'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.'

def data_files(self):

    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
