write operation stats to log
        :param mode: 'open', 'saveas', 'listtree'
        :param infostr: string to put into info_st
        :param stat: 'OK' or 'ERR'

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