code: def save(df, path):
          
          buffer = []
          size = _reset_df_and_get_size(df)
          with get_tqdm(total=size) as pbar:
              for dp in df:
                  buffer.append(dp)
                  pbar.update()
          np.savez_compressed(path, buffer=np.asarray(buffer, dtype=np.object))
docs: Args:
              df (DataFlow): the DataFlow to serialize.
              path (str): output npz file.