def binary_search(list,item):
    low = 0
    high = len(list) -1#low和high用于跟踪要在其中查找的列表部分

    while low <= high:
        mid = (low + high) // 2 #//表示整数除法
        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))

print(binary_search(my_list, -1))

报错原因：列表索引必须是整型或者切片

解决办法：
1.将mid = (low + high) / 2改写为mid = (low + high) // 2
注：" / “就表示 浮点数除法，返回浮点结果;” // "表示整数除法
2.强行转换为整型：
将mid = (low + high) / 2改写为mid = int((low + high) / 2)