def parseOptions(self, options=None):
        """
        The guts of the command-line parser.
        """

        if options is None:
            options = sys.argv[1:]

        # we really do need to place the shell completion check here, because
        # if we used an opt_shell_completion method then it would be possible
        # for other opt_* methods to be run first, and they could possibly
        # raise validation errors which would result in error output on the
        # terminal of the user performing shell completion. Validation errors
        # would occur quite frequently, in fact, because users often initiate
        # tab-completion while they are editing an unfinished command-line.
        if len(options) > 1 and options[-2] == "--_shell-completion":
            from twisted.python import _shellcomp
            cmdName = path.basename(sys.argv[0])
            _shellcomp.shellComplete(self, cmdName, options,
                                     self._shellCompFile)
            sys.exit(0)

        try:
            opts, args = getopt.getopt(options,
                                       self.shortOpt, self.longOpt)
        except getopt.error as e:
            raise UsageError(str(e))

        for opt, arg in opts:
            if opt[1] == '-':
                opt = opt[2:]
            else:
                opt = opt[1:]

            optMangled = opt
            if optMangled not in self.synonyms:
                optMangled = opt.replace("-", "_")
                if optMangled not in self.synonyms:
                    raise UsageError("No such option '%s'" % (opt,))

            optMangled = self.synonyms[optMangled]
            if isinstance(self._dispatch[optMangled], CoerceParameter):
                self._dispatch[optMangled].dispatch(optMangled, arg)
            else:
                self._dispatch[optMangled](optMangled, arg)

        if (getattr(self, 'subCommands', None)
            and (args or self.defaultSubCommand is not None)):
            if not args:
                args = [self.defaultSubCommand]
            sub, rest = args[0], args[1:]
            for (cmd, short, parser, doc) in self.subCommands:
                if sub == cmd or sub == short:
                    self.subCommand = cmd
                    self.subOptions = parser()
                    self.subOptions.parent = self
                    self.subOptions.parseOptions(rest)
                    break
            else:
                raise UsageError("Unknown command: %s" % sub)
        else:
            try:
                self.parseArgs(*args)
            except TypeError:
                raise UsageError("Wrong number of arguments.")

        self.postOptions()

        The guts of the command-line parser.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/usage.py