l = [1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31]

def find(l ,aim ,start = 0,end = None):

    end = len(l) if end is None else end    #end的问题解决

    mid_index = ( end - start )//2 + start  #中间数下标问题的解决

    if start <=end:                         #找不到问题的解决

        if l[mid_index]<aim:

            return find(l ,aim ,start = mid_index+1,end = end )

        elif l[mid_index]>aim:

            return find(l,aim, start = start,end = mid_index - 1)

        else:

            return mid_index

    else:

        return '没有找到'

#l = [1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31]

ret = find(l,27)

print(ret)

#注意，是len(l)，还有end = end ,start = start，+1-1要注意