Generating random, valid, polish tax payer identification number with simple Python snippet

RandomNIP.py

def random_nip(self):
  res = ''
  sum = 0
  weights = [6, 5, 7, 2, 3, 4, 5, 6, 7]
  for i in range(8):
    k = self.rand.randint(1 if i < 3 else 0, 9)
    sum += weights[i] * k
    res += str(k)
  k = self.rand.randint(0, 9)
    if (sum + (k * weights[8])) % 11 == 10:
      k += (1 if k + 1 < 10 else -1)
  res += str(k)
  sum += k * weights[8]
  res += str(sum % 11)
  return res