Teardown of the event_loop fixture no longer replaces the event loop policy

README.rst

@@ -164,6 +164,10 @@ Only test coroutines will be affected (by default, coroutines prefixed by
 
 Changelog
 ---------
+0.17.0 (UNRELEASED)
+~~~~~~~~~~~~~~~~~~~
+- `pytest-asyncio` no longer alters existing event loop policies. `#168 <https://github.com/pytest-dev/pytest-asyncio/issues/168>`_, `#188 <https://github.com/pytest-dev/pytest-asyncio/issues/168>`_
+
 0.16.0 (2021-10-16)
 ~~~~~~~~~~~~~~~~~~~
 - Add support for Python 3.10

pytest_asyncio/plugin.py

@@ -80,8 +80,11 @@ def get_and_strip_from(self, name, data_dict):
 def pytest_fixture_post_finalizer(fixturedef, request):
     """Called after fixture teardown"""
     if fixturedef.argname == "event_loop":
-        # Set empty loop policy, so that subsequent get_event_loop() provides a new loop
-        asyncio.set_event_loop_policy(None)
+        policy = asyncio.get_event_loop_policy()
+        policy.get_event_loop().close()  # Clean up existing loop to avoid ResourceWarnings
+        new_loop = policy.new_event_loop()  # Replace existing event loop
+        # Ensure subsequent calls to get_event_loop() succeed
+        policy.set_event_loop(new_loop)
 
 
 @pytest.hookimpl(hookwrapper=True)

tests/respect_event_loop_policy/conftest.py

@@ -0,0 +1,16 @@
+"""Defines and sets a custom event loop policy"""
+import asyncio
+from asyncio import DefaultEventLoopPolicy, SelectorEventLoop
+
+
+class TestEventLoop(SelectorEventLoop):
+    pass
+
+
+class TestEventLoopPolicy(DefaultEventLoopPolicy):
+    def new_event_loop(self):
+        return TestEventLoop()
+
+
+# This statement represents a code which sets a custom event loop policy
+asyncio.set_event_loop_policy(TestEventLoopPolicy())

tests/respect_event_loop_policy/test_respects_event_loop_policy.py

@@ -0,0 +1,16 @@
+"""Tests that any externally provided event loop policy remains unaltered."""
+import asyncio
+
+import pytest
+
+
+@pytest.mark.asyncio
+async def test_uses_loop_provided_by_custom_policy():
+    """Asserts that test cases use the event loop provided by the custom event loop policy"""
+    assert type(asyncio.get_event_loop()).__name__ == "TestEventLoop"
+
+
+@pytest.mark.asyncio
+async def test_custom_policy_is_not_overwritten():
+    """Asserts that any custom event loop policy stays the same across test cases"""
+    assert type(asyncio.get_event_loop()).__name__ == "TestEventLoop"