def _get_parent_cache_dir_url():
    """Get parent cache dir url from `petastorm.spark.converter.parentCacheDirUrl`
    We can only set the url config once.
    """
    global _parent_cache_dir_url  # pylint: disable=global-statement

    conf_url = _get_spark_session().conf \
        .get(SparkDatasetConverter.PARENT_CACHE_DIR_URL_CONF, None)

    if conf_url is None:
        raise ValueError(
            "Please set the spark config {}.".format(SparkDatasetConverter.PARENT_CACHE_DIR_URL_CONF))

    conf_url = normalize_dir_url(conf_url)
    _check_parent_cache_dir_url(conf_url)
    _parent_cache_dir_url = conf_url
    logger.info(
        'Read %s %s', SparkDatasetConverter.PARENT_CACHE_DIR_URL_CONF, _parent_cache_dir_url)

    return _parent_cache_dir_url
Get parent cache dir url from `petastorm.spark.converter.parentCacheDirUrl`
    We can only set the url config once.
    
petastorm/spark/spark_dataset_converter.py