def test_assertIsNotError(self):
        """
        L{assertIsNot} fails if two objects are identical.
        """
        a = MockEquality("first")
        self.assertRaises(self.failureException, self.assertIsNot, a, a)

        L{assertIsNot} fails if two objects are identical.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_assertions.py