Calculate EPO Check Digit given the application number as a string

checkdigit.py

def calculate_checkdigit(appln_number):
    """Calculate EPO checkdigit."""
    def digits_of(n):
        return [int(d) for d in str(n)]
    digits = digits_of(appln_number)
    even_digits = digits[-1::-2]
    odd_digits = digits[-2::-2]
    checksum = 0
    checksum += sum(odd_digits)
    for d in even_digits:
        checksum += sum(digits_of(d*2))
    check_digit = 10 - (checksum % 10)
    if check_digit == 10:
        check_digit = 0
    return check_digit