def test_idempotentChallenge(self):
        """
        The same L{CramMD5Credentials} will always provide the same challenge,
        no matter how many times it is called.
        """
        c = CramMD5Credentials()
        chal = c.getChallenge()
        self.assertEqual(chal, c.getChallenge())

        The same L{CramMD5Credentials} will always provide the same challenge,
        no matter how many times it is called.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/cred/test/test_cramauth.py