Calculating the checksum digit with the Luhn-algorithm

I'm doing some exercises in Python and this one is about using the Luhn-algorithm to calculate the checksum digit in Swedish personal identification numbers.

The description is the same as this question:


Given a string of 9 digits, abcdefghi compute:

array = [a*2, b, c*2, d, e*2, f, g*2, h, i*2]

Then you compute the sum of the digits in this array, so for example
if a*2 is 16, then that part counts as 1 + 6


Then the result is how much more you have to add to make it evenly
divisible by 10. For example, if sum == 54 then the result is 6 as
60 - 54 = 6.

def control_numb(nbr_string):
    cut_string = nbr_string[:9]  ## We need 9 numbers to calculate the 10th
    mult_numb = [2,1]            ## we should alternate multiplying 2 and 1 starting with 2
    i = 0                        ## holds the index of which mult_numt to use. starting with index 0

    sum_of_digits = 0
    for s in cut_string:         ## loop through the string of digits
        for d in str(int(s) * mult_numb[i]):  ## use s as int to do the multiplication. then turn back to str to loop through digit
            sum_of_digits += int(d)
        i = (i+1)%2

    return (10 - sum_of_digits % 10) % 10

I'm pretty new to Python and would very much like feedback, if there are any pythonic way to do things please spread the knowledge. Or just tear the code apart completely. Any comments are welcome!

Some comments:

• Variable names could be improved a little bit. For example, I think nbr_string could be better as swedish_id

• Please add a docstring (PEP257)

• itertools.cycle is useful to iterate multiple times over the same sequence (in this case, the multiplication factors)

• Use list comprehensions where possible instead of for loops

Things that might be different from the question's code:

• I'm assuming the initial string has 9 digits, but if it has already 10, then go with your code to keep only the first 9 numbers, but note that you can use some assertions to be sure the data passed is correct.

• I've calculated partial sums to apply the algorithm as in the explanation, but doing the sums as in the question should be fine.

Please find my solution based on your code below:

from itertools import cycle

def control_number(swedish_id):
    """Calculate control number in Swedish personal IDs

    :param swedish_id: A Swedish personal ID
    :type swedish_id: str

    """
    assert swedish_id.isdigit()
    assert len(swedish_id) == 9

    # Multiplication factors for each digit depending on its position
    mult_factors = cycle([2, 1])

    def partial_sum(number, mult_factor):
        """Calculate partial sum ofr a single digit."""
        quotient, remainder = divmod(number * mult_factor, 10)
        return quotient + remainder

    final_sum = sum(
        partial_sum(int(character), mult_factor)
        for character, mult_factor in zip(swedish_id, mult_factors))

    # Calculate control number based on partial sums
    control_number = -final_sum % 10

    return control_number

I hope this helps.

StackExchange Code Review Q#57916, answer score: 10