GH-95736: fix IsolatedAsyncioTestCase to initialize Runner before calling setup functions (#95898)

Lib/test/test_unittest/test_async_case.py

@@ -434,6 +434,21 @@ async def cleanup(self, fut):
         test.doCleanups()
         self.assertEqual(events, ['asyncSetUp', 'test', 'cleanup'])
 
+    def test_setup_get_event_loop(self):
+        # See https://github.com/python/cpython/issues/95736
+        # Make sure the default event loop is not used
+        asyncio.set_event_loop(None)
+
+        class TestCase1(unittest.IsolatedAsyncioTestCase):
+            def setUp(self):
+                asyncio.get_event_loop_policy().get_event_loop()
+
+            async def test_demo1(self):
+                pass
+
+        test = TestCase1('test_demo1')
+        result = test.run()
+        self.assertTrue(result.wasSuccessful())
 
 if __name__ == "__main__":
     unittest.main()

Lib/unittest/async_case.py

@@ -116,6 +116,10 @@ def _setupAsyncioRunner(self):
         assert self._asyncioRunner is None, 'asyncio runner is already initialized'
         runner = asyncio.Runner(debug=True)
         self._asyncioRunner = runner
+        # Force loop to be initialized and set as the current loop
+        # so that setUp functions can use get_event_loop() and get the
+        # correct loop instance.
+        runner.get_loop()
 
     def _tearDownAsyncioRunner(self):
         runner = self._asyncioRunner

Misc/NEWS.d/next/Library/2022-08-11-18-22-29.gh-issue-95736.LzRZXe.rst

@@ -0,0 +1 @@
+Fix :class:`unittest.IsolatedAsyncioTestCase` to set event loop before calling setup functions. Patch by Kumar Aditya.