Don't close event loop if the loop doesn't exist

README.rst

@@ -259,6 +259,7 @@ Changelog
 0.17.1 (UNRELEASED)
 ~~~~~~~~~~~~~~~~~~~
 - Fixes a bug that prevents async Hypothesis tests from working without explicit ``asyncio`` marker when ``--asyncio-mode=auto`` is set. `#258 <https://github.com/pytest-dev/pytest-asyncio/issues/258>`_
+- Fixed a bug that closes the default event loop if the loop doesn't exist `#257 <https://github.com/pytest-dev/pytest-asyncio/issues/257>`_
 
 0.17.0 (22-01-13)
 ~~~~~~~~~~~~~~~~~~~

pytest_asyncio/plugin.py

@@ -169,8 +169,13 @@ def pytest_fixture_post_finalizer(fixturedef, request):
     """Called after fixture teardown"""
     if fixturedef.argname == "event_loop":
         policy = asyncio.get_event_loop_policy()
-        # Clean up existing loop to avoid ResourceWarnings
-        policy.get_event_loop().close()
+        try:
+            loop = policy.get_event_loop()
+        except RuntimeError:
+            loop = None
+        if loop is not None:
+            # Clean up existing loop to avoid ResourceWarnings
+            loop.close()
         new_loop = policy.new_event_loop()  # Replace existing event loop
         # Ensure subsequent calls to get_event_loop() succeed
         policy.set_event_loop(new_loop)

tests/test_event_loop_scope.py

@@ -29,3 +29,9 @@ def test_3():
     current_loop = asyncio.get_event_loop_policy().get_event_loop()
     # Now the event loop from test_2 should have been cleaned up
     assert loop is not current_loop
+
+
+def test_4(event_loop):
+    # If a test sets the loop to None -- pytest_fixture_post_finalizer()
+    # still should work
+    asyncio.get_event_loop_policy().set_event_loop(None)