def test_failIfUnknownService(self):
        """
        If the user requests a service that we don't support, the
        authentication should fail.
        """
        packet = NS(b'foo') + NS(b'') + NS(b'password') + chr(0) + NS(b'foo')
        self.authServer.clock = task.Clock()
        d = self.authServer.ssh_USERAUTH_REQUEST(packet)
        return d.addCallback(self._checkFailed)

        If the user requests a service that we don't support, the
        authentication should fail.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/test/test_userauth.py