docs: def get_index_from_alias(alias_name, index_client=None):
      """Retrieve the base index name from an alias
  
      Args:
          alias_name (str) Name of the alias
          index_client (Elasticsearch.IndicesClient) an Elasticsearch index
              client. Optional, will create one if not given
  
      Returns: (str) Name of index
      """
      index_client = index_client or indices_client()
      if not index_client.exists_alias(name=alias_name):
          return None
      return list(index_client.get_alias(name=alias_name).keys())[0]