python - for ループが None を返すのはなぜですか?

Question

私は Code Academy から演習を行って、shopping_list. 次のコードの結果が余分な結果を返すのはなぜNoneですか?

shopping_list = ["banana","apple"]

stock = { "banana": 6,
    "apple": 0,
    "orange": 32,
    "pear": 15
}

prices = { "banana": 4,
    "apple": 2,
    "orange": 1.5,
    "pear": 3
}

# Write your code below!
def compute_bill(food):
    total = 0
    for x in food:
        print x
        total += prices[x]
compute_bill(shopping_list)

Answer 1

shopping_list = ["banana","apple"]

stock = { "banana": 6,
    "apple": 0,
    "orange": 32,
    "pear": 15
}

prices = { "banana": 4,
    "apple": 2,
    "orange": 1.5,
    "pear": 3
}

# Write your code below!
def compute_bill(food):
    total = 0
    for x in food:
        print x
        total += prices[x]
    return total
print(compute_bill(shopping_list))

return関数の結果を取得するには use ステートメントが必要ですcompute_bill。