code: def key_for_namespace(cls, namespace):
      
      if namespace:
        return model.Key(cls.KIND_NAME, namespace)
      else:
        return model.Key(cls.KIND_NAME, cls.EMPTY_NAMESPACE_ID)
docs: Return the Key for a namespace.
  
      Args:
        namespace: A string giving the namespace whose key is requested.
  
      Returns:
        The Key for the namespace.