Execution of Unit Tests Dead Locked After Upgrade to Python 3.10.7

[silenum]

I experienced a problem described on StackOverflow.

Obviously, nest_asyncio does not properly stop the event loop in Python 3.10.7 as it used to do in Python 3.8.3 when executing unit tests. I am using FastAPI and within the event loop of FastAPI, I called another API with asyncio calls. Therefore I used nested_asyncio to achieve those calls. After the Python upgrade the nested event loop did not stop and so pytest did not stop either.

[erdewit]

Do you have an isolated test case that demonstrates the problem.

[silenum]

Hi @erdewit

Yes sure, see Demo Project.
It's the minimal setup I created for demonstrating the mentioned behaviour.

[erdewit]

Thank you for the very clear test setup.

It appears FastAPI uses a threadpool for servicing requests and that the actual deadlock is in one of the threads:

Exception ignored in: <module 'threading' from '/usr/lib64/python3.11/threading.py'>
Traceback (most recent call last):
  File "/usr/lib64/python3.11/threading.py", line 1574, in _shutdown
    lock.acquire()

My guess is that nest_asyncio changes the order of how cleanup is done in FastAPI. This is an issue within FastAPI itself.

The deadlock can be circumvented by not using Depends in main.py:

import nest_asyncio
from fastapi import FastAPI, Depends

from application.dependencies import get_service
from application.service import Service

app = FastAPI()
nest_asyncio.apply()

service: Service = get_service()

@app.get("/asyncio-demo")
async def get_cat_facts():  # service: Service = Depends(get_service)):
    return {"facts": (service.collect_cat_facts())}