def test_assertEqualMessage(self):
        """
        When a message is passed to L{assertEqual} it is included in the error
        message.
        """
        message = 'message'
        exception = self.assertRaises(
            self.failureException, self.assertEqual,
            'foo', 'bar', message
        )
        self.assertIn(message, str(exception))

        When a message is passed to L{assertEqual} it is included in the error
        message.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_assertions.py