def test_authenticatorChallengeResponse(self):
        """
        L{PLAINAuthenticator.challengeResponse} returns challenge strings of
        the form::

            NUL<authn-id>NUL<secret>
        """
        username = b'testuser'
        secret = b'secret'
        chal = b'challenge'
        cAuth = imap4.PLAINAuthenticator(username)
        response = cAuth.challengeResponse(secret, chal)
        self.assertEqual(response, b'\0' + username + b'\0' + secret)

        L{PLAINAuthenticator.challengeResponse} returns challenge strings of
        the form::

            NUL<authn-id>NUL<secret>
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_imap.py