牛乳パック1つと卵があれば6つ買うをやるシステム

main.py

from shop import Item, Shop

shop = Shop('イオン ドームシティ店')

cart = shop.cart()
cart.add(Item.MILK)

# TODO implement more

receipt = shop.paycheck(cart)
print(receipt)

shop.py

from enum import Enum


class Item(Enum):
    EGG = '卵'
    MILK = '牛乳パック'


class Cart:
    def __init__(self, shop):
        self.shop = shop
        self.items = []

    def add(self, item):
        if self.shop.stock_of(item) >= 1:
            self.items.append(item)
            self.shop.stock[item] -= 1
        else:
            raise Exception("%sの在庫がありません" % item.value)

    def summary(self):
        s = dict()
        for item in self.items:
            if item not in s:
                s[item] = 0
            s[item] += 1
        return s


class Shop:
    def __init__(self, name):
        self.name = name
        self.stock = dict()

        # デフォルトの在庫を 100 とする
        self.stocks(Item.MILK, 100)
        self.stocks(Item.EGG, 100)

    def stocks(self, item, amount):
        self.stock[item] = amount

    def cart(self):
        return Cart(self)

    def stock_of(self, item):
        return self.stock[item] if item in self.stock else 0

    def paycheck(self, cart):
        receipt = ["%s でのお買い上げ:" % self.name]
        for (item, amount) in cart.summary().items():
            receipt.append("  %s ... %d点" % (item.value, amount,))
        return "\n".join(receipt)