python 策略模式

strategy_best.py

Strategy pattern -- function-based implementation

selecting best promotion from static list of functions

""" >>> 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)] >>> Order(joe, cart, fidelity_promo) <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) <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>

BEGIN STRATEGY_BEST_TESTS

>>> Order(joe, long_order, best_promo)  # <1>
<Order total: 10.00 due: 9.30>
>>> Order(joe, banana_cart, best_promo)  # <2>
<Order total: 30.00 due: 28.50>
>>> Order(ann, cart, best_promo)  # <3>
<Order total: 42.00 due: 39.90>

END STRATEGY_BEST_TESTS

"""

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())

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 = [globals()[name] for name in globals() if name.endswith('_promo') and name != 'best_promo'] def best_promo(order): # <2> """Select best discount available """ return max(promo(order) for promo in promos) # <3>

END STRATEGY_BEST