Run until complete in thread / nested event loops

[davidbrochart]

It would be nice to have the "opposite" of anyio.to_thread.run_sync(), which would run an async function until complete in a thread with its own event loop. Something like anyio.to_thread.run_async():

from anyio import to_thread

async def foo():
    print('Hello, world!')

def main():
    to_thread.run_async(foo)

main()

It would help to solve the problem where you want to run an async function in a blocking way but you don't know if you are being called from an already running event loop, so you cannot run until complete. An alternative is to use nest-asyncio, but it has issues too.

Maybe it could also be made transparent, such as the following code works:

from anyio import run

async def foo():
    print('Hello, world!')

async def main():
    run(foo)

run(main)

This currently gives a RuntimeError: Already running asyncio in this thread.

[graingert]

there's https://anyio.readthedocs.io/en/stable/api.html#anyio.from_thread.start_blocking_portal

[davidbrochart]

Just in case someone reads this issue, this is the way. Thanks!

from anyio import run, from_thread

async def foo():
    print('Hello, world!')

async def main():
    with from_thread.start_blocking_portal() as portal:
        portal.call(foo)

run(main)

[davidbrochart]

As its name implies, start_blocking_portal will block. What if I want to run a new event loop in a separate thread, as a task in the event loop thread? This seems to work but I'm not sure it's the correct way:

from functools import partial
from anyio import create_task_group, run, sleep, to_thread

async def thread():
    while True:
        await sleep(1)
        print("From thread")

async def main():
    async with create_task_group() as tg:
        tg.start_soon(to_thread.run_sync, partial(run, thread))
        while True:
            await sleep(1)
            print("From main")

run(main)

[gschaffner]

Just in case someone reads this issue, this is the way. Thanks!

...

fyi, this currently only works if using anyio.run(main, backend="asyncio"). it's broken on trio; see #523 and #525.