1. 使用函数实现‘策略’模式
from collections import namedtuple
Customer = namedtuple('Customer', 'name fidelity')
class LineItem():
def __init__(self, product, quantity, price):
self.product = product
self.quantity = quantity
self.price = price
def total(self):
return self.price * self.quantity
class Order(): #上下文
def __init__(self, customer, cart, promotion=None):
self.customer = customer
self.cart = list(cart)
self.promotion = promotion
def total(self):
if not hasattr(self, '__total'):
self.__total = sum(item.total() for item in self.cart)
return self.__total
def due(self):
if self.promotion == None:
discount = 0
else:
discount = self.promotion(self)
return self.total() - discount
def __repr__(self):
return f'<Customer: {self.customer.name}\n order total: {self.total()}, due: {self.due()}>'
def fidelity_promo(order):
"""为积分1000以上的客户提供5%折扣"""
return order.total() * 0.05 if order.customer.fidelity >= 1000 else 0
def bulk_item_promo(order):
"""单个商品为20个或以上提供10%折扣"""
discount = 0
for item in order.cart:
if item.quantity >= 20:
discount += item.total() * 0.1
return discount
def large_order_promo(order):
"""订单中不同商品达到10个或以上时提供7%折扣"""
distinct_items = {item.product for item in order.cart}
if len(distinct_items) >= 10:
return order.total() * 0.07
return 0
jason = Customer('jason', 1100)
cart = [LineItem('banana', 30, 0.5), LineItem('apple', 10, 1.5)]
print(Order(jason, cart, fidelity_promo))
print(Order(jason, cart, bulk_item_promo))
print(Order(jason, cart, large_order_promo))
2. 选择最佳策略
promos = [globals()[name] for name in globals() if name.endswith('_promo') and name != 'best_promo']
def best_promo(order): #与其他几个 *_promo函数一样， best_promo 函数的参数也是一个 Order 实例
"""选择可用的最佳折扣"""
return max(promo(order) for promo in promos)
print(Order(jason, cart, best_promo))
3. 装饰器基础知识
装饰器是可调用对象，其参数是另一个函数（被装饰的函数）。装饰器可能会处理被装饰的函数，然后把它返回，或者将其替换成另一个函数或可调用对象。
示例：
def deco(func):
def inner():
print('running inner()')
return inner
@deco
def target():
print ('running target()')
target()
输出: running inner()
现在我们使用装饰器改进‘策略’模式
promos = []
def promotion(promo_func):
promos.append(promo_func)
return promo_func
@promotion
def fidelity(order):
"""为积分1000以上的客户提供5%折扣"""
return order.total() * 0.05 if order.customer.fidelity >= 1000 else 0
@promotion
def bulk_item(order):
"""单个商品为20个或以上提供10%折扣"""
discount = 0
for item in order.cart:
if item.quantity >= 20:
discount += item.total() * 0.1
return discount
@promotion
def large_order(order):
"""订单中不同商品达到10个或以上时提供7%折扣"""
distinct_items = {item.product for item in order.cart}
if len(distinct_items) >= 10:
return order.total() * 0.07
return 0
def best_promo(order):
"""选择可用的最佳折扣"""
return max(promo(order) for promo in promos)
这个方案有几个优点：
(1). 促销策略函数无需使用特殊的名称(即不用以_promo结尾) (2). 临时禁用某个促销策略，只需要把装饰器注释掉
