def opt_log_level(self, levelName):
        """
        Set default log level.
        (options: {options}; default: "{default}")
        """
        try:
            self["logLevel"] = LogLevel.levelWithName(levelName)
        except InvalidLogLevelError:
            raise UsageError("Invalid log level: {}".format(levelName))

        Set default log level.
        (options: {options}; default: "{default}")
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/application/twist/_options.py