convert isbn10 to isbn13 and accounts for 8,9 digit isbn without leading zeroes

isbns.py

def check_digit_10(isbn):
    assert len(isbn) == 9
    sum = 0
    for i in range(len(isbn)):
        c = int(isbn[i])
        w = i + 1
        sum += w * c
    r = sum % 11
    if r == 10: return 'X'
    else: return str(r)

def check_digit_13(isbn):
    assert len(isbn) == 12
    sum = 0
    for i in range(len(isbn)):
        c = int(isbn[i])
        if i % 2: w = 3
        else: w = 1
        sum += w * c
    r = 10 - (sum % 10)
    if r == 10: return '0'
    else: return str(r)

def convert_10_to_13(isbn):

    char_len = len(isbn)
    if char_len == 13:
        return isbn
    elif char_len == 8:
        prefix = '97800' + isbn[:-1]
        check = check_digit_13(prefix)
        return prefix + check
    elif char_len == 9:
        prefix = '9780' + isbn[:-1]
        check = check_digit_13(prefix)
        return prefix + check
    elif char_len == 10:
        prefix = '978' + isbn[:-1]
        check = check_digit_13(prefix)
        return prefix + check


fname = 'isbns.txt'
with open(fname) as f:
    isbns = f.readlines()

for isbn in isbns:
    print convert_10_to_13(isbn.rstrip().lstrip())