Python Practice 17/22

Fix the bug: the second cart is not empty

A small store's checkout script. add_item puts one item in a cart, and if you do not hand it a cart it starts a fresh one. Ada shops first and gets apples. Ben shops next and gets bread. Run it and read Ben's cart: Ada's apples are in it.

Nothing raised. Both calls returned a list, which is exactly what they promised to return. The only thing wrong is the value - and the value is what reaches the customer, on the receipt and in the total. A traceback would have stopped the deploy. This ships.

The data is not doing anything clever. Two shoppers, one ordinary string each, no None, no empty input, nothing to trip over. The first call is perfect, which is exactly why nobody catches this in review. What is adversarial here is the second call. Ada and Ben share nothing in this program except add_item itself, so whatever is leaking between them is living on the function.

Your task: fix add_item so a call that passes no cart starts from an empty one, while a call that does pass a cart still adds to that cart. Ben's cart should be ['bread'].

You'll practice:

Reading a clean run that returns a wrong value as a bug, not a quirk
Tracing leaked state back to when a default argument is actually created

# Ada shops, then Ben shops. Ben's cart comes back holding Ada's apples. Fix add_item.
def add_item(item, cart=[]):
    cart.append(item)
    return cart

print(add_item("apples"))  # Ada's cart
print(add_item("bread"))   # Ben's cart - should be ['bread']