Python设计模式: 最佳的"策略"模式实践代码
今天抽空看了下流畅的python，发现里面介绍了不少python自带的库的使用实例，用起来非常的优雅。
平时用Python来写爬虫比较多，所以最近一直在看设计模式的内容。刚好这本书里面有一章单独讲设计模式的，讲的还不错，特意摘录出来。
这段代码的需求背景是电商平台常用的促销策略：
- 用户的积分超过1000时，订单总价优惠5%的金额
- 购买商品的种类超过10种时，订单总价优惠7%的金额
- 单个商品购买数量超过20件时，该商品获得10%的折扣
具体代码如下：
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: # the Context
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 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())
promos = []
def promotion(promo_func):
promos.append(promo_func)
return promo_func
@promotion
def fidelity_promo(order): # <3>
"""5% discount for customers with 1000 or more fidelity points"""
return order.total() * .05 if order.customer.fidelity >= 1000 else 0
@promotion
def bulk_item_promo(order):
"""10% discount for each LineItem with 20 or more units"""
discount = 0
for item in order.cart:
if item.quantity >= 20:
discount += item.total() * .1
return discount
@promotion
def large_order_promo(order):
"""7% discount for orders with 10 or more distinct items"""
distinct_items = {item.product for item in order.cart}
if len(distinct_items) >= 10:
return order.total() * .07
return 0
def best_promo(order):
return max(promo(order) for promo in promos)
# # BEGIN STRATEGY_TESTS
#
# >>> joe = Customer('John Doe', 0) # <1>
# >>> ann = Customer('Ann Smith', 1100)
# >>> cart = [LineItem('banana', 4, .5),
# ... LineItem('apple', 10, 1.5),
# ... LineItem('watermellon', 5, 5.0)]
# >>> Order(joe, cart, fidelity_promo) # <2>
# <Order total: 42.00 due: 42.00>
# >>> Order(ann, cart, fidelity_promo)
# <Order total: 42.00 due: 39.90>
# >>> banana_cart = [LineItem('banana', 30, .5),
# ... LineItem('apple', 10, 1.5)]
# >>> Order(joe, banana_cart, bulk_item_promo) # <3>
# <Order total: 30.00 due: 28.50>
# >>> long_order = [LineItem(str(item_code), 1, 1.0)
# ... for item_code in range(10)]
# >>> Order(joe, long_order, large_order_promo)
# <Order total: 10.00 due: 9.30>
# >>> Order(joe, cart, large_order_promo)
# <Order total: 42.00 due: 42.00>
# best promo========================================
# >>> Order(joe, long_order, best_promo)
# <Order total: 10.00 due: 9.30>
# >>> Order(joe, banana_cart, best_promo)
# <Order total: 30.00 due: 28.50>
# >>> Order(ann, cart, best_promo)
# <Order total: 42.00 due: 39.90>
# # END STRATEGY_TESTS
# """
# BEGIN STRATEGY这段代码得益于装饰器的使用，实现起来非常优雅，而且逻辑清晰、代码简洁，仅用了77行代码，就实现了策略模式。
但是，这个代码是经过了3次迭代、优化之后的。
第一版：使用ABC抽象基类来实现
在第一个版本中，用了ABC抽象基类来实现，代码非常的啰嗦，感觉像在写JAVA
# 这里只贴核心逻辑代码
from abc import ABC, abstractmethod
class Promotion(ABC): # 策略：抽象基类
@abstractmethod
def discount(self, order):
"""Return discount as a positive dollar amount"""
class FidelityPromo(Promotion): # 第一个策略
"""5% discount for customers with 1000 or more fidelity points
拥有1000或更高积分的客户可享受5％的折扣
"""
def discount(self, order):
return order.total() * .05 if order.customer.fidelity >= 1000 else 0
class BulkItemPromo(Promotion): # 第二个策略
"""10% discount for each LineItem with 20 or more units
单件商品购买超过20个,商品总价获得10％折扣"""
def discount(self, order):
discount = 0
for item in order.cart:
if item.quantity >= 20:
discount += item.total() * .1
return discount
class LargeOrderPromo(Promotion): # 第三个策略
"""7% discount for orders with 10 or more distinct items
购买商品的各类超过10个,获得7%的折扣
"""
def discount(self, order):
distinct_items = {item.product for item in order.cart}
if len(distinct_items) >= 10:
return order.total() * .07
return 0
第二版：使用函数实现，更加扁平化，可复用
def fidelity_promo(order):
"""5% discount for customers with 1000 or more fidelity points"""
return order.total() * .05 if order.customer.fidelity >= 1000 else 0
def bulk_item_promo(order):
"""10% discount for each LineItem with 20 or more units"""
discount = 0
for item in order.cart:
if item.quantity >= 20:
discount += item.total() * .1
return discount
def large_order_promo(order):
"""7% discount for orders with 10 or more distinct items"""
distinct_items = {item.product for item in order.cart}
if len(distinct_items) >= 10:
return order.total() * .07
return 0
# BEGIN STRATEGY_BEST
promos = [fidelity_promo, bulk_item_promo, large_order_promo] # <1>
def best_promo(order): # <2>
"""Select best discount available
"""
return max(promo(order) for promo in promos) # <3>好一些，主要的变动是抽象类撤掉了，改成函数来使用，实现更加的扁平化。而且不用实例化那么多对象。
第三版：把促销逻辑封装到一个模块当中
import inspect
import promotions
# 把上面那3个促销策略代码挪到promotions模块中
# 这个模块里只有这3个函数
promos = [func for name, func in inspect.getmembers(promotions, inspect.isfunction)]
def best_promo(order):
"""Select best discount available
"""
return max(promo(order) for promo in promos)其实第三版已经很优雅了，所有的促销逻辑全部都封装到promotions中，以后要增加新的促销策略，只需要去改promotions文件里面的代码就好了。
参考资料：