Deprecate closing of existing loops

docs/source/reference/changelog.rst

@@ -5,7 +5,10 @@ Changelog
 UNRELEASED
 =================
 - Drop compatibility with pytest 6.1. Pytest-asyncio now depends on pytest 7.0 or newer.
-- event_loop fixture teardown emits a ResourceWarning when the current event loop has not been closed.
+- pytest-asyncio cleans up any stale event loops when setting up and tearing down the
+  event_loop fixture. This behavior has been deprecated and pytest-asyncio emits a
+  DeprecationWarning when tearing down the event_loop fixture and current event loop
+  has not been closed.
 
 0.20.3 (22-12-08)
 =================

pytest_asyncio/plugin.py

@@ -397,7 +397,9 @@ def pytest_fixture_setup(
             if old_loop is not loop:
                 old_loop.close()
         except RuntimeError:
-            # Swallow this, since it's probably bad event loop hygiene.
+            # Either the current event loop has been set to None
+            # or the loop policy doesn't specify to create new loops
+            # or we're not in the main thread
             pass
         policy.set_event_loop(loop)
         return
@@ -420,11 +422,13 @@ def _add_finalizers(fixturedef: FixtureDef, *finalizers: Callable[[], object]) -
 
 _UNCLOSED_EVENT_LOOP_WARNING = dedent(
     """\
-    unclosed event loop %r.
-    Possible causes are:
-        1. A custom "event_loop" fixture is used which doesn't close the loop
-        2. Your code or one of your dependencies created a new event loop during
-           the test run
+    pytest-asyncio detected an unclosed event loop when tearing down the event_loop
+    fixture: %r
+    pytest-asyncio will close the event loop for you, but future versions of the
+    library will no longer do so. In order to ensure compatibility with future
+    versions, please make sure that:
+        1. Any custom "event_loop" fixture properly closes the loop after yielding it
+        2. Your code does not modify the event loop in async fixtures or tests
     """
 )
 
@@ -436,14 +440,10 @@ def _close_event_loop() -> None:
     except RuntimeError:
         loop = None
     if loop is not None:
-        # Emit ResourceWarnings in the context of the fixture/test case
-        # rather than waiting for the interpreter to trigger the warning when
-        # garbage collecting the event loop.
         if not loop.is_closed():
             warnings.warn(
                 _UNCLOSED_EVENT_LOOP_WARNING % loop,
-                ResourceWarning,
-                source=loop,
+                DeprecationWarning,
             )
         loop.close()
 

tests/test_event_loop_fixture_finalizer.py

@@ -88,15 +88,14 @@ async def test_async_with_explicit_fixture_request(event_loop):
     result.assert_outcomes(passed=1)
 
 
-def test_event_loop_fixture_finalizer_raises_warning_when_loop_is_unclosed(
+def test_event_loop_fixture_finalizer_raises_warning_when_fixture_leaves_loop_unclosed(
     pytester: Pytester,
 ):
     pytester.makepyfile(
         dedent(
             """\
             import asyncio
             import pytest
-            import pytest_asyncio
 
             pytest_plugins = 'pytest_asyncio'
 
@@ -114,3 +113,25 @@ async def test_ends_with_unclosed_loop():
     result = pytester.runpytest("--asyncio-mode=strict", "-W", "default")
     result.assert_outcomes(passed=1, warnings=1)
     result.stdout.fnmatch_lines("*unclosed event loop*")
+
+
+def test_event_loop_fixture_finalizer_raises_warning_when_test_leaves_loop_unclosed(
+    pytester: Pytester,
+):
+    pytester.makepyfile(
+        dedent(
+            """\
+            import asyncio
+            import pytest
+
+            pytest_plugins = 'pytest_asyncio'
+
+            @pytest.mark.asyncio
+            async def test_ends_with_unclosed_loop():
+                asyncio.set_event_loop(asyncio.new_event_loop())
+            """
+        )
+    )
+    result = pytester.runpytest("--asyncio-mode=strict", "-W", "default")
+    result.assert_outcomes(passed=1, warnings=1)
+    result.stdout.fnmatch_lines("*unclosed event loop*")