def save_session(
        self, app: Flask, session: SessionMixin, response: Response
    ) -> None:
        """This is called at the end of each request, after generating
        a response, before removing the request context. It is skipped
        if :meth:`is_null_session` returns ``True``.
        """
        raise NotImplementedError()
This is called at the end of each request, after generating
        a response, before removing the request context. It is skipped
        if :meth:`is_null_session` returns ``True``.
        
src/flask/sessions.py