Luhn checkbit creator for text strings - Made Python3 compatible from https://wiki.openmrs.org/display/docs/Check+Digit+Algorithm

text_luhn.py

# 99.99 based on https://wiki.openmrs.org/display/docs/Check+Digit+Algorithm

def get_checkdigit(id_without_check):

    # allowable characters within identifier
    valid_chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVYWXZ_"

    # remove leading or trailing whitespace, convert to uppercase
    id_without_checkdigit = id_without_check.strip().upper()

    # this will be a running total
    sum = 0;

    # loop through digits from right to left
    for n, char in enumerate(reversed(id_without_checkdigit)):

        if not valid_chars.count(char):
            raise Exception('InvalidIDException')

        # our "digit" is calculated using ASCII value - 48
        digit = ord(char) - 48

        # weight will be the current digit's contribution to
        # the running total
        weight = None
        if (n % 2 == 0):

            # for alternating digits starting with the rightmost, we
            # use our formula this is the same as multiplying x 2 and
            # adding digits together for values 0 to 9.  Using the
            # following formula allows us to gracefully calculate a
            # weight for non-numeric "digits" as well (from their
            # ASCII value - 48).
            ## Use_sparingly: In Python 3, '/' makes floats. '//' fixes it for Python 3.
            ## For cross compatibility, simply int() the result
            ##                     VVVVVVVVVVVVV
            weight = (2 * digit) - int(digit / 5) * 9
        else:
            # even-positioned digits just contribute their ascii
            # value minus 48
            weight = digit

        # keep a running total of weights
        ## Use_sparingly: removed maths.fabs()
        ## abs() is sufficient
        sum += weight

    # avoid sum less than 10 (if characters below "0" allowed,
    # this could happen)
    sum = abs(sum) + 10

    # check digit is amount needed to reach next number
    # divisible by ten. Return an integer
    return int((10 - (sum % 10)) % 10)