kata09.py

# http://codekata.com/kata/kata09-back-to-the-checkout/ implemenation using python3 with type-hints and mypy
from collections import defaultdict
from typing import NamedTuple, Dict, List
import unittest


class Product(NamedTuple):
    price: int


class Discount(NamedTuple):
    price: int
    quantity: int


class Store:
    def __init__(self,
                 products: Dict[str, Product],
                 discounts: Dict[str, Discount]):
        self.products = products
        self.discounts: Dict[Product, Discount] = {}
        for sku, discount in discounts.items():
            product = products[sku]
            self.discounts[product] = discount


class Cart:

    def __init__(self, store: Store):
        self.store = store
        self.products: List[Product] = []
        self.total = 0
        self._sum_products()

    def _sum_products(self):
        co = make_cart_checkout(self)
        self.total = co.total

    def scan(self, sku: str):
        self.products.append(self.store.products[sku])
        self._sum_products()


class Checkout(NamedTuple):
    quantities: Dict[Product, int]
    discounts: List[Discount] = []
    total: int = 0


def make_cart_checkout(cart: Cart) -> Checkout:
    quantities: Dict[Product, int] = defaultdict(lambda: 0)

    for product in cart.products:
        quantities[product] += 1

    total = 0
    discounts: List[Discount] = []

    for product, qty in quantities.items():
        if product in cart.store.discounts:
            discount = cart.store.discounts[product]
            discountable, qty = divmod(qty, discount.quantity)
            discounts += [discount] * discountable
            total += discountable * discount.price

        total += product.price * qty

    return Checkout(quantities, discounts, total)


class TestKata09(unittest.TestCase):

    def setUp(self):

        self.store = Store(
            products={
                'A': Product(price=50),
                'B': Product(price=30),
                'C': Product(price=20),
                'D': Product(price=15),
            },
            discounts={
                'A': Discount(price=130, quantity=3),
                'B': Discount(price=45, quantity=2),
            })

    def _make_checkout(self, skus):
        cart = Cart(store=self.store)
        for sku in skus:
            cart.scan(sku)

        return cart.total

    def test_totals(self):
        self.assertEqual(0, self._make_checkout(""))
        self.assertEqual(50, self._make_checkout("A"))
        self.assertEqual(80, self._make_checkout("AB"))
        self.assertEqual(115, self._make_checkout("CDBA"))

        self.assertEqual(100, self._make_checkout("AA"))
        self.assertEqual(130, self._make_checkout("AAA"))
        self.assertEqual(180, self._make_checkout("AAAA"))
        self.assertEqual(230, self._make_checkout("AAAAA"))
        self.assertEqual(260, self._make_checkout("AAAAAA"))

        self.assertEqual(160, self._make_checkout("AAAB"))
        self.assertEqual(175, self._make_checkout("AAABB"))
        self.assertEqual(190, self._make_checkout("AAABBD"))
        self.assertEqual(190, self._make_checkout("DABABA"))

    def test_incremental(self):
        co = Cart(self.store)
        co.scan("A")
        self.assertEqual(50, co.total)

        co.scan("B")
        self.assertEqual(80, co.total)

        co.scan("A")
        self.assertEqual(130, co.total)

        co.scan("A")
        self.assertEqual(160, co.total)

        co.scan("B")
        self.assertEqual(175, co.total)


if __name__ == '__main__':
    # mypy.main()
    unittest.main()