got Future <Future pending> attached to a different loop

[krishsharma0413]

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• I already checked if it is not related to FastAPI but to ReDoc.

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Example Code

def main():
  uvicorn.run(app,host="0.0.0.0",port=8090,loop="asyncio")
loop = asyncio.get_event_loop()
def run():
  loop.run_until_complete(main())

def keep_alive1():
    t = threading.Thread(target=run)
    t.start()

keep_alive1()


client.run('discord bot token')

Description

i was using discord client with fastapi i used another thread for running uvicorn.
the bot worked fine but when i used the fastapi endpoints it gave me an error

RuntimeError: Task <Task pending name='Task-32' coro=<RequestResponseCycle.run_asgi() running at /opt/virtualenvs/python3/lib/python3.8/site-packages/uvicorn/protocols/http/h11_impl.py:369> cb=[set.discard()]> got Future <Future pending> attached to a different loop

the endpoint that gave me this error was asynchronous using mongodb module motor, other non asynchronous endpoints worked fine like the /docs.

when i removed the discord client with just the fastapi it still didnt worked with uvicorn.run() but did worked with the cmd command uvicorn main:app not sure why tho.

Operating System

Windows

Operating System Details

No response

FastAPI Version

0.66.0

Python Version

3.9.4

Additional Context

No response

[frankie567]

This is a quite common issue when using Motor (unfortunately, their implementation doesn't always play well with some event loops patterns). I wrote some details about this some time ago here: fastapi-users/fastapi-users#663 (reply in thread)

When using Motor, if you want to invoke Uvicorn programmatically you have to make sure of two things:

1. Put the invokation in a separate file (like main.py) alone, not besides the app instance;
2. Don't import app to pass it to uvicorn: prefer the import path as a string. This way, you'll ensure Uvicorn imports the module by itself with the right loop attached:

# main.py

import uvicorn

if __name__ == "__main__":
    # Assuming your `app` variable is in a module named `app.py`
    uvicorn.run("app:app", host="0.0.0.0", port=8000)

BTW, I see in your example that you do loop.run_until_complete(main()) on the function that calls Uvicorn. You don't need to do this (see above), Uvicorn takes care of this.

[Kludex]

Is this related to encode/uvicorn#1110 ?

[frankie567]

From what I understand of the issue, yes, this is what probably causes the problem.

However, I also think that Motor could greatly improve things on their side: contrary to most database drivers, they don't provide a connect/disconnect mechanism we could trigger on FastAPI startup; which would be then in the right loop context.

[Kludex]

Yeah, that PR solves an issue that should never happen. :)

[krishsharma0413]

thanks it worked, tho you should add a little note in the docs about the motor connector.

[rbrisita]

The issue could also be creating asyncio.Queue() outside of the current event loop. These queues should be created inside the main coroutine so they access the current running loop.

[kevinddchen]

Thank you. I got an error similar to this one, and not creating a queue until inside the event loop fixed the root issue.