registrasion/controllers/cart.py
def _add_discount(self, product, quantity, discounts):
        ''' Applies the best discounts on the given product, from the given
        discounts.'''

        def matches(discount):
            ''' Returns True if and only if the given discount apples to
            our product. '''
            if isinstance(discount.clause, conditions.DiscountForCategory):
                return discount.clause.category == product.category
            else:
                return discount.clause.product == product

        def value(discount):
            ''' Returns the value of this discount clause
            as applied to this product '''
            if discount.clause.percentage is not None:
                return discount.clause.percentage * product.price
            else:
                return discount.clause.price

        discounts = [i for i in discounts if matches(i)]
        discounts.sort(key=value)

        for candidate in reversed(discounts):
            if quantity == 0:
                break
            elif candidate.quantity == 0:
                # This discount clause has been exhausted by this cart
                continue

            # Get a provisional instance for this DiscountItem
            # with the quantity set to as much as we have in the cart
            discount_item = commerce.DiscountItem.objects.create(
                product=product,
                cart=self.cart,
                discount=candidate.discount,
                quantity=quantity,
            )

            # Truncate the quantity for this DiscountItem if we exceed quantity
            ours = discount_item.quantity
            allowed = candidate.quantity
            if ours > allowed:
                discount_item.quantity = allowed
                discount_item.save()
                # Update the remaining quantity.
                quantity = ours - allowed
            else:
                quantity = 0

            candidate.quantity -= discount_item.quantity
def _add_discount(self, product, quantity, discounts):
        ''' Applies the best discounts on the given product, from the given
        discounts.'''

        def matches(discount):
            ''' Returns True if and only if the given discount apples to
            our product. '''
            if isinstance(discount.clause, conditions.DiscountForCategory):
                return discount.clause.category == product.category
            else:
                return discount.clause.product == product

        def value(discount):
            ''' Returns the value of this discount clause
            as applied to this product '''
            if discount.clause.percentage is not None:
                return discount.clause.percentage * product.price
            else:
                return discount.clause.price

        discounts = [i for i in discounts if matches(i)]
        discounts.sort(key=value)

        for candidate in reversed(discounts):
            if quantity == 0:
                break
            elif candidate.quantity == 0:
                # This discount clause has been exhausted by this cart
                continue

            # Get a provisional instance for this DiscountItem
            # with the quantity set to as much as we have in the cart
            discount_item = commerce.DiscountItem.objects.create(
                product=product,
                cart=self.cart,
                discount=candidate.discount,
                quantity=quantity,
            )

            # Truncate the quantity for this DiscountItem if we exceed quantity
            ours = discount_item.quantity
            allowed = candidate.quantity
            if ours > allowed:
                discount_item.quantity = allowed
                discount_item.save()
                # Update the remaining quantity.
                quantity = ours - allowed
            else:
                quantity = 0

            candidate.quantity -= discount_item.quantity
Applies the best discounts on the given product, from the given
        discounts.