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

The typing module defines the Generator type, which you can use like:

Generator[yield_type, send_type, return_type]

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