pubsub1.py

class MessageBroker:

    def __init__(self):
        self.__subscribers = {}

    def publish(self, topic, message):
        for callback in self.__subscribers.get(topic, []):
            callback(message)
            
    def subscribe(self, topic, callback):
        subscribers = self.__subscribers.setdefault(topic, [])
        subscribers.append(callback)

    def unsubscribe(self, topic, callback):
        subscribers = self.__subscribers[topic]
        subscribers.remove(callback)