How to Implement an In-Memory Pub/Sub System in Python

class PubSub:
    def __init__(self):
        self.topics = {}

    def subscribe(self, topic, callback):
        if topic not in self.topics:
            self.topics[topic] = []
        self.topics[topic].append(callback)
        return lambda: self.unsubscribe(topic, callback)

    def unsubscribe(self, topic, callback):
        if topic in self.topics and callback in self.topics[topic]:
            self.topics[topic].remove(callback)
            if not self.topics[topic]:
                del self.topics[topic]

    def publish(self, topic, message):
        if topic in self.topics:
            for callback in list(self.topics[topic]):
                callback(message)


if __name__ == "__main__":
    pubsub = PubSub()

    received = []
    unsubscribe_foo = pubsub.subscribe("news", lambda m: received.append(f"news: {m}"))
    pubsub.subscribe("sports", lambda m: received.append(f"sports: {m}"))

    pubsub.publish("news", "Hello world")
    pubsub.publish("sports", "Game update")
    publish_again = pubsub.publish("news", "Second article")

    unsubscribe_foo()

    pubsub.publish("news", "After unsubscribe")

    print(received)