python3二进制搜索无法正常工作

Question

我跟随有关python3算法的书。 但我的代码不起作用。 它看起来与本书完全相同。哪里错了？

def binary_search(list, item):
    low = 0
    high = len(list)-1

    while low <= high:
        mid = (low+high)//2
        guess = list[mid]

        if guess == item:
            return mid
        if guess > item:
            high = mid - 1
        else :
            low = mid + 1
    return None

my_list = [1, 3, 5, 7, 9]

print(binary_search(my_list, 3))