def test_concat_two_carts_with_two_differents_items():
result_cart = Cart.concat(
cart(item(reference='x', quantity=1)),
cart(item(reference='y', quantity=1)))
assert Cart.create(
item(reference='x', quantity=1),
item(reference='y', quantity=1)) == result_cart
def test_emptycart_is_neutral_element():
neutral = cart()
element = cart(item())
assert Cart.concat(neutral, element) == element
assert Cart.concat(element, neutral) == element
def test_concat_two_carts_with_same_items():
result_cart = Cart.concat(
cart(item(reference='x', quantity=3)),
cart(item(reference='x', quantity=2)))
assert Cart.create(item(reference='x', quantity=5)) == result_cart
def test_associativity():
x = cart(item())
y = cart(item())
z = cart(item())
assert Cart.concat(Cart.concat(x, y), z) == Cart.concat(x, Cart.concat(y, z))
