老生常谈的算法了。

#!/usr/bin/python
# -*- coding:utf-8 -*-
# Filename: demo.py

# 用python实现二分查找
def binarySearch(a, ll, rr, val):
    while ll <= rr:
        mid = int((ll + rr) / 2)
        if a[mid] == val:
            return mid
        elif a[mid] > val:
            rr = mid - 1
        else:
            ll = mid + 1
    return -1

if __name__ == '__main__':
    a = int(input('请输入要查找的值:'))
    flag = binarySearch([1, 2, 3, 4, 5, 6, 7, 8], 0, 7, a)
    if flag == -1:
        print('数组中不存在%d.' % a)
    else:
        print('%d的下标是%d.' % (a, flag))

如有不当之处欢迎指出!