docs: def _interval_string_to_seconds(interval_string):
      """Convert internal string like 1M, 1Y3M, 3W to seconds.
  
      :type interval_string: str
      :param interval_string: Interval string like 1M, 1W, 1M3W4h2s...
          (s => seconds, m => minutes, h => hours, D => days,
           W => weeks, M => months, Y => Years).
  
      :rtype: int
      :return: The conversion in seconds of interval_string.
  
      """
      interval_exc = "Bad interval format for {0}".format(interval_string)
      interval_dict = {"s": 1, "m": 60, "h": 3600, "D": 86400,
                       "W": 7*86400, "M": 30*86400, "Y": 365*86400}
  
      interval_regex = re.compile("^(?P<num>[0-9]+)(?P<ext>[smhDWMY])")
      seconds = 0
  
      while interval_string:
          match = interval_regex.match(interval_string)
          if match:
              num, ext = int(match.group("num")), match.group("ext")
              if num > 0 and ext in interval_dict:
                  seconds += num * interval_dict[ext]
                  interval_string = interval_string[match.end():]
              else:
                  raise Exception(interval_exc)
          else:
              raise Exception(interval_exc)
      return seconds