用python验证6174猜想时出现这样的问题，是什么导致不能循环呢

def f(n):      #将四位数各位取出放入列表
    x=n//1000
    y=n//100%10
    z=n%100//10
    m=n%10
    l=[]
    l.append(x)
    l.append(y)
    l.append(z)
    l.append(m)
    return l
def min(l):
     l.sort(reverse=True)
     n1 = ''
     for i in l:
         n1 += str(i)
     return int(n1)

def max(l):

     l.sort(reverse=False)
     n2 = ''
     for i in l:
         n2 += str(i)
     return int(n2)


def jian(n1,n2):
     t=(n2-n1)
     return t

n=int(input('请输入一个四位数（各个数位不相等）：'))
l=f(n)
x=min(l)
y=max(l)
a=jian(x,y)
j=1
if a!=6174:
    print("Round[{}]：".format(j))
    print('the maxone is:{}，'.format(x))
    print('the minone is:{}，'.format(y))
    print('the diff is:{}'.format(a))
    list=f(a)
    x = min(list)
    y = max(list)
    a = jian(x, y)
    j+=1
else:
    print("Round[{}]：". format(j))
    print('the maxone is:{}，'.format(x))
    print('the minone is:{}，'.format(y))
    print('the diff is:{}'.format(a))

你的for循环或while循环哪去了？