装饰者（或修饰）模式（Decorator）：

增加一个修饰类包裹原来的类，装饰过的对象可替代原始对象。

原则：

类应该对扩展开放，对修改关闭。

案例：

奶茶店有多种饮料：奶茶、果汁、咖啡；每种饮料可搭配若干配料：珍珠、布丁、牛奶；要能根据顾客的任意搭配下单并计算价钱；

代码：

#!/usr/bin/python

class Beverage:
    description = "Unknown Beverage"

    def get_description(self):
        return self.description

    def cost(self):
        pass


class CondimentDecorator(Beverage):
    def get_description(self):
        pass


class MilkyTea(Beverage):
    def __init__(self):
        self.description = "MilkyTea"

    def cost(self):
        return 1.99


class FruitJuice(Beverage):
    def __init__(self):
        self.description = "FruitJuice"

    def cost(self):
        return 1.80


class Coffee(Beverage):
    def __init__(self):
        self.description = "Coffee"

    def cost(self):
        return 2.00


class Pearl(CondimentDecorator):
    def __init__(self, beverage):
        self.beverage = beverage

    def get_description(self):
        return self.beverage.get_description() + " + Pearl"

    def cost(self):
        return 1.50 + self.beverage.cost()


class Pudding(CondimentDecorator):
    def __init__(self, beverage):
        self.beverage = beverage

    def get_description(self):
        return self.beverage.get_description() + " + Pudding"

    def cost(self):
        return 1.60 + self.beverage.cost()


class Milk(CondimentDecorator):
    def __init__(self, beverage):
        self.beverage = beverage

    def get_description(self):
        return self.beverage.get_description() + " + Milk"

    def cost(self):
        return 2.10 + self.beverage.cost()


if __name__ == '__main__':
    b = FruitJuice()
    print "%s = $%s\n" % (b.get_description(), b.cost())

    b = MilkyTea()
    b = Pearl(b)
    b = Pudding(b)
    print "%s = $%s\n" % (b.get_description(), b.cost())

    b = Coffee()
    b = Pearl(b)
    b = Milk(b)
    print "%s = $%s\n" % (b.get_description(), b.cost())

输出：

FruitJuice = $1.8

MilkyTea + Pearl + Pudding = $5.09

Coffee + Pearl + Milk = $5.6

参考：

《Head First 设计模式》