docs: def data_files(self):
      """Returns a python list of all (sharded) data subset files.
  
      Returns:
        python list of all (sharded) data set files.
      Raises:
        ValueError: if there are not data_files matching the subset.
      """
      tf_record_pattern = os.path.join(FLAGS.data_dir, '%s-*' % self.subset)
      data_files = tf.gfile.Glob(tf_record_pattern)
      if not data_files:
        print('No files found for dataset %s/%s at %s' % (self.name,
                                                          self.subset,
                                                          FLAGS.data_dir))
  
        self.download_message()
        exit(-1)
      return data_files