In our example, before instantiating an order, the system would somehow select a promotional discount strategy and pass it to the Order constructor.

ABC.py

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: # 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.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): # the Strategy: an Abstract Base Class
 @abstractmethod
 def discount(self, order):
 """Return discount as a positive dollar amount"""
class FidelityPromo(Promotion): # first Concrete Strategy
 """5% discount for customers with 1000 or more fidelity points"""
 def discount(self, order):
 return order.total() * .05 if order.customer.fidelity >= 1000 else 0
class BulkItemPromo(Promotion): # second Concrete Strategy
 """10% discount for each LineItem with 20 or more units"""
 def discount(self, order):
 discount = 0
 for item in order.cart:
 if item.quantity >= 20:
 discount += item.total() * .1
 return discount
class LargeOrderPromo(Promotion): # third Concrete Strategy
 """7% discount for orders with 10 or more distinct items"""
 def discount(self, order):
 distinct_items = {item.product for item in order.cart}
 if len(distinct_items) >= 10:
 return order.total() * .07
 return 0