Helper for lpushx and rpushx, that only index the new values if the list
        existed when the command was called

def _pushx(self, command, *args, **kwargs):
        """
        Helper for lpushx and rpushx, that only index the new values if the list
        existed when the command was called
        """
        result = self._traverse_command(command, *args, **kwargs)
        if self.indexable and result:
            self.index(args)
        return result