dado un numero entero positivos imrimir los digitos que no se encuentren dentro del numero

def missing_numbers(a):
  #a is a positive integer
  n = len(str(a))
  numbers = set([i for i in xrange(1, n+1)])
  appearing = set([int(x) for x in list(str(a))])
  return numbers.difference(appearing)

print missing_numbers(123)