ISBN-10 and ISBN-13 Check-digit calculator

isbn.py

#!/usr/bin/env python3

"""
https://en.wikipedia.org/wiki/International_Standard_Book_Number

Test values:
    978-1-4860-1097-4
    0 9596129 9 8
    978-1-4405-9334-5
    1-4405-9334-5
    978-0-522-86467-0
    0-19-506160-8
    0-415-01011-X

More test data from http://www.unionsverlag.com/isbn_2/default.html
"""

while True:
    try:
        text = input('Enter an ISBN: ')
    except EOFError:
        break

    if len(text) == 0:
        break

    isbn = ''
    for char in text:
        if char == ' ' or char == '-':
            continue
        isbn += char

    if len(isbn) == 10:
        i = 0
        total = 0
        while i < 9:
            digit = int(isbn[i])
            total += digit * (i + 1)
            i += 1
        
        if isbn[9] == 'x' or isbn[9] == 'X':
            check = 10
        else:
            check = int(isbn[9])
        
        calculated_check = (total % 11)

        if calculated_check == check:
            print('ISBN-10 {} is correct.'.format(text))
        else:
            if calculated_check < 10:
                check_char = str(calculated_check)
            else:
                check_char = 'X'
            print('ISBN-10 {} contains an error. The calculated check character is {}.' \
                .format(text, check_char))

    elif len(isbn) == 13:
        i = 0
        total = 0
        while i < 12:
            digit = int(isbn[i])

            if (i % 2) == 0:
                mod = 1
            else:
                mod = 3
            total += (digit * mod)
            i += 1
        
        check = int(isbn[12])
        calculated_check = (10 - total) % 10

        if calculated_check == check:
            print('ISBN-13 {} is correct.'.format(text))
        else:
            check_char = str(calculated_check)
            print('ISBN-13 {} contains an error. The calculated check character is {}.' \
                .format(text, check_char))

    else:
        print('ISBN must be 10 or 13 characters long.')