def parse_if_range_header(value):
    if not value:
        return IfRange()
    date = parse_date(value)
    if date is not None:
        return IfRange(date=date)
    # drop weakness information
    return IfRange(unquote_etag(value)[0])
Parses an if-range header which can be an etag or a date.  Returns
dataset/ETHPy150Open Eforcers/gae-flask-todo/lib/werkzeug/http.py/parse_if_range_header