composite.py

class Discount:
    def price_after_discount(self, product):
        return product.price

class PercentDiscount:
    def __init__(self, discount, pct):
        self.pct = pct
        self.discount = discount

    def price_after_discount(self, product):
        baseprice = self.discount.price_after_discount(product)
        return baseprice * (1 - self.pct)

class BuyXGetY:
    def __init__(self, discount, minprice, reduction):
        self.discount = discount
        self.minprice = minprice
        self.reduction = reduction

    def price_after_discount(self, product):
        baseprice = self.discount.price_after_discount(product)

        if baseprice > self.minprice:
            return baseprice - self.reduction
        else:
            return baseprice


class ShoppingCart:
    def __init__(self, products):
        self.products = products

    def price(self):
        return sum([x.price for x in self.products])

class Product:
    def __init__(self, name, price):
        self.name = name
        self.price = price

    def add_discount(self, discount):
        self.price = discount.price_after_discount(self)


class Bundle:
    def __init__(self, products):
        self.price = sum([x.price for x in products])

    def add_discount(self, discount):
        self.price = discount.price_after_discount(self)


iPhoneDeal = Bundle([
    Product('iPhone', 750),
    Product('iPhone case', 20),
    Bundle([
        Product('Support', 50),
        Product('Cool Apps', 20),
    ])
])

discount = PercentDiscount(BuyXGetY(Discount(), 20, 10), 0.5)

iPhoneDeal.add_discount(discount)

s = ShoppingCart([
    iPhoneDeal,
    Product('Galaxy', 550),
])

print(s.price())