scrambler function not working properly, might be because of difference calculator

Question

I made a python scrambler but whenever I run my program, it strangely does not print anything, however, when I close my program, the python dialogue says the program is still running. This is my code:

def differ(a1, a2):
    diff = 0
    for i in range(len(a2)):
        if a1[i] != a2[i]:
            diff += 1
    return diff

import random

def scramble(arr):
    scrambled = arr
    while differ(arr, scrambled) < len(arr)-1:
        popped = scrambled.pop(random.randrange(1, len(scrambled)))
        scrambled.insert(popped, 0)
    return scrambled

print(scramble([1,2,3]))

does anybody know what's currently happening and knows how to fix it and possibly cleanse the code out?