Does this not work if you put the assert on the line right below inspect.currentframe()? (it wouldn't surprise me if not, mypy is fragile with this kind of type narrowing)

Technically assert's are used for asserting things that must be true. In this case that is the case (because the reason for None is the below), so the change is correct - just unfortunate the same assert must be used twice and inside a for-loop.

CPython implementation detail: This function relies on Python stack frame support in the interpreter, which isn’t guaranteed to exist in all implementations of Python. If running in an implementation without Python stack frame support this function returns None.

Yeah, unfortunately it doesn't work. If I move the assertion to the line below current_frame = inspect.currentframe(), mypy raises an error.