docs: def get_option_default(self, opt_name):
          """ Return the default value of a given option
          
          :param opt_name: option name
          :type opt_name: str
          
          :returns: the default value of the option
          """
          if not self.has_option(opt_name):
              raise ValueError("Unknow option name (%s)" % opt_name)
          return self._options[opt_name].default