[3.9] GH-89074: Fixed IsolatedAsyncioTestCase from throwing an exception on leaked tasks (GH-27765)

Lib/unittest/async_case.py

@@ -134,7 +134,7 @@ def _tearDownAsyncioLoop(self):
                 task.cancel()
 
             loop.run_until_complete(
-                asyncio.gather(*to_cancel, loop=loop, return_exceptions=True))
+                asyncio.gather(*to_cancel, return_exceptions=True))
 
             for task in to_cancel:
                 if task.cancelled():

Lib/unittest/test/test_async_case.py

@@ -278,6 +278,26 @@ async def test_cancel(self):
         output = test.run()
         self.assertFalse(output.wasSuccessful())
 
+    def test_cancellation_hanging_tasks(self):
+        cancelled = False
+        class Test(unittest.IsolatedAsyncioTestCase):
+            async def test_leaking_task(self):
+                async def coro():
+                    nonlocal cancelled
+                    try:
+                        await asyncio.sleep(1)
+                    except asyncio.CancelledError:
+                        cancelled = True
+                        raise
+
+                # Leave this running in the background
+                asyncio.create_task(coro())
+
+        test = Test("test_leaking_task")
+        output = test.run()
+        self.assertTrue(cancelled)
+
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Library/2021-08-14-00-55-16.bpo-44911.uk3hYk.rst

@@ -0,0 +1 @@
+:class:`~unittest.IsolatedAsyncioTestCase` will no longer throw an exception while cancelling leaked tasks. Patch by Bar Harel.