Setup default loop in test utilities (#2804)

* Make set_event_loop(loop) for pytest runners

* Code cleanup

* Update docs

* Add versionchanged

* Add changelog

Author: asvetlov

CHANGES/2804.feature

@@ -0,0 +1,3 @@
+Install a test event loop as default by
+``asyncio.set_event_loop()``. The change affects aiohttp test utils
+but backward compatibility is not broken for 99.99% of use cases.

aiohttp/pytest_plugin.py

@@ -203,8 +203,8 @@ def loop(loop_factory, fast, loop_debug):
     with loop_context(loop_factory, fast=fast) as _loop:
         if loop_debug:
             _loop.set_debug(True)  # pragma: no cover
+        asyncio.set_event_loop(_loop)
         yield _loop
-    asyncio.set_event_loop(None)
 
 
 @pytest.fixture

aiohttp/test_utils.py

@@ -410,7 +410,7 @@ def setup_test_loop(loop_factory=asyncio.new_event_loop):
     once they are done with the loop.
     """
     loop = loop_factory()
-    asyncio.set_event_loop(None)
+    asyncio.set_event_loop(loop)
     if sys.platform != "win32":
         policy = asyncio.get_event_loop_policy()
         watcher = asyncio.SafeChildWatcher()

docs/testing.rst

@@ -788,6 +788,20 @@ Utilities
    The caller should also call teardown_test_loop, once they are done
    with the loop.
 
+   .. note::
+
+      As side effect the function changes asyncio *default loop* by
+      :func:`asyncio.set_event_loop` call.
+
+      Previous default loop is not restored.
+
+      It should not be a problem for test suite: every test expects a
+      new test loop instance anyway.
+
+   .. versionchanged:: 3.1
+
+      The function installs a created event loop as *default*.
+
 .. function:: teardown_test_loop(loop)
 
    Teardown and cleanup an event_loop created by setup_test_loop.

tests/test_loop.py

@@ -29,4 +29,11 @@ async def on_startup_hook(self, app):
 
     @unittest_run_loop
     async def test_on_startup_hook(self):
-        assert self.startup_loop is not None
+        self.assertIsNotNone(self.startup_loop)
+
+    def test_default_loop(self):
+        self.assertIs(self.loop, asyncio.get_event_loop())
+
+
+def test_default_loop(loop):
+    assert asyncio.get_event_loop() is loop