https://www.codewars.com/kata/5659c6d896bc135c4c00021e

next_smaller.py

def list2Int(l):
    n = 0
    for i in range(len(l)):
        n += l[i] * (10 ** (len(l)-i-1))
    return n

def next_smaller(n):
    n = list(str(n))
    n = [int(x) for x in n]

    for s in range(2, len(n)+1):
        c = len(n)-s
        if n[c] > n[c+1]:
            l = n[c:]
            l.sort(reverse=True)
            i = 0
            while(l[i] >= n[c]):
                i += 1
            n[c] = l[i]
            l = l[:i] + l[i+1:]
            n[c+1:] = l
            if(n[0] != 0):
                return list2Int(n)
            else:
                break
    return -1