class Cart:
    def __init__(self):
        self.products = []
    def add(self, obj):
        self.products.append(obj)
    def notify(self):
        for obj in self.products:
            obj.notify(len(self.products))
    def price(self):
        for obj in self.products:
            print(obj.price)

class Checkout():
    def __init__(self,cart):
        self.cart = cart
        self.products = self.cart.products
        self.totals = 0
        self.accounting()
    def total(self):
        print(self.totals)
        return (self.totals)
    def accounting(self):
        for obj in self.products:
            self.totals = self.totals + obj.price
    def bill(self):
        pass

class Shipping():
    def __init__(self,checkout,ship):
        self.ship = ship
        checkout.totals = checkout.totals + ship.getCost()
    def cost(self):
        print(self.ship.getCost())

class UPS():
    def __init__(self):
        self.cost = 15.2
    def getCost(self):
        return self.cost

class FedEx(Shipping):
    def __init__(self):
        self.cost = 10
    def getCost(self):
        return self.cost

class Payment():
    def __init__(self, checkout):
        pass
    def payable(self):
        pass
    def tax(self):
        pass
		
		

			
                         +-------------+     +-----------------------+
User -> Goods -> Cart -> | Delivery    |  -> | Promotions components | -> Checkout
                         +-------------+     +-----------------------+
                         | rule A      |     | Promotion rule 1      |
                         | rule B      |     | Promotion rule 2      |
                         | rule C      |     | Promotion rule 3      |
                         | rule D      |     | Promotion rule 4      |
                         | rule E      |     | Promotion rule 5      |
                         +-------------+     +-----------------------+
			
			

資料庫設計

			
 +--------------+
 | shipping     |
 +--------------+
 | id           |o---+
 | name         |    |
 | ...          |    |
 +--------------+    |    +----------------------+
                     |    | shipping_rule        |
 +--------------+    |    +----------------------+
 | zone         |    |    | id                   |
 +--------------+    +--->| shipping_id          |
 | id           |o------->| zone_id              |
 | name         |         | plugin               |
 |              |         | ...                  |
 +--------------+         +----------------------+

 +--------------+
 | delivery     |
 +--------------+
 | id           |
 | user_id      |o------>
 | address_id   |o------>
 +--------------+