def test_str(self):
        """
        The string representation of a L{SMTPClientError} instance includes
        the response code and response string.
        """
        err = smtp.SMTPClientError(123, "some text")
        self.assertEqual(str(err), "123 some text")

        The string representation of a L{SMTPClientError} instance includes
        the response code and response string.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_smtp.py