bpo-23890: Fix ref cycle in TestCase.assertRaises (#858)

unittest.TestCase.assertRaises() now manually breaks a
reference cycle to not keep objects alive longer than expected.
(cherry picked from commit bbd3cf8f1e)

Lib/unittest/test/test_case.py

@@ -1273,6 +1273,19 @@ test case
         with self.assertRaises(TypeError):
             self.assertRaises((ValueError, object))
 
+    def testAssertRaisesRefcount(self):
+        # bpo-23890: assertRaises() must not keep objects alive longer
+        # than expected
+        def func() :
+            try:
+                raise ValueError
+            except ValueError:
+                raise ValueError
+
+        refcount = sys.getrefcount(func)
+        self.assertRaises(ValueError, func)
+        self.assertEqual(refcount, sys.getrefcount(func))
+
     def testAssertRaisesRegex(self):
         class ExceptionMock(Exception):
             pass