Concerning Dictionaries and += in Python 2.x

[Luke Tomaneng]

I have been having a bit of trouble with the things mentioned in the title. I have written the following script for a Codecademy course:
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]
            stock[item] = stock[item] - 1
            return total
Whenever I run this script, "4" is returned. It does not seem to matter what in in the list the script is run on. I have tried this on the Codecademy interpreter/emulator (I'm not sure which they use) and the repl.it interpreter, but for the same result. If anyone could find the glitch in my code, please let me know. Thanks!