How to annotate a generator in python3?

While Generator[x, y, z] exists, most of the time, you might want to use the less verbose Iterator:

def fn(x: int) -> Iterator[int]:
    return (n for n in range(x) if n%2 == 0)

Also works for yield

def fn(x: int) -> Iterator[int]:
    for n in range(x):
        yield n