strategy_pattern.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:
    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):
    @abstractmethod
    def discount(self, order):
        """Return discount as a positivve dolar amount"""

class FidelityPromo(Promotion):
    """First concrete strategy,5% discount for customers with 1000 or more point"""
    def discount(self, order):
        return order.total()*0.05 if order.customer.fidelity >= 1000 else 0

class BulkItemPromo(Promotion):
    """10%discount for each lineitem with 20 or more units"""
    def discount(self, order):
        discount = 0
        for item in order.cart:
            if item.quantity >= 0:
                discount += item.total()*.1
        return discount

class LargeOrderPromo(Promotion):
    """7% discount for orders 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() * 0.07
        return 0