code: def error(self, msg, repeat_at_end=True):
          
          print(msg, file=sys.stderr)
          if repeat_at_end:
              self.error_log.append(msg)
docs: Log a non-fatal error message to stderr, which is repeated at program termination.
  
          :param msg: Message to be printed.
          :param repeat_at_end: Set to false if the message should be printed, but not repeated at program termination.