def _parse_optional(self, arg_string):
        # if it's an empty string, it was meant to be a positional
        if not arg_string:
            return None

        # if it doesn't start with a prefix, it was meant to be positional
        if not arg_string[0] in self.prefix_chars:
            return None

        # if the option string is present in the parser, return the action
        if arg_string in self._option_string_actions:
            action = self._option_string_actions[arg_string]
            return action, arg_string, None

        # if it's just a single character, it was meant to be positional
        if len(arg_string) == 1:
            return None

        # if the option string before the "=" is present, return the action
        if '=' in arg_string:
            option_string, explicit_arg = arg_string.split('=', 1)
            if option_string in self._option_string_actions:
                action = self._option_string_actions[option_string]
                return action, option_string, explicit_arg

        # search through all possible prefixes of the option string
        # and all actions in the parser for possible interpretations
        option_tuples = self._get_option_tuples(arg_string)

        # if multiple actions match, the option string was ambiguous
        if len(option_tuples) > 1:
            options = ', '.join([option_string
                for action, option_string, explicit_arg in option_tuples])
            tup = arg_string, options
            self.error(_('ambiguous option: %s could match %s') % tup)

        # if exactly one action matched, this segmentation is good,
        # so return the parsed action
        elif len(option_tuples) == 1:
            option_tuple, = option_tuples
            return option_tuple

        # if it was not found as an option, but it looks like a negative
        # number, it was meant to be positional
        # unless there are negative-number-like options
        if self._negative_number_matcher.match(arg_string):
            if not self._has_negative_number_optionals:
                return None

        # if it contains a space, it was meant to be a positional
        if ' ' in arg_string:
            return None

        # it was meant to be an optional but there is no such option
        # in this parser (though it might be a valid option in a subparser)
        return None, arg_string, None
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/argparse/argparse.py/ArgumentParser._parse_optional/original