def rpop(self, key):
        """Emulate lpop."""
        redis_list = self._get_list(key, 'RPOP')

        if self._encode(key) not in self.redis:
            return None

        try:
            value = redis_list.pop()
            if len(redis_list) == 0:
                self.delete(key)
            return value
        except (IndexError):
            # Redis returns nil if popping from an empty list
            return None
Emulate lpop.