How to implement functionality that allows users to subscribe to updates from other users?

To implement the functionality of users subscribing to each other, you can use a data structure that stores the relationships between users — who follows whom. In Python, this is often implemented through subscription tables in databases or through dictionaries/sets in memory.

Example in Python using a dictionary, where the key is the user and the value is a set of users they follow:

class SubscriptionService:
    def __init__(self):
        self.subscriptions = {}  # {user_id: set of user_ids}

    def subscribe(self, follower_id, followee_id):
        if follower_id == followee_id:
            return  # cannot subscribe to oneself
        self.subscriptions.setdefault(follower_id, set()).add(followee_id)

    def unsubscribe(self, follower_id, followee_id):
        if follower_id in self.subscriptions:
            self.subscriptions[follower_id].discard(followee_id)

    def get_followees(self, user_id):
        return self.subscriptions.get(user_id, set())

# Usage
service = SubscriptionService()
service.subscribe('user1', 'user2')
service.subscribe('user1', 'user3')
print(service.get_followees('user1'))  # {'user2', 'user3'}

In a real application, a database with a subscriptions table is usually used, where each record contains follower_id and followee_id. This allows for efficient storage and querying of subscriptions.