1. 面向对象版的策略模式

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:

    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 = '<Order total: {:.2f} due: {:.2f}>'
        return fmt.format(self.total(), self.due())


class Promotion(ABC):
    @abstractmethod
    def discount(self, order):
        """Return discount as a positive dollar amount"""


class FidelityPromo(Promotion):
    """有 1000 或以上积分的顾客，每个订单享 5% 折扣"""
    def discount(self, order):
        return order.total() * 0.05 if order.customer.fidelity >= 1000 else 0


class BulkItemPromo(Promotion):
    """同一订单中，单个商品的数量达到 20 个或以上，享 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，享 7% 折扣"""
    def discount(self, order):
        distinct_items = {item.product for item in order.cart}
        if len(distinct_items) >= 10:
            return order.total() * 0.07
        return 0


def run():
    joe = Customer('John Doe', 0)
    ann = Customer('Ann Smith', 1100)

    cart = [LineItem('banana', 4, .5),
            LineItem('apple', 10, 1.5),
            LineItem('watermellon', 5, 5.0)]
    print(Order(joe, cart, FidelityPromo()))
    print(Order(ann, cart, FidelityPromo()))

    banane_cart = [LineItem('banana', 30, .5),
                   LineItem('apple', 10, 1.5)]
    print(Order(joe, banane_cart, BulkItemPromo()))

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


if __name__ == '__main__':
    run()

2. 函数版的策略模式

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:

    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 = '<Order total: {:.2f} due: {:.2f}>'
        return fmt.format(self.total(), self.due())


def fidelity_promo(order):
    return order.total() * 0.05 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() * 0.1
    return discount


def large_order_promo(order):
    distinct_items = {item.product for item in order.cart}
    if len(distinct_items) >= 10:
        return order.total() * 0.07
    return 0


def run():
    print(Order(joe, cart, fidelity_promo))
    print(Order(ann, cart, fidelity_promo))

    print(Order(joe, banane_cart, bulk_item_promo))

    print(Order(joe, long_order, large_order_promo))


if __name__ == '__main__':
    joe = Customer('John Doe', 0)
    ann = Customer('Ann Smith', 1100)

    cart = [LineItem('banana', 4, .5),
            LineItem('apple', 10, 1.5),
            LineItem('watermellon', 5, 5.0)]

    banane_cart = [LineItem('banana', 30, .5),
                   LineItem('apple', 10, 1.5)]

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

    run()

计算最优折扣

promos = [fidelity_promo, bulk_item_promo, large_order_promo]

def best_promo(order):
    """选择可用的最佳折扣"""
    return max(promo(order) for promo in promos)

def best_run():
    print(Order(joe, long_order, best_promo))
    print(Order(joe, banane_cart, best_promo))
    print(Order(ann, cart, best_promo))

使用 `inspect.getmembers` 内省独立模块

import inspect
import promotions

# inspect.getmembers 获取对象(这里是模块)的属性
promos = [func for name, func in
          inspect.getmembers(promotions, inspect.isfunction)]

promotions.py 里面是策略函数:

def fidelity_promo(order):
    return order.total() * 0.05 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() * 0.1
    return discount


def large_order_promo(order):
    distinct_items = {item.product for item in order.cart}
    if len(distinct_items) >= 10:
        return order.total() * 0.07
    return 0