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