docs: def _key(key=''):
      '''
      Returns a Datastore key object, prefixed with the NAMESPACE.
      '''
      if not isinstance(key, datastore.Key):
          # Switchboard uses ':' to denote one thing (parent-child) and datastore
          # uses it for another, so replace ':' in the datastore version of the
          # key.
          safe_key = key.replace(':', '|')
          key = datastore.Key(os.path.join(NAMESPACE, safe_key))
      return key