Changes to fix pytest-dev#168

To take into account user case that sets a specific event loop policy before runnins pytest asyncio tests:
- Rollover changes in V0.14 (that clears event loop policy in pytest_fixture_post_finalizer)
- include a new unit test that checks if policy has been changed (fails with V0.14 and pass with this PR).

pytest_asyncio/plugin.py

@@ -72,12 +72,6 @@ def get_and_strip_from(self, name, data_dict):
             del data_dict[name]
         return result
 
-@pytest.hookimpl(trylast=True)
-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)
 
 
 
@@ -88,7 +82,14 @@ def pytest_fixture_setup(fixturedef, request):
         outcome = yield
         loop = outcome.get_result()
         policy = asyncio.get_event_loop_policy()
+        try:
+            old_loop = policy.get_event_loop()
+        except RuntimeError as exc:
+            if 'no current event loop' not in str(exc):
+                raise
+            old_loop = None
         policy.set_event_loop(loop)
+        fixturedef.addfinalizer(lambda: policy.set_event_loop(old_loop))
         return
 
     if isasyncgenfunction(fixturedef.func):

tests/test_change_of_policy.py

@@ -0,0 +1,22 @@
+import asyncio
+import pytest
+
+
+@pytest.fixture(scope="module")
+def old_policy():
+    policy = asyncio.get_event_loop_policy()
+    yield policy
+
+
+def test_1(old_policy):
+    assert old_policy == asyncio.get_event_loop_policy()
+
+@pytest.mark.asyncio
+async def test_2(old_policy):
+    assert old_policy == asyncio.get_event_loop_policy()
+
+@pytest.mark.asyncio
+async def test_3(old_policy):
+    # This test fails with pytest-asyncio V0.14
+    # due to a teardown change of event loop policy
+    assert old_policy == asyncio.get_event_loop_policy()