List[int]=cast(Any, None)) -> int: if l is None: l = [

It's possible to work around without forcing wrong type on parameter def f(l: List[int]=cast(Any, None)) -> int: if l is None: l = [] ... f() Using casts is useful for all sorts of things like this when sentinel values/objects are involved. People often feel it's somehow 'wrong', but it's fine to trick the type checker as long as it's localized to neighbouring lines of code :)