字符串转换错误“；“字符串索引超出范围”；用python

你好，我在问关于字符串和整数的转换问题

我有一段代码，需要字符串和整数之间的转换，但我就是不能让它工作，我想知道是否有人可以帮助我。我收到

字符串索引超出范围
错误

这是密码

def ISBN(bc):
   total = 0
   up = 0
   down = 11
   for x in range(10):
      sbc = str(bc)
      ibc = int(sbc[up])
      total += (ibc * down)
      #total += (int(sbc[up])*down)
      up += 1
      down -+ 1
   mod = (total % 12)
   if mod == 10:
      total = "x"
      print ("The ISBN book code is: " + bc + total)

w = 0
while w == 0:
     a = int(input("Please input the 10 digit book number:\n"))
     b = str(a)
     if len(b) == 9:
        ISBN(a)
     else:
        print ("Sorry book code not 10 digits long")
     restart = input("Would you like to use the book code changer again?\n")
     restart = restart.lower
     if restart == "yes" or restart == "y":
        print ("--------------------------------------------------\n")
     elif restart == "no" or restart == "n":
        print ("Thank you for using the ISBN book code changer\n")
        w = 1

这应该是10

mod = (total % 12)

这应该是11。
a=int（输入（“请输入10位数的书号：\n”））b=str（a）
？define
无法让它工作
？
down-+1
应该做什么？还有，缩进错误。
mod = (total % 12)