Unless you're using exclusively Java-style checked exceptions, then there literally (literally!) is huge difference. That is:

    def main():
        try:
            my_a = a()
        except ExA, ExB:
            my_a = None
        ...
         
    def a() -> A:
        ...
        my_b = b()
        ...

    def b() -> B:
        ...

If b changes its signature to return type C, then it is a type error in a. main doesn't need to worry about what b returns; only a does.

BUT if b begins raising ExC instead of ExB, then that will break main at runtime. That is, main needs to be aware of what b could raise, even though it doesn't directly call it.