force the (default) value of an option.
        The option is then no more listed by :func:`get_options()`.
        
        :param opt_name: option name
        :type opt_name: str
        :param value: option value

def force_option_value(self, opt_name, value):
        """ force the (default) value of an option.
        The option is then no more listed by :func:`get_options()`.
        
        :param opt_name: option name
        :type opt_name: str
        :param value: option value
        """
        if not self.has_option(opt_name):
            raise ValueError("Unknow option name (%s)" % opt_name)
        self._options[opt_name].default = value # also change the value
        self._options[opt_name].hidden = True