def maybe_download_dataset(self, tmp_dir, dataset_split):
    """Downloads the appropriate dataset file and returns its path."""
    # Get the dataset url for the split requested.
    url = self.DATA_URLS.get(dataset_split, None)

    # Sanity check.
    if url is None:
      tf.logging.fatal("Unknown dataset_split passed: {}".format(dataset_split))

    # Download the data, if it doesn't already exist.
    return generator_utils.maybe_download(tmp_dir,
                                          self._extract_filename_from_url(url),
                                          url)
Downloads the appropriate dataset file and returns its path.
tensor2tensor/data_generators/program_search.py