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 how to represent a given “type” can be represented, and this time, I have a function that returns a generator:

def myfunc(x: [int]) -> "generator that returns ints":
    #                     ^~~~~~~~~~~~~~~~~~~~~~~~~~
    return (n for n in x if n%2 == 0)

How should I annotate the return value? Is there any reference I can consult to?

Asked By: Yosh

||

Answer 1

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

Generator[yield_type, send_type, return_type]

See also PEP 0484.

Answered By: pfp.meijers

Answer 2

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

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