getting "Future attached to a different" loop error #663

Hi Frank,

Sorry for the delay in reply.
I found the problem. it appears when I run uvicorn from inside the python script, I can this error. assuming that probably uvicorn creates the second loop, but the futures were defined in the main look.

The error disappeared when I run uvicorn from the terminal.

Does that make sense to you?

Thanks,
Tigran

Hmm... I made some tests and I found out it happens when you directly pass the app instance instead of the path module. This works:

import uvicorn

if __name__ == "__main__":
    uvicorn.run("app:app", host="0.0.0.0", port=5000, log_level="info")

This provokes the "attached to a different loop" error:

import uvicorn
from app import app

if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=5000, log_level="info")

My theory is the following:

• When we import app from the app module, it automatically loads the AsyncIOMotorClient instance, which is binded to the default asyncio loop.
• When Uvicorn starts, it automatically creates a new loop and set it as the new default.
• Problem: Motor was already instantiated with another loop; hence the error.

When we let Uvicorn import the app by itself (with the "app:app" argument), it also creates a new loop and set it by default; but it imports Motor after, so they are binded to the same loop.

Another solution is to prevent Uvicorn to setup the loop by itself using the loop argument, but I don't know what could be the potential gotchas around this:

import uvicorn
from app import app

if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=5000, log_level="info", loop="none")

My final advice:

• Prefer the command line invokation
• If you really need to invoke it programmatically, put it in another file and use the import style instead of importing the application yourself.

thanks for the answer.
indeed I was starting uvicorn from a separate file, but by just telling "app" in the param list.

and thanks for suggestions, thats helpful

is this a bug? Should it matter how uvicorn is started?