摘要：前言合理使用作為一等對(duì)象的函數(shù)，可以使某種設(shè)計(jì)模式得以簡(jiǎn)化。本例中這個(gè)抽象類扮演這個(gè)角色。此外，策略實(shí)例沒有狀態(tài)實(shí)例屬性使用函數(shù)代替抽象類，每個(gè)策略都是函數(shù)，不必實(shí)例化，拿來(lái)即用。

合理使用作為一等對(duì)象的函數(shù)，可以使某種設(shè)計(jì)模式得以簡(jiǎn)化。

定義一系列算法，把它們一一封裝起來(lái)，并且使它們可以相互替換。

規(guī)則

1 有1000或以上積分的客戶，每個(gè)訂單享受5%的折扣。
2 同一個(gè)訂單中，單個(gè)商品的數(shù)量達(dá)到20個(gè)或以上，享受10%折扣。
3 訂單中的不同商品數(shù)達(dá)到10個(gè)或以上，享受7%折扣。

策略模式

上下文

把一些計(jì)算委托給實(shí)現(xiàn)不同算法的可互換組件，它提供服務(wù)。本例中，上下文是Order，它會(huì)根據(jù)不同的算法計(jì)算促銷折扣

策略

實(shí)現(xiàn)不同算法的組件共同的接口。本例中Promotion這個(gè)抽象類扮演這個(gè)角色。

具體策略

策略的具體子類。本例中為 fidelityPromo, BulkPromo, LargeOrderPromo三個(gè)子類。

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# @File    : ex1.py
# @Time    : 18/10/09 17:01

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 = 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 = ""
        return fmt.format(self.total(), self.due())


class Promotion(ABC):  # 策略:抽象基類

    @abstractmethod
    def discount(self, order):
        """
        返回折扣金額(正值)
        :param order:
        :return:
        """
class FidelityPromo(Promotion):  # 第一個(gè)具體策略
    """
    為積分1000或以上的顧客提供5%折扣
    """

    def discount(self, order):
        return order.total() * .05 if order.customer.fidelity >= 1000 else 0

class BulkItemPromo(Promotion): # 第二個(gè)具體策略
    """
    單個(gè)商品為20個(gè)或以上時(shí)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): # 第三個(gè)具體策略
    """
    訂單中的不同商品達(dá)到10個(gè)或以上時(shí)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

# 兩個(gè)顧客:joe的積分為0，ann的積分是1100
joe = Customer("John Doe", 0)
ann = Customer("Ann Smith", 1100)

# 有3個(gè)商品的購(gòu)物車
cart = [LineItem("banana", 4, .5), LineItem("apple", 10, 1.5), LineItem("watermellon", 5, 5.0)]

# joe未享受到折扣，ann享受到了5%折扣
ex1 = Order(joe, cart, FidelityPromo())
ex2 = Order(ann, cart, FidelityPromo())

# banana數(shù)量超過20個(gè),joe享受到了10%的折扣
banana_cart = [LineItem("banana", 30, .5), LineItem("apple", 10, 1.5)]
ex3 = Order(joe, banana_cart, BulkItemPromo())

# 商品數(shù)量超過了10個(gè)，為joe提供了7%的折扣
long_order = [LineItem(str(item_code), 1, 1.0) for item_code in range(10)]
ex4 = Order(joe, long_order, LargeOrderPromo())
ex5 = Order(joe, cart, LargeOrderPromo())

print(ex1)
print(ex2)
print(ex3)
print(ex4)
print(ex5)

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# @File    : ex2.py
# @Time    : 18/10/10 10:46

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 = 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 = ""
        return fmt.format(self.total(), self.due())

def fidelity_promo(order):

    return order.total() * .05 if order.customer.fidelity >= 1000 else 0

def bulk_item_promo(order):

     discount = 0
     for item in order.cart:
         if item.quantity >= 20:
             discount += item.total() * .1
     return discount

def large_order_promo(order):

     distinct_items = {item.product for item in order.cart}
     if len(distinct_items) >= 10:
         return order.total() * .07
     return 0


# 兩個(gè)顧客:joe的積分為0，ann的積分是1100
joe = Customer("John Doe", 0)
ann = Customer("Ann Smith", 1100)

# 有3個(gè)商品的購(gòu)物車
cart = [LineItem("banana", 4, .5), LineItem("apple", 10, 1.5), LineItem("watermellon", 5, 5.0)]

eg1 = Order(joe, cart, fidelity_promo)
eg2 = Order(ann, cart, fidelity_promo)

banana_cart = [LineItem("banana", 30, .5), LineItem("apple", 10, 1.5)]
eg3 = Order(joe, banana_cart, bulk_item_promo)

long_order = [LineItem(str(item_code), 1, 1.0) for item_code in range(10)]
eg4 = Order(joe, long_order, large_order_promo)

print(eg1,"
", eg2, "
",  eg3, "
", eg4)

經(jīng)典模式中每個(gè)具體策略都是一個(gè)類，而且只定義了一個(gè)方法，即discount。此外，策略實(shí)例沒有狀態(tài)(實(shí)例屬性)

使用函數(shù)代替抽象類，每個(gè)策略都是函數(shù)，不必實(shí)例化，拿來(lái)即用。新的Order類使用起來(lái)更簡(jiǎn)單，代碼行數(shù)更少。

<<流暢的Python>>