How to use mixed-scoped fixtures in an async test?

[nmacholl]

Using: pytest v8.2.0 pytest-asyncio v0.23.6

I have a fixture that stands up an asyncio subprocesses which my tests connect to over TCP. Re-instantiating this subprocesses for every test is time-consuming and unnecessary. I'd like for my subprocesses fixture to have a module scope. However, I am running into problems because some of my other fixtures are function scoped and I end up with different event loops running for the same test.

Here is a minimal example:

conftest.py

from asyncio.events import AbstractEventLoop
from collections.abc import AsyncGenerator
import pytest_asyncio
import asyncio


@pytest_asyncio.fixture(name="module_scoped", scope="module")
async def fixture_module_scoped() -> AsyncGenerator[AbstractEventLoop, None]:
    loop = asyncio.get_running_loop()
    print("module_scoped", id(loop))
    yield loop


@pytest_asyncio.fixture(name="function_scoped")
async def fixture_function_scoped() -> AsyncGenerator[AbstractEventLoop, None]:
    loop = asyncio.get_running_loop()
    print("function_scoped", id(loop))
    yield loop

test_fixtures.py

from asyncio import AbstractEventLoop
import asyncio

import pytest


@pytest.mark.asyncio(scope="module")
async def test_fixtures(
    function_scoped: AbstractEventLoop,
    module_scoped: AbstractEventLoop,
) -> None:
    test_loop = asyncio.get_running_loop()
    print("test", id(test_loop))
    assert test_loop is function_scoped
    assert test_loop is module_scoped

The last assertion fails:

...
>       assert test_loop is module_scoped
E       assert <_UnixSelectorEventLoop running=True closed=False debug=False> is <_UnixSelectorEventLoop running=False closed=False debug=False>

The IDs printed show that the test, and function scoped fixture, are using the same loop. However, the module scoped fixture has a different loop.

module_scoped 4332745600
function_scoped 4337806432
test 4337806432

I am wondering how to achieve a single module-scoped event loop for this test. Appreciate any help with this.

[nmacholl]

My current solution is to pin to pytest-asyncio v0.21.1 and override the event loop fixture to have module scope.

[seifertm]

Pytest-asyncio v0.23 wrongly assumes that the fixture caching scope (defined with scope=…) is the same as the event loop scope in which the async fixture should run. Your example requires both fixtures and the test to run in a module-scope event loop, but fixture_function_scoped should be reevaluated for every function. This is currently unavailable, but is hopefully fixed in the next version. See #871

Your solution of staying on pytest-asycnio v0.21 is the right choice in your case.

[nmacholl]

Thank you for the reply @seifertm. 🙏