[refactor] Run multiloop test inside Pytester to avoid the custom eve…

…nt loop implementation to pollute the test environment.

Signed-off-by: Michael Seifert <[email protected]>

tests/test_multiloop.py

@@ -0,0 +1,70 @@
+from textwrap import dedent
+
+from pytest import Pytester
+
+
+def test_event_loop_override(pytester: Pytester):
+    pytester.makeconftest(
+        dedent(
+            '''\
+            import asyncio
+
+            import pytest
+
+
+            @pytest.fixture
+            def dependent_fixture(event_loop):
+                """A fixture dependent on the event_loop fixture, doing some cleanup."""
+                counter = 0
+
+                async def just_a_sleep():
+                    """Just sleep a little while."""
+                    nonlocal event_loop
+                    await asyncio.sleep(0.1)
+                    nonlocal counter
+                    counter += 1
+
+                event_loop.run_until_complete(just_a_sleep())
+                yield
+                event_loop.run_until_complete(just_a_sleep())
+
+                assert counter == 2
+
+
+            class CustomSelectorLoop(asyncio.SelectorEventLoop):
+                """A subclass with no overrides, just to test for presence."""
+
+
+            @pytest.fixture
+            def event_loop():
+                """Create an instance of the default event loop for each test case."""
+                loop = CustomSelectorLoop()
+                yield loop
+                loop.close()
+            '''
+        )
+    )
+    pytester.makepyfile(
+        dedent(
+            '''\
+            """Unit tests for overriding the event loop."""
+            import asyncio
+
+            import pytest
+
+
+            @pytest.mark.asyncio
+            async def test_for_custom_loop():
+                """This test should be executed using the custom loop."""
+                await asyncio.sleep(0.01)
+                assert type(asyncio.get_event_loop()).__name__ == "CustomSelectorLoop"
+
+
+            @pytest.mark.asyncio
+            async def test_dependent_fixture(dependent_fixture):
+                await asyncio.sleep(0.1)
+            '''
+        )
+    )
+    result = pytester.runpytest_subprocess("--asyncio-mode=strict")
+    result.assert_outcomes(passed=2)