how-to-cart-mod.rst

How to create a Cart modifier

Cart modifiers are simple plugins that allow you to define rules according to which carts should be modified (and in what order).

Generally, this is how you implement a "bulk rebate" module, for instance.

Writing a simple cart modifier for the whole cart

Let's assume you want to provide a discount of 10% off the cart's total to clients that buy more than 500$ of goods.

This will affect the price of the whole cart. We will therefore override the :meth:`get_extra_cart_price_field` method of :class:`shop.cart.cart_modifiers_base.BaseCartModifier`:

from shop.cart.cart_modifiers_base import BaseCartModifier
from decimal import Decimal # We need this because python's float are confusing

class MyModifier(BaseCartModifier):
    """
    An example class that will grant a 10% discount to shoppers who buy
    more than 500 worth of goods.
    """
    def get_extra_cart_price_field(self, cart):
        ten_percent = Decimal('10') / Decimal('100')
        # Now we need the current cart total. It's not just the subtotal field
        # because there may be other modifiers before this one
        total = cart.current_total

        if total > Decimal('500'):
            rebate_amount = total * ten_percent
            rebate_amount = - rebate_amount # a rebate is a negative difference
            return ('My awesome rebate', rebate_amount)
        else:
            return None # None is no-op: it means "do nothing"

Adding this cart modifier to your SHOP_CART_MODIFIERS setting will enable it, and you should be able to already test that your cart displays a rebate when the total for the order is over 500.

Note

When using cart.extra_price_fields.append('your label', price) you might want to use from django.utils.translation import ugettext as _ for your label in multilingual projects. Please make sure that you use gettext