docs: def open(filename, mode="rb", format = None):
      """Shorthand for WARCFile(filename, mode).
  
      Auto detects file and opens it.
  
      """
      if format == "auto" or format == None:
          format = detect_format(filename)
  
      if format == "warc":
          return WARCFile(filename, mode)
      elif format == "arc":
          return ARCFile(filename, mode)
      else:
          raise IOError("Don't know how to open '%s' files"%format)