def test_isChecker(self):
        """
        Verifies that strcred.makeChecker('sshkey') returns an object
        that implements the L{ICredentialsChecker} interface.
        """
        sshChecker = strcred.makeChecker('sshkey')
        self.assertTrue(checkers.ICredentialsChecker.providedBy(sshChecker))
        self.assertIn(
            credentials.ISSHPrivateKey, sshChecker.credentialInterfaces)

        Verifies that strcred.makeChecker('sshkey') returns an object
        that implements the L{ICredentialsChecker} interface.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/cred/test/test_strcred.py