def get_lfn_key(lfn_obj: dict) -> str:
    """get either lfn key or file key from a file description"""
    if not lfn_obj or not isinstance(lfn_obj, dict): return ''
    if "lfn" in lfn_obj: return lfn_obj["lfn"]
    if "file" in lfn_obj: return lfn_obj["file"]
    return ''