code: def _get_content_range(start: Optional[int], end: Optional[int], total: int) -> str:
      
      start = start or 0
      end = (end or total) - 1
      return "bytes %s-%s/%s" % (start, end, total)
docs: Returns a suitable Content-Range header:
  
      >>> print(_get_content_range(None, 1, 4))
      bytes 0-0/4
      >>> print(_get_content_range(1, 3, 4))
      bytes 1-2/4
      >>> print(_get_content_range(None, None, 4))
      bytes 0-3/4