'''二分查找有序不重复数组
BinarySearch(unfinished).py'''

# 未完成版
def BinarySearch(sortedlist):
    num = int(input('Number:\t'))
    counter = 0  # 记录循环次数
    mid = 0  # 记录中间位置

    if num in [sortedlist[0], sortedlist[len(sortedlist) - 1]]:  # 切割时取不到首尾
        print('Find it at beginning or end.')
    else:
        while True:  # 切割数组
            mid = len(sortedlist) // 2
            if sortedlist[mid] == num:
                print('Find number {}.'.format(num))
                break
            elif sortedlist[mid] > num:
                sortedlist = sortedlist[:(mid + 1)]
            else:
                sortedlist = sortedlist[mid:]
            print(sortedlist, mid)

            counter += 1
            if counter > len(sortedlist) + 1:  # 超过迭代次数即停止
                print('Don\'t find it.')
                break

sortedlist = [-2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]
BinarySearch(sortedlist)

补充个增序或者减序都能用的版本：

# 二分查找（非递归版本）
# isAsc为True时，列表增序，否则减序
def binarySearch(sortedList, key, isAsc=True):
    left = 0                                             # 查找范围最左边的下标
    right = len(sortedList) - 1                          # 查找范围最右边的下标
    while left <= right:                                 #
        mid = left + right >> 1                          # 中间数下标（向下取整）
        if isAsc and sortedList[mid] < key \
                or not isAsc and sortedList[mid] > key:  # key在中间数右边时，查找范围变为右边
            left = mid + 1
        elif isAsc and sortedList[mid] > key \
                or not isAsc and sortedList[mid] < key:  # key在中间数左边时，查找范围变为左边
            right = mid - 1
        else:
            return mid                                   # key为中间数时，返回其下标
    return -1