Close #19880: Fix a reference leak in unittest.TestCase. Explicitly break

reference cycles between frames and the _Outcome instance.

Lib/unittest/test/test_case.py

@@ -1533,6 +1533,32 @@ test case
             del case
             self.assertFalse(wr())
 
+    def test_no_exception_leak(self):
+        # Issue #19880: TestCase.run() should not keep a reference
+        # to the exception
+        class MyException(Exception):
+            ninstance = 0
+
+            def __init__(self):
+                MyException.ninstance += 1
+                Exception.__init__(self)
+
+            def __del__(self):
+                MyException.ninstance -= 1
+
+        class TestCase(unittest.TestCase):
+            def test1(self):
+                raise MyException()
+
+            @unittest.expectedFailure
+            def test2(self):
+                raise MyException()
+
+        for method_name in ('test1', 'test2'):
+            testcase = TestCase(method_name)
+            testcase.run()
+            self.assertEqual(MyException.ninstance, 0)
+
 
 if __name__ == "__main__":
     unittest.main()