code: def can_subscribe_to_topic(self, topic, user):
          
          # A user can subscribe to topics if they are authenticated and if they have the permission
          # to read the related forum. Of course a user can subscribe only if they have not already
          # subscribed to the considered topic.
          return (
              user.is_authenticated and
              not topic.has_subscriber(user) and
              self._perform_basic_permission_check(topic.forum, user, 'can_read_forum')
          )
docs: Given a topic, checks whether the user can add it to their subscription list.