Running concurrent event loops

[hyansuper]

To test the basic usage of websockets, I want to start a server and test the return string from client, I think server and client should be in separate thread.

The below code passed, but I just want to know if there's a proper/generalized way to do it?

import pytest
import asyncio
import threading
import websockets

async def server(stop_sig):
    async def echo(ws):
        async for msg in ws:
            await ws.send(msg)
    async with websockets.serve(echo, "localhost", 8000):
        await stop_sig

@pytest.fixture
def threaded_server():
    policy = asyncio.get_event_loop_policy()
    loop = policy.new_event_loop()
    sig = asyncio.Future(loop=loop)
    def run_loop(loop, coro):
        loop.run_until_complete(coro)
        loop.close()
    thread = threading.Thread(target=run_loop, args=(loop, server(sig)))
    thread.start()
    yield
    loop.call_soon_threadsafe(sig.set_result, None)
    thread.join()

async def test_client_echo(threaded_server):
    async with websockets.connect('ws://localhost:8000') as ws:
        await ws.send("hello")
        assert await ws.recv() == 'hello'

[seifertm]

I'm not aware of a better solution. asyncio is limited to at most one event loop per thread. This is by design. If you need concurrent event loops, you need to resort to multiple processes or multiple threads like you did.

You might be able to work around that using https://github.com/erdewit/nest_asyncio, but it should only be used for tests.

Does this answer you question?

[hyansuper]

thank you
I think it will be convenient if there’s a decorator to run async function in a separate loop in its own thread, like

@pytest.mark.run_in_threaded_loop
async def test_server():
….

But i don’t know if it’s a very needed feature

[seifertm]

It's worth investigating if this functionality can be added to the new asyncio_event_loop mark (see #620).
However, there should also be a discussion if this should be added.