How to annotate a generator in python3?

Question

Python 3.x supports (optional) function annotations:

def add_ints(x:int, y:int) -> int :
    return x+y

I sometimes encounter problems as to

Answer 1

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

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

Also works for yield

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

Answer 2

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

Generator[yield_type, send_type, return_type] 

See also PEP 0484.