expand the run_coroutine_threadsafe recipies

[graingert]

I see a lot of people having trouble using run_coroutine_threadsafe correctly:

• they struggle running a loop in a thread then accessing that loop from the parent thread
• stopping the loop from another thread
• stashing the event loop somewhere when spawning a new thread
• calling back into the event loop from another thread

I've expanded the examples to cover these cases.

---

📚 Documentation preview 📚: https://cpython-previews--127576.org.readthedocs.build/

[kumaraditya303]

FWIW I think the above simpler example is sufficient, this one looks very contrived to me involving thread pool executor and all.

[graingert]

It's used to get any failures out, if they happen when constructing the loop

[kumaraditya303]

I meant that can we avoid mixing concurrent futures executors with loop? Starting loop in a thread should be sufficient here no? I don't want to encourage such code which creates loop in another thread but runs code from another thread especially using concurrent futures.

[graingert]

You need to be able to start and stop the thread, and collect errors that get raised from asyncio.run, it's complicated to do that with threading.Thread

[kumaraditya303]

I am thinking of something like this:

from threading import Thread
import contextlib
import asyncio
import concurrent.futures


@contextlib.contextmanager
def get_loop():
    loop_fut = concurrent.futures.Future()
    stop = asyncio.Event()

    async def runner():
        loop_fut.set_result(asyncio.get_event_loop())
        await stop.wait()

    try:
        t = Thread(target=lambda: asyncio.run(runner()))
        t.start()
        loop = loop_fut.result()
        yield loop
    finally:
        loop.call_soon_threadsafe(stop.set)
        t.join()


with get_loop() as loop:
    r = asyncio.run_coroutine_threadsafe(
        asyncio.sleep(1, result="hello"), loop
    ).result()
    print(r)

[graingert]

But ThreadPoolExecutor(1) does all that for you, and collects any result from asyncio.run. Eg your code will hang if the OS has run out of file descriptors, mine will propagate the exception

[kumaraditya303]

Okay I take it, I am not really a fan of this pattern but I am fine if you wish.