python二分法

l1 = [1, 2, 3, 4, 5, 6, 7, 8, 9]
left = 0
right = len(l1) - 1
a = int(input("Please input a number:"))

while left <= right:
    mid = (left + right) // 2
    if a > l1[mid]:
        left = mid + 1
    elif a < l1[mid]:
        right = mid - 1
    else:
        print(str(a)+"is in the index of "+str(mid))
        break
else:
    print('404 NOT FOUND')