def test_assertEqualNoneMessage(self):
        """
        If a message is specified as L{None}, it is not included in the error
        message of L{assertEqual}.
        """
        exceptionForNone = self.assertRaises(
            self.failureException, self.assertEqual, 'foo', 'bar', None
        )
        exceptionWithout = self.assertRaises(
            self.failureException, self.assertEqual, 'foo', 'bar'
        )
        self.assertEqual(str(exceptionWithout), str(exceptionForNone))

        If a message is specified as L{None}, it is not included in the error
        message of L{assertEqual}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_assertions.py