code: def was_modified_since(header=None, mtime=0, size=0):
      
      try:
          if header is None:
              raise ValueError
          matches = re.match(r"^([^;]+)(; length=([0-9]+))?$", header,
                             re.IGNORECASE)
          header_mtime = parse_http_date(matches.group(1))
          header_len = matches.group(3)
          if header_len and int(header_len) != size:
              raise ValueError
          if int(mtime) > header_mtime:
              raise ValueError
      except (AttributeError, ValueError, OverflowError):
          return True
      return False
docs: Was something modified since the user last downloaded it?
      header
        This is the value of the If-Modified-Since header.  If this is None,
        I'll just return True.
      mtime
        This is the modification time of the item we're talking about.
      size
        This is the size of the item we're talking about.