code: def _new_device_id(self, key):
          
          device_id = Id.SERVER + 1
          if key in self._key2deviceId:
              return self._key2deviceId[key]
          while device_id in self._clients:
              device_id += 1
          return device_id
docs: Generate a new device id or return existing device id for key
  
          :param key: Key for device
          :type key: unicode
          :return: The device id
          :rtype: int