[FIXED] How to run a coroutine and wait it result from a sync func when the loop is running?

Issue

I have a code like the foolowing:

def render():
    loop = asyncio.get_event_loop()

    async def test():
        await asyncio.sleep(2)
        print("hi")
        return 200

    if loop.is_running():
        result = asyncio.ensure_future(test())
    else:
        result = loop.run_until_complete(test())

When the loop is not running is quite easy, just use loop.run_until_complete and it return the coro result but if the loop is already running (my blocking code running in app which is already running the loop) I cannot use loop.run_until_complete since it will raise an exception; when I call asyncio.ensure_future the task gets scheduled and run, but I want to wait there for the result, does anybody knows how to do this? Docs are not very clear how to do this.

I tried passing a concurrent.futures.Future calling set_result inside the coro and then calling Future.result() on my blocking code, but it doesn't work, it blocks there and do not let anything else to run. ANy help would be appreciated.

Solution

To implement runner with the proposed design, you would need a way to single-step the event loop from a callback running inside it. Asyncio explicitly forbids recursive event loops, so this approach is a dead end.

Given that constraint, you have two options:

1. make render() itself a coroutine;
2. execute render() (and its callers) in a thread different than the thread that runs the asyncio event loop.

Assuming #1 is out of the question, you can implement the #2 variant of render() like this:

def render():
    loop = _event_loop  # can't call get_event_loop()

    async def test():
        await asyncio.sleep(2)
        print("hi")
        return 200

    future = asyncio.run_coroutine_threadsafe(test(), loop)
    result = future.result()

Note that you cannot use asyncio.get_event_loop() in render because the event loop is not (and should not be) set for that thread. Instead, the code that spawns the runner thread must call asyncio.get_event_loop() and send it to the thread, or just leave it in a global variable or a shared structure.

Answered By - user4815162342