test.py

shopping_list = ["banana", "orange", "apple"]

stock = {
    "banana": 6,
    "apple": 0,
    "orange": 32,
    "pear": 15
}
    
prices = {
    "banana": 4,
    "apple": 2,
    "orange": 1.5,
    "pear": 3
}

def compute_bill(food):
    total = 0
    for item in food:
        if stock[item] > 0:
            total = prices[item] + total
            stock[item] = stock[item] - 1
    return total
    
print compute_bill(shopping_list)