code: def _del_subscription(self, a_filter, session):
          
          deleted = 0
          try:
              subscriptions = self._subscriptions[a_filter]
              for index, (sub_session, qos) in enumerate(subscriptions):
                  if sub_session.client_id == session.client_id:
                      self.logger.debug("Removing subscription on topic '%s' for client %s" %
                                        (a_filter, format_client_message(session=session)))
                      subscriptions.pop(index)
                      deleted += 1
                      break
          except KeyError:
              # Unsubscribe topic not found in current subscribed topics
              pass
          finally:
              return deleted
docs: Delete a session subscription on a given topic
          :param a_filter:
          :param session:
          :return: