第六章 - 使用一等函数实现设计模式
6.1 案例分析： 重构“策略”模式
如果合理利用一等对象的函数，某些设计模式可以简化，“策略”模式就是其中一个很好的例子。
6.1.1 经典的“策略”模式
《设计模式： 可复用面向对象软件的基础》 一书是这样概述“策略”模式的:
定义一些列的算法，把它们一一封装起来，并且使它们可以相互替换。 本模式使得算法可以独立于使用它的客户而变化。
实现Order类， 支持插入式折扣策略
from abc import ABC, abstractmethod from collections import namedtuple Customer = namedtuple("Customer", "name, fidelity") class LineItem(object): """产品线""" def __init__(self, product, quantity, price): """ :param product: 产品 :param quantity: 数量 :param price: 价格 """ self.product = product self.quantity = quantity self.price = price def total(self): return self.price * self.quantity class Order(object): """上下文管理""" def __init__(self, customer, cart, promotion=None): """ :param customer: 顾客 :param cart: 购物车 :param promotion:促销 """ self.customer = customer self.cart = list(cart) self.promotion = promotion self._total = None 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 is None: discount = 0 else: discount = self.promotion.discount(self) return self.total() - discount def __repr__(self): fmt = "<Order total:{:.2f} due:{:.2f}>" return fmt.format(self.total(), self.due()) class Promotion(ABC): """策略：抽象基类""" @abstractmethod def discount(self, order): """返回折扣金额""" class FidelityPromo(Promotion): """为积分超过1000的顾客提供5%折扣""" def discount(self, order): return order.total() * 0.05 if order.customer.fidelity >= 1000 else 0 class BulkItemPromo(Promotion): """单个商品为20个或以上时提供10%折扣""" def discount(self, order): discount = 0 for item in order.cart: if item.quantity >= 20: discount += item.total() * 0.1 return discount class LargeOrderPromo(Promotion): """订单中的不同商品达到10个或以上时提供7%折扣""" def discount(self, order): discount_items = {item.product for item in order.cart} # 集合生成式 if len(discount_items) >= 10: return order.total() * 0.07 return 0 joe = Customer("John Doe", 0) ann = Customer("Ann Smith", 1100) cart = [LineItem("banana", 4, 0.5), LineItem("apple", 10, 1.5), LineItem("watermellon", 5, 5.0)] print(Order(joe, cart, FidelityPromo())) print(Order(ann, cart, FidelityPromo())) banana_cart = [LineItem("banana", 30, 0.5), LineItem("apple", 10, 1.5)] print(Order(joe, banana_cart, BulkItemPromo())) long_cart = [LineItem(str(item_code), 1, 1.0) for item_code in range(10)] print(Order(joe, long_cart, LargeOrderPromo())) print(Order(joe, cart, LargeOrderPromo()))
函数实现的促销折扣的Order类示例
from collections import namedtuple Customer = namedtuple("Customer", "name, fidelity") class LineItem(object): def __init__(self, product, quantity, price): self.product = product self.quantity = quantity self.price = price def total(self): return self.quantity * self.price class Order(object): def __init__(self, customer, cart, promotion=None): self.customer = customer self.cart = list(cart) self.promotion = promotion def total(self): self.__total = sum(item.total() for item in self.cart) return self.__total def due(self): """折扣计算""" if self.promotion is None: discount = 0 else: discount = self.promotion(self) return self.total() - discount def __repr__(self): fmt = "<Order total: {:.2f} due: {:.2f}>" return fmt.format(self.total(), self.due()) def fidelity_promotion(order): """积分1000或以上，折扣5%""" return order.total() * 0.05 if order.customer.fidelity >= 1000 else 0 def bulk_item_promotion(order): """单个商品数量为20或以上，折扣10%""" discount = 0 for item in order.cart: if item.quantity >= 20: discount += item.total() * 0.1 return discount def large_order_promotion(order): """订单中不同商品达到10个或以上时，折扣7%""" distinct_item = {item.product for item in order.cart} if len(distinct_item) >= 10: return order.total() * 0.07 return 0 joe = Customer("John Doe", 0) ann = Customer("Ann Smith", 1100) cart = [LineItem("banana", 4, 0.5), LineItem("apple", 10, 1.5), LineItem("watermellon", 5, 5.0)] print(Order(joe, cart, fidelity_promotion)) print(Order(ann, cart, fidelity_promotion)) banana_cart = [LineItem("banana", 30, 0.5), LineItem("apple", 10, 1.5)] long_cart = [LineItem(str(item_code), 1, 1.0) for item_code in range(10)] print(Order(joe, banana_cart, bulk_item_promotion)) print(Order(joe, long_cart, large_order_promotion)) print(Order(joe, cart, large_order_promotion))
best_promo迭代一个函数列表， 并找出折扣额度最大的
from collections import namedtuple Customer = namedtuple("Customer", "name, fidelity") class LineItem(object): def __init__(self, product, quantity, price): self.product = product self.quantity = quantity self.price = price def total(self): return self.quantity * self.price class Order(object): def __init__(self, customer, cart, promotion=None): self.customer = customer self.cart = list(cart) self.promotion = promotion def total(self): self.__total = sum(item.total() for item in self.cart) return self.__total def due(self): """折扣计算""" if self.promotion is None: discount = 0 else: discount = self.promotion(self) return self.total() - discount def __repr__(self): fmt = "<Order total: {:.2f} due: {:.2f}>" return fmt.format(self.total(), self.due()) def fidelity_promotion(order): """积分1000或以上，折扣5%""" return order.total() * 0.05 if order.customer.fidelity >= 1000 else 0 def bulk_item_promotion(order): """单个商品数量为20或以上，折扣10%""" discount = 0 for item in order.cart: if item.quantity >= 20: discount += item.total() * 0.1 return discount def large_order_promotion(order): """订单中不同商品达到10个或以上时，折扣7%""" distinct_item = {item.product for item in order.cart} if len(distinct_item) >= 10: return order.total() * 0.07 return 0 promos = [fidelity_promotion, bulk_item_promotion, large_order_promotion] def best_promo(order): """选择可用的最佳折扣""" return max(promo(order) for promo in promos) joe = Customer("John Doe", 0) ann = Customer("Ann Smith", 1100) cart = [LineItem("banana", 4, 0.5), LineItem("apple", 10, 1.5), LineItem("watermellon", 5, 5.0)] print(Order(joe, cart, best_promo)) print(Order(ann, cart, best_promo)) banana_cart = [LineItem("banana", 30, 0.5), LineItem("apple", 10, 1.5)] long_cart = [LineItem(str(item_code), 1, 1.0) for item_code in range(10)] print(Order(joe, banana_cart, best_promo)) print(Order(joe, long_cart, best_promo)) print(Order(joe, cart, best_promo))
内省单独的promotions模块， 构建promos列表,
通过inspect.getmembers, inspect.isfunction获取函数
def fidelity_promotion(order): """积分为1000或以上， 5%折扣""" discount = 0 if order.customer.fidelity >= 1000: discount = order.total() * 0.05 return discount def bulk_promotion(order): """单个商品数量为20个或以上， 10%折扣""" discount = 0 for item in order.cart: if item.quantity >= 20: discount += item.total() * 0.01 return discount def large_order_promotion(order): """订单中不同的商品达到10个或以上， 7%折扣""" discount = 0 distinct_items = {item.product for item in order.cart} if len(distinct_items) >= 10: discount = order.total() * 0.07 return discount
from collections import namedtuple import promotions import inspect def best_promo(order): """选择可用的最佳折扣""" promos = [func for name, func in inspect.getmembers(promotions, inspect.isfunction)] return max(promo(order) for promo in promos) Customer = namedtuple("Customer", "name, fidelity") class LineItem(object): def __init__(self, product, quantity, price): self.product = product self.quantity = quantity self.price = price def total(self): return self.quantity * self.price class Order(object): def __init__(self, customer, cart, promotion=None): self.customer = customer self.cart = list(cart) self.promotion = promotion def total(self): self.__total = sum(item.total() for item in self.cart) return self.__total def due(self): """折扣计算""" if self.promotion is None: discount = 0 else: discount = self.promotion(self) return self.total() - discount def __repr__(self): fmt = "<Order total: {:.2f} due: {:.2f}>" return fmt.format(self.total(), self.due()) joe = Customer("John Doe", 0) ann = Customer("Ann Smith", 1100) carts = [LineItem("banana", 4, 0.5), LineItem("apple", 10, 1.5), LineItem("water", 5, 5.0)] print(Order(joe, carts, best_promo)) print(Order(ann, carts, best_promo)) banana_carts = [LineItem("banana", 30, 0.5), LineItem("apple", 10, 1.5)] long_carts = [LineItem(str(item_code), 1, 1.0) for item_code in range(10)] print(Order(joe, banana_carts, best_promo)) print(Order(joe, long_carts, best_promo)) print(Order(joe, carts, best_promo))
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