def handle_storage_class(vol):
    """
    vol: dict (send from the frontend)

    If the fronend sent the special values `{none}` or `{empty}` then the
    backend will need to set the corresponding storage_class value that the
    python client expects.
    """
    if "class" not in vol:
        return None
    if vol["class"] == "{none}":
        return ""
    if vol["class"] == "{empty}":
        return None
    else:
        return vol["class"]