Huge 👍 to adding this.

I think it would be helpful to have an example rather than just saying "assign y = x and use y".

You may need to annotate the type of the reassigned variable, depending on your case. Here's a minimal example of what I mean:

def outer(x: typing.Union[None, int, float]):
    if isinstance(x, float):
        x_: typing.Optional[int] = int(x)
    else:
        x_ = x

    def inner() -> int:
        if x_:
            return x_ + 1
        else:
            return 0

    return inner

Without x_ annotated, mypy will deduce its type -- correctly -- from the first assignment as int. But we're really trying to narrow the union down to Optional[int] in this case.

So maybe it would be best to advise people that you "may need to annotate the reassignment in some cases" or something?