I am new to python and I am stuck. it shows error TypeError: unsupported operand type(s) for +=: 'int' and 'str' : 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:
        total += item
    return total
for key in prices:
    print compute_bill(key)