def test_registerCheckerWithInterface(self):
        """
        If a specific interface is passed into
        L{SSHProtocolChecker.registerChecker}, that interface should be
        registered instead of what the checker specifies in
        credentialIntefaces.
        """
        checker = checkers.SSHProtocolChecker()
        self.assertEqual(checker.credentialInterfaces, [])
        checker.registerChecker(checkers.SSHPublicKeyDatabase(),
                                IUsernamePassword)
        self.assertEqual(checker.credentialInterfaces, [IUsernamePassword])
        self.assertIsInstance(checker.checkers[IUsernamePassword],
                              checkers.SSHPublicKeyDatabase)

        If a specific interface is passed into
        L{SSHProtocolChecker.registerChecker}, that interface should be
        registered instead of what the checker specifies in
        credentialIntefaces.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/test/test_checkers.py