code: 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])
docs: Parses an if-range header which can be an etag or a date.  Returns
      a :class:`~werkzeug.datastructures.IfRange` object.
  
      .. versionadded:: 0.7