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的問題解決
    mid_index = ( end - start )//2 + start  #中間數下標問題的解決
    if start <=end:                         #找不到問題的解決
        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 '沒有找到'
#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)，還有end = end ,start = start，+1-1要注意