Estonian bank invoice reference number calculator

calculate_reference_number.py

"""
See https://www.pangaliit.ee/settlements-and-standards/reference-number-of-the-invoice/check-digit-calculator-of-domestic-account-number
"""
def calculate_reference_number(n):
    """
    Calculates Estonian bank invoice reference number.
    
    Argument:
    n - a string of numbers, e.g. invoice number or any other identifier chosen by the invoicer.
    
    >>> calculate_reference_number('12131295')
    '121312952'
    """
    checksum = sum(a * int(b) for a, b in zip((7, 3, 1) * 10, reversed(n)))
    checkdigit = int(round(checksum, -1)) - checksum
    return n + str(checkdigit)

if __name__ == '__main__':
    import doctest
    doctest.testmod()