isbn13.py

def check_isbn13(isbn, debugging=False):
    """
    # Since ISBN13s have 978 at the start, we can check for this and calculate it seperately.
    Also, we dont need any hyphens as the order of hyphens does not matter after the first one.

    https://isbn-information.com/the-13-digit-isbn.html

    """
    isbn_str = remove_hyphens(str(isbn))
    if len(isbn_str) != 13:

        if debugging:
            print("isbn13 check: Invalid length")

        return False

    if debugging:
        print(isbn, isbn_str)

    first_three = isbn_str[:3]
    if first_three != "978":
        return False
    remaining = isbn_str[3:-1]
    base, tripled, normal = 38, 0, 0  # 38 is the constant from calculating values from the starting 978

    if debugging:
        print("\nSTATUS", "i", "x", "x*3")
    for i in range(0, len(remaining)):
        if (i + 1) % 2 == 1:
            tripled = tripled + int(remaining[i]) * 3

            if debugging:
                print("TRIPLE", i, remaining[i], int(remaining[i]) * 3)
        else:
            normal = normal + int(remaining[i])

            if debugging:
                print("NORMAL", i, remaining[i], remaining[i])

    total = base + tripled + normal
    check_digit = int(isbn_str[12])  # last character of the ISBN provided
    mod = total % 10

    if debugging:
        print("\n100 mod", total, "=", mod)

    calculated_digit = 10 - mod

    if debugging:
        print("calculation:", calculated_digit, "\nlast character:", check_digit)
    if calculated_digit == check_digit:
        if debugging:
            print("\nEQUAL\n==========")
        return True
    else:
        if debugging:
            print("\nNOT EQUAL\n==========")
        return False


def remove_hyphens(text, debugging=False):
    out_text = text.replace("-", "")
    if debugging:
        print(text, ">>>", out_text)
    return out_text