[FIXED] Closing a nested async generator prematurely

Issue

I'm looking how to close a nested async generator without fully iterating it. For example:

import asyncio


async def gen1():
    yield 1
    yield 2


async def gen2():
    async for nr in gen1():
        yield nr


async def main():
    stream = gen2()
    print(await stream.__anext__())  # 1
    await stream.aclose()


asyncio.run(main())

However, this results in a RuntimeError:

unhandled exception during asyncio.run() shutdown
task: <Task finished name='Task-2' coro=<<async_generator_athrow without __name__>()> exception=RuntimeError("can't send non-None value to a just-started coroutine")>
RuntimeError: can't send non-None value to a just-started coroutine

Solution

The original issue was present when testing with Python versions 3.7.4 and 3.8.0b4. It has since been fixed in 3.8.0rc1:

https://bugs.python.org/issue38013

Answered By - Jaanus Varus