code: def _lookup_model(cls, kind, default_model=None):
      
      modelclass = cls._kind_map.get(kind, default_model)
      if modelclass is None:
        raise KindError(
            "No model class found for kind '%s'. Did you forget to import it?" %
            kind)
      return modelclass
docs: Get the model class for the kind.
  
      Args:
        kind: A string representing the name of the kind to lookup.
        default_model: The model class to use if the kind can't be found.
  
      Returns:
        The model class for the requested kind.
      Raises:
        KindError: The kind was not found and no default_model was provided.