Can i subscribe to a topic after i started loop_forever() in paho_mqtt?

Question

Do i need to subscribe to all topics i am interested in before i execute the loop_forever() function? E.g. to somehow dynamically add more subscriptions during the life time of the MQTT client.

Answer 1

I found out that it is possible. I implemented a class which hides the mqtt stuff. In the constructor i connect to the broker and start the loop_forever() with a seperate thread. And afterwards that class subscribes to some topics and register a callback for each topic, which are called from on_message.

c = comm("localhost")
c.register_handler("topic1", first_handler)
c.register_handler("topic2", second_handler)

class comm:
    def __init__(self, broker_address):
        self.client = mqtt.Client("")
        self.client.on_message = self.on_message
        self.callbacks = dict()
        self.client.connect(broker_address)
        threading.Thread(target=self.__run, daemon=True).start()
        
    def __run(self):
        self.client.loop_forever()

    def on_message(self, client, userdata, msg):
        self.callbacks[msg.topic](jsonpickle.decode(msg.payload))

    def on_connect(self, client, userdata, flags, rc):
        for topic in self.callbacks:
           self.client.subscribe(topic)    

    def register_handler(self, topic: str, handler: Callable[[Dict], None]):
        self.callbacks[topic] = handler
        self.client.subscribe(topic)