AsyncGenerator needs to include None in SendType?

[pfaion-soundhound]

Hey all,

I'm running into an issue with type annotations for an AsyncGenerator. The issue is that the generator needs to be "primed" with sending None once. Otherwise this causes a TypeError at runtime. But this value will never appear in the Generator function. I would like to keep the function annotations without that None, otherwise I have to do None-checks all the time...

Not sure if this is a "bug" or a "feature"? Is there an easy way around this? Feels kind of weird to be honest, like as if there should be .start() method that does the priming.

I.e. here's an example that should explain all of this (playground link).

from collections.abc import AsyncGenerator
import asyncio

async def pure_int_generator() -> AsyncGenerator[int, int]:
    for i in range(10):
        incoming = yield(i)
        print(i * incoming)

async def A():
    gen = pure_int_generator()
    await gen.asend(None)  # Argument of type "None" cannot be assigned to parameter "value" of type "int" in function "asend"

async def B():
    gen = pure_int_generator()
    await gen.asend(0)     # runtime TypeError: can't send non-None value to a just-started async generator


asyncio.run(A())
asyncio.run(B())


async def cumbersome_generator() -> AsyncGenerator[int, int | None]:
    for i in range(10):
        incoming = yield(i)
        if incoming is None:
            raise ValueError
        print(i * incoming)

async def C():
    gen = cumbersome_generator()
    await gen.asend(None)


asyncio.run(C())     # works, but cumbersome non-check!

[傅立业（Chris Fu） (Azureblade3808)]

Would this work for you?

async def A():
    gen = pure_int_generator()
    await anext(gen)

[pfaion-soundhound]

Yeah that works, thanks!