def test_registerChecker(self):
        """
        L{SSHProcotolChecker.registerChecker} should add the given checker to
        the list of registered checkers.
        """
        checker = checkers.SSHProtocolChecker()
        self.assertEqual(checker.credentialInterfaces, [])
        checker.registerChecker(checkers.SSHPublicKeyDatabase(), )
        self.assertEqual(checker.credentialInterfaces, [ISSHPrivateKey])
        self.assertIsInstance(checker.checkers[ISSHPrivateKey],
                              checkers.SSHPublicKeyDatabase)

        L{SSHProcotolChecker.registerChecker} should add the given checker to
        the list of registered checkers.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/test/test_checkers.py