code: def _cmd(self, command, uid=None):
          
          if not uid:
              uid = self.uid
          self._client_send(json.dumps({'cmd': command, 'uid': uid}))
          return self._client_receive()
docs: Send a command to the server.
  
          Args:
              command: str, The name of the command to execute.
              uid: int, the uid of the session to send the command to.
  
          Returns:
              The line that was written back.