LESSON 408 OF 410 · PROJECTS: OBJECT-ORIENTED

Shopping Cart in Python in Python

Practice

A shop keeps its orders in a cart. Build two classes that work together: a LineItem is one row of the cart (a product, its price and how many were bought), and a Cart holds many rows and adds them up. The cart has line items (composition), it is not one.

LineItem

__init__(self, name, price, quantity) stores the three values in self.name, self.price and self.quantity.
subtotal(self) returns price * quantity, the money this one row adds up to.

Cart

__init__(self) creates an empty list in self._items (the leading underscore means: internal, don't touch it from outside).
add(self, item) appends a LineItem to the list.
total(self) returns the sum of the subtotals of all items. Use a plain loop: start with total = 0, add each item.subtotal(), then return it.

Rules:

The total is worked out every time it is asked for, from the items in the cart at that moment. Adding an item later changes the next total.
An empty cart has a total of 0.
An item with price 5 and quantity 3 has a subtotal of 15.
Prices in the driver are whole numbers.

The driver wraps the numbers in int(...) so a whole-number total prints as 75.

The driver (already in the starter code, don't change it) creates an empty cart, checks one subtotal, then fills a cart and prints the total twice:

cart = Cart()
cart.add(apple)
cart.add(LineItem("Bread", 20, 3))
print("Total: " + str(int(cart.total())))
cart.add(LineItem("Milk", 10, 2))
print("Total: " + str(int(cart.total())))

Expected output:

Empty: 0
Subtotal: 15
Total: 75
Total: 95

Start with class LineItem: and its __init__, then subtotal. Then class Cart: with the list in __init__.

For total: total = 0, then for item in self._items: add item.subtotal() to it, then return total.

class LineItem:
    def __init__(self, name, price, quantity):
        self.name = name
        self.price = price
        self.quantity = quantity

    def subtotal(self):
        return self.price * self.quantity

class Cart:
    def __init__(self):
        self._items = []

    def add(self, item):
        self._items.append(item)

    def total(self):
        total = 0
        for item in self._items:
            total += item.subtotal()
        return total

empty = Cart()
print("Empty: " + str(int(empty.total())))
apple = LineItem("Apple", 5, 3)
print("Subtotal: " + str(int(apple.subtotal())))
cart = Cart()
cart.add(apple)
cart.add(LineItem("Bread", 20, 3))
print("Total: " + str(int(cart.total())))
cart.add(LineItem("Milk", 10, 2))
print("Total: " + str(int(cart.total())))

STEP 1 OF 17

Guided

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