def parse_cache_control_header(value, on_update=None, cls=None):
    """Parse a cache control header.  The RFC differs between response and
    request cache control, this method does not.  It's your responsibility
    to not use the wrong control statements.

    .. versionadded:: 0.5
       The `cls` was added.  If not specified an immutable
       :class:`~werkzeug.datastructures.RequestCacheControl` is returned.

    :param value: a cache control header to be parsed.
    :param on_update: an optional callable that is called every time a value
                      on the :class:`~werkzeug.datastructures.CacheControl`
                      object is changed.
    :param cls: the class for the returned object.  By default
                :class:`~werkzeug.datastructures.RequestCacheControl` is used.
    :return: a `cls` object.
    """
    if cls is None:
        cls = RequestCacheControl
    if not value:
        return cls(None, on_update)
    return cls(parse_dict_header(value), on_update)
dataset/ETHPy150Open Eforcers/gae-flask-todo/lib/werkzeug/http.py/parse_cache_control_header/original