Log attribute to file and pass the message to underlying logger.

        :param int level: logging level
        :param str attr_name: attribute's name to be logged

def log_attr(self, level, attr_name):
        '''Log attribute to file and pass the message to underlying logger.

        :param int level: logging level
        :param str attr_name: attribute's name to be logged
        '''
        msg = self.write(attr_name)
        self.log(level, msg)