code: def update_stat(self, mode='open', infostr='', stat=''):
          
          self._update_stat[mode](mode, infostr, stat)
docs: write operation stats to log
          :param mode: 'open', 'saveas', 'listtree'
          :param infostr: string to put into info_st
          :param stat: 'OK' or 'ERR'