loop on notifications until killed with a control-C,

The notify command arms two deferred, the usual deferred for the reply to the notify command: d. The response for this command will actually much later, when the client sends the stop_notif data to the server. Only at this point, the server will send the OK reply which acknowledges the completion of the notif command.

from twisted.internet import reactor, protocol, defer
from twisted.protocols import basic

class Client(basic.LineReceiver):
    
    # Internal
    def lineReceived(self, data):
        self.d.callback(data)
        
    def command(self, cmd):
        self.sendLine(cmd)
        self.d = defer.Deferred()
        return self.d

    # public API
    def random(self): 
        def gotRandom(number):
            return int(number)
        return self.command("random?").addCallback(gotRandom)

    def classified(self): 
        return self.command("classified?")

    waiting_notif = False

    def waitNotif(self):
        def cbWaitNotif(data):
            self.waitingNotif = False if data == 'OK' else True
            self.d.callback(data)
            
        self.notif_d = defer.Deferred()
        self.waitingNotif =True
        return self.notif_d

    def notify(self, notifCallback):
        def _cbNotify(data, notifCallback ):
            notifCallback(data)
        self.waiting
        return self.command("notif").addCallback(_cbNotify, notifCallback)

    def stopNotify(self, _):
        return self.command("stop_notif")

    # User code, this is actually the main()
    @defer.inlineCallbacks
    def gotConnection(self):
        print (yield self.random())
        print (yield self.classified())        
        print (yield self.notify(gotNotification))
        gotNotification((yield self.waitNotif()))
        gotNotification((yield self.waitNotif()))
        gotNotification((yield self.waitNotif()))

    def gotNotification(notif):
        print "a notif:", notif

factory = protocol.ClientFactory()
factory.protocol = Client
reactor.connectTCP("localhost", 6789, factory)
reactor.run()