def test_publickey_without_privatekey(self):
        """
        If the SSHUserAuthClient doesn't return anything from signData,
        the client should start the authentication over again by requesting
        'none' authentication.
        """
        authClient = ClientAuthWithoutPrivateKey(b'foo',
                                                 FakeTransport.Service())

        authClient.transport = FakeTransport(None)
        authClient.transport.sessionID = b'test'
        authClient.serviceStarted()
        authClient.tryAuth(b'publickey')
        authClient.transport.packets = []
        self.assertIsNone(authClient.ssh_USERAUTH_PK_OK(b''))
        self.assertEqual(authClient.transport.packets, [
                (userauth.MSG_USERAUTH_REQUEST, NS(b'foo') + NS(b'nancy') +
                 NS(b'none'))])

        If the SSHUserAuthClient doesn't return anything from signData,
        the client should start the authentication over again by requesting
        'none' authentication.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/test/test_userauth.py