def get_rich_menu_id_of_user(self, user_id, timeout=None):
        """Call get rich menu ID of user API.

        https://developers.line.me/en/docs/messaging-api/reference/#get-rich-menu-id-of-user

        :param str user_id: IDs of the user
        :param timeout: (optional) How long to wait for the server
            to send data before giving up, as a float,
            or a (connect timeout, read timeout) float tuple.
            Default is self.http_client.timeout
        :type timeout: float | tuple(float, float)
        :rtype: str
        :return: rich menu id
        """
        response = self._get(
            '/v2/bot/user/{user_id}/richmenu'.format(user_id=user_id),
            timeout=timeout
        )

        return response.json.get('richMenuId')
Call get rich menu ID of user API.

        https://developers.line.me/en/docs/messaging-api/reference/#get-rich-menu-id-of-user

        :param str user_id: IDs of the user
        :param timeout: (optional) How long to wait for the server
            to send data before giving up, as a float,
            or a (connect timeout, read timeout) float tuple.
            Default is self.http_client.timeout
        :type timeout: float | tuple(float, float)
        :rtype: str
        :return: rich menu id