code: def broadcast(self, channel, event, data):
          
          payload = self._server.serialize_event(event, data)
          for socket_id in self.subscriptions.get(channel, ()):
              rv = self._server.sockets.get(socket_id)
              if rv is not None:
                  rv.socket.send(payload)
docs: Broadcasts an event to all sockets listening on a channel.