Python--策略模式

电商领域有个功能明显可以使用“策略”模式，即根据客户的属性或订单中的商品计算折扣。

假如一个网店制定了下述折扣规则。

有 1000 或以上积分的顾客，每个订单享 5% 折扣。

同一订单中，单个商品的数量达到 20 个或以上，享 10% 折扣。

订单中的不同商品达到 10 个或以上，享 7% 折扣。

1.使用类实现的折扣策略

from abc import ABC, abstractmethod
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 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（正值）"""
        pass
class FidelityPromo(Promotion): # 第一个具体策略
"""为积分为1000或以上的顾客提供5%折扣"""
    def discount(self, order):
        return order.total() * .05 if order.customer.fidelity >= 1000 else 0
class BulkItemPromo(Promotion): # 第二个具体策略
"""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() * .1
        return discount
class LargeOrderPromo(Promotion): # 第三个具体策略
"""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):
        distinct_items = {item.product for item in order.cart}
        if len(distinct_items) >= 10:
            return order.total() * .07
        return 0

 

2.使用函数实现的折扣策略

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 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_promo(order): 
    """为积分为1000或以上的顾客提供5%折扣"""
        return order.total() * .05 if order.customer.fidelity >= 1000 else 0
    def bulk_item_promo(order):
    """单个商品为20个或以上时提供10%折扣"""
        discount = 0
        for item in order.cart:
            if item.quantity >= 20:
                discount += item.total() * .1
        return discount
    def large_order_promo(order):
    """订单中的不同商品达到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() * .07
        return 0

 

3.选择最佳策略

promos = [globals()[name] for name in globals() if name.endswith('_promo') and name != 'best_promo']  #globals返回一个字典，始终针对当前模块
def best_promo(order):
"""选择可用的最佳折扣"""
    return max(promo(order) for promo in promos)

或者使用inspect

promos = [func for name, func in inspect.getmembers(promotions, inspect.isfunction)]
def best_promo(order):
"""选择可用的最佳折扣"""
    return max(promo(order) for promo in promos)

 

4.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):
"""为积分为1000或以上的顾客提供5%折扣"""
    return order.total() * .05 if order.customer.fidelity >= 1000 else 0
@promotion
def bulk_item(order):
"""单个商品为20个或以上时提供10%折扣"""
    discount = 0
    for item in order.cart:
        if item.quantity >= 20:
            discount += item.total() * .1
    return discount
@promotion
def large_order(order):
"""订单中的不同商品达到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() * .07
    return 0
def best_promo(order): 
"""选择可用的最佳折扣"""
    return max(promo(order) for promo in promos)