ISBN Checknumber Calculator

isbn.py

# -*- coding: UTF-8 -*-
import sys

class InvalidInput(Exception):
    def __init__(self, error):
        self.error = error
    def __str__(self):
        return self.error

try:
    ISBNInput = input("ISBN Nummer: ")
    
    if len(str(ISBNInput)) != 9:
        raise InvalidInput(ISBNInput)
    
    a, b, c, d, e, f, g, h, i = str(ISBNInput)
    
    a = int(a)
    b = int(b)
    c = int(c)
    d = int(d)
    e = int(e)
    f = int(f)
    g = int(g)
    h = int(h)
    i = int(i)
    
    checknumcalc = (a*1+b*2+c*3+d*4+e*5+f*6+g*7+h*8+i*9)%11
    
    if checknumcalc == 10:
        pruefziffer = "X"
    else:
        pruefziffer = str(checknumcalc)
    print("ISBN Prüfziffer: " + pruefziffer)
except KeyboardInterrupt:
    sys.exit(1)
except InvalidInput:
    print "Deine Eingabe ist ungültig."
    sys.exit(1)