`RuntimeError` when the event loop is unset between tests

[tjkuson]

When using a shared event loop (e.g., when loop_scope="session") and asyncio.set_event_loop(None) is called from a test, any subsequent async test raises RuntimeError: There is no current event loop in thread 'MainThread'.

This will happen in any test that calls asyncio.Runner or asyncio.run, which call asyncio.set_event_loop(None) as part of its clean-up code.

To reproduce:

import asyncio
import pytest


@pytest.mark.asyncio
async def test_before() -> None:
    pass


def test_set_event_loop_none() -> None:
    asyncio.set_event_loop(None)


@pytest.mark.asyncio
async def test_after() -> None:
    pass

$ pytest -o 'asyncio_default_test_loop_scope=module' test_mre.py

The expected behaviour is for all tests to pass, but an exception is raised instead. The exception is not raised if either test_before or test_after are skipped.

I found issue #658 which is related; however, that issue is closed and related to fixtures (whereas this is not), so decided to create a new one instead.

I have a patch that resolves the issue by reinstating the event loop if needed. Let me know if a PR would be welcome!

Update: I bisected the issue to the v0.23 release, so this isn't a recent regression.