with CartController.operations_batch(self.cart.user) as mark:

                mark.mark = True  # Marker that we've modified the cart

    # Marks the carts that are currently in batches

    _FOR_USER = {}

    _BATCH_COUNT = collections.defaultdict(int)

    _MODIFIED_CARTS = set()

    class _ModificationMarker(object):

        pass

    @classmethod

    @contextlib.contextmanager

    def operations_batch(cls, user):

        ''' Marks the boundary for a batch of operations on a user's cart.

        These markers can be nested. Only on exiting the outermost marker will

        a batch be ended.

        When a batch is ended, discounts are recalculated, and the cart's

        revision is increased.

        '''

        if user not in cls._FOR_USER:

            _ctrl = cls.for_user(user)

            cls._FOR_USER[user] = (_ctrl, _ctrl.cart.id)

        ctrl, _id = cls._FOR_USER[user]

        cls._BATCH_COUNT[_id] += 1

        try:

            success = False

            marker = cls._ModificationMarker()

            yield marker

            if hasattr(marker, "mark"):

                cls._MODIFIED_CARTS.add(_id)

            success = True

        finally:

            cls._BATCH_COUNT[_id] -= 1

            # Only end on the outermost batch marker, and only if

            # it excited cleanly, and a modification occurred

            modified = _id in cls._MODIFIED_CARTS

            outermost = cls._BATCH_COUNT[_id] == 0

            if modified and outermost and success:

                ctrl._end_batch()

                cls._MODIFIED_CARTS.remove(_id)

            # Clear out the cache on the outermost operation

            if outermost:

                del cls._FOR_USER[user]