docs: def send(self, data, namespace=None, callback=None):
          """Send a message to one or more connected clients.
  
          This function emits an event with the name ``'message'``. Use
          :func:`emit` to issue custom event names.
  
          :param data: The data to send to the client or clients. Data can be of
                       type ``str``, ``bytes``, ``list`` or ``dict``. If a
                       ``list`` or ``dict``, the data will be serialized as JSON.
          :param namespace: The Socket.IO namespace for the event. If this
                            argument is omitted the event is emitted to the
                            default namespace.
          :param callback: If given, this function will be called to acknowledge
                           the the client has received the message. The arguments
                           that will be passed to the function are those provided
                           by the client. Callback functions can only be used
                           when addressing an individual client.
          """
          self.emit('message', data=data, namespace=namespace,
                    callback=callback)