The anext_awaitable object returned by anext(..., default) does not support .send()/.throw(). It only supports __next__(). So we can pass messages from the suspending coroutine to the event loop but not from the event loop to the suspending coroutine. trio and curio rely on both directions working. (I don't know about asyncio.) For example, this trio code fails: import trio async def produce(): for v in range(3): await trio.sleep(1) yield v async def consume(): p = produce() while True: print(await anext(p, 'finished')) trio.run(consume) raising AttributeError: 'anext_awaitable' object has no attribute 'send'. I realise that any awaitable that wants to await another awaitable must return not an iterator from __await__() but something that implements the full PEP-342 generator protocol. Should PEP-492 section on __await__()[1] say something about that? [1] https://www.python.org/dev/peps/pep-0492/#await-expression

it also fails with asyncio.create_task ``` import asyncio async def agen(): yield async def main(): p = agen() await asyncio.create_task(anext(p, 'finished')) asyncio.run(main()) ``` ``` Traceback (most recent call last): File "/home/graingert/projects/datapraxis/analysis/foo.py", line 10, in <module> asyncio.run(main()) File "/usr/lib/python3.10/asyncio/runners.py", line 44, in run return loop.run_until_complete(main) File "/usr/lib/python3.10/asyncio/base_events.py", line 641, in run_until_complete return future.result() File "/home/graingert/projects/asyncio-demo/foo.py", line 8, in main await asyncio.create_task(anext(p, 'finished')) File "/usr/lib/python3.10/asyncio/tasks.py", line 337, in create_task task = loop.create_task(coro) File "/usr/lib/python3.10/asyncio/base_events.py", line 433, in create_task task = tasks.Task(coro, loop=self, name=name) TypeError: a coroutine was expected, got <anext_awaitable object at 0x7f67f5587550> ``` there's also a problem with cancelling and aclosing an async gen this way: ``` import asyncio import contextlib async def agen(): await asyncio.sleep(1) yield async def main(): async with contextlib.aclosing(agen()) as p: asyncio.current_task().cancel() try: await anext(p, 'finished') except asyncio.CancelledError: pass asyncio.run(main()) ``` ``` an error occurred during closing of asynchronous generator <async_generator object agen at 0x7f2061a61dc0> asyncgen: <async_generator object agen at 0x7f2061a61dc0> Traceback (most recent call last): File "/usr/lib/python3.10/asyncio/runners.py", line 44, in run return loop.run_until_complete(main) File "/usr/lib/python3.10/asyncio/base_events.py", line 641, in run_until_complete return future.result() File "/home/graingert/projects/asyncio-demo/foo.py", line 9, in main async with contextlib.aclosing(agen()) as p: File "/usr/lib/python3.10/contextlib.py", line 366, in __aexit__ await self.thing.aclose() RuntimeError: aclose(): asynchronous generator is already running During handling of the above exception, another exception occurred: RuntimeError: aclose(): asynchronous generator is already running Traceback (most recent call last): File "/home/graingert/projects/datapraxis/analysis/foo.py", line 16, in <module> asyncio.run(main()) File "/usr/lib/python3.10/asyncio/runners.py", line 44, in run return loop.run_until_complete(main) File "/usr/lib/python3.10/asyncio/base_events.py", line 641, in run_until_complete return future.result() File "/home/graingert/projects/asyncio-demo/foo.py", line 9, in main async with contextlib.aclosing(agen()) as p: File "/usr/lib/python3.10/contextlib.py", line 366, in __aexit__ await self.thing.aclose() RuntimeError: aclose(): asynchronous generator is already running ```

I am marking this as a release blocker, given that this is probably going to trip a lot of users.

 New changeset 533e725821b15e2df2cd4479a34597c1d8faf616 by Pablo Galindo Salgado in branch 'main': bpo-44963: Implement send() and throw() methods for anext_awaitable objects (GH-27955) https://github.com/python/cpython/commit/533e725821b15e2df2cd4479a34597c1d8faf616 

 New changeset adc80a58f9683468e0ba5a6eed72040f7f6ba405 by Miss Islington (bot) in branch '3.10': bpo-44963: Implement send() and throw() methods for anext_awaitable objects (GH-27955) https://github.com/python/cpython/commit/adc80a58f9683468e0ba5a6eed72040f7f6ba405