Calculate FIK 71 checksum with python

def _calculate_ocr_checksum(ocr_code):
    """
        Inspired from https://danskebank.dk/PDF/Blanketter/ERHVERV/Indbetalingskort/Fremstilling_af_71-kort.pdf
    :param ocr_code: string
    :return: int
    """
    counter = 0
    should_multiply = (len(ocr_code) % 2) == 1

    for c in ocr_code:
        v = int(c) * 2 if should_multiply else int(c)
        counter += sum(map(int, str(v)))  # sum of digits
        should_multiply = not should_multiply

    return 10 - (counter % 10)

be aware that this code will return '10' instead of '0'