运行我的代码时出现索引错误

Question

I am fairly new to Python and I am writing a program that can convert a seven digit number into a GTIN-8 code with a check digit also. It allows me to run it, but after I enter my number it gives me the error:

IndexError: String index out of range

My code is as follows:

sevenNum = ""
gtinNum = ""
checkDigit = ""
total = ""

a = 0
b = 0
c = 0
d = 0
e = 0
f = 0
g = 0

def GTINCalc():
    a = int(sevenNum[0])*3
    b = int(sevenNum[1])*1
    c = int(sevenNum[2])*3
    d = int(sevenNum[3])*1
    e = int(sevenNum[4])*3
    f = int(sevenNum[5])*1
    g = int(sevenNum[6])*3

    total = int(a+b+c+d+e+f+g)

    checkDigit = (total + 9) // 10 * 10 - total

    print("GTIN-8 Code: {0}{1}{2}{3}{4}{5}{6}{7}".format(a, b, c, d, e, f, g, checkDigit))


def sevenNumAsk():
    sevenNum = input("Enter a 7 digit number to be converted into a GTIN-8 Number")
    if sevenNum.isdigit() == True and len(sevenNum) == 7:
        print("Valid Number - Calculating GTIN-8...")
        GTINCalc()
    else:
        print("The number is not valid - please re-enter ")
        sevenNumAsk()

sevenNumAsk()

The part that is highlighted is:

a = int(sevenNum[0])*3

Any help is appreciated. Thanks.

Answer 1

sevenNum is a local variable inside sevenNumAsk and doesn't affect the global variable you created at the top. Do this:

def sevenNumAsk():
    global sevenNum
    sevenNum = ...

and it will act as you expect. Better yet, use a class or pass sevenNum as a parameter. Global variables are generally bad and this is one of the reasons why.