ISBN 12 to 13 digit

I'm trying to return the ISBN-13 check digit from a 12-digit ISBN but have encountered a bizarre error.

Take the following as my 12-digit ISBN - 978311020318

Each digit is alternatively multiplied by 1 or 3.

9*1 + 7*3 + 8*1 + 3*3 + 1*1 + 1*3 + 0*1 + 2*3 + 0*1 + 3*3 + 1*1 + 8*3 = 91

91 % 10 = 1

10 - 1 = 9, which is our ISBN-13 check digit.

This is what I have so far...

def isbn_check_digit(isbn):
    s = 0
    for i, d in enumerate(isbn):
        if i % 2 == 0:
            s += int(d*1)
        else:
            s += int(d*3)
            print(s)
    return (10 - (s % 10))

print(isbn_check_digit("978311020318"))

Which outputs the following...

I broke it up to see what was happening

    if i % 2 == 0:
        s += int(d*1)
        print(s)
    else:
        s += 0

9
17
18
18
18
19
1

Multiples of 1 are working fine, but why is it behaving strangely with multiples of 3?

Solution

Because, at the time that the expression int(d * 3) is evaluated, d is not a number — it's a string consisting of one character, which is a digit. As a result, "multiplying" it by 3 will repeat the digit three times: for instance, 3 will turn into 333, rather than 9.

You want to multiply by 3 after converting the string to an integer:

int(d) * 3