code: def _key_for_namespace(namespace, app):
    
    if namespace:
      return db.Key.from_path(metadata.Namespace.KIND_NAME,
                              namespace,
                              _app=app)
    else:
      return db.Key.from_path(metadata.Namespace.KIND_NAME,
                              metadata.Namespace.EMPTY_NAMESPACE_ID,
                              _app=app)
docs: Return the __namespace__ key for a namespace.
  
    Args:
      namespace: The namespace whose key is requested.
      app: The id of the application that the key belongs to.
  
    Returns:
      A db.Key representing the namespace.