guard_game.py

def digit(number):
    """Add digits as long as there is more than one digit

    >>> digit(2)
    2
    >>> digit(12)
    4
    >>> digit(99)
    9
    >>> digit(83928)
    3
    >>> digit(123456789)
    9
    >>> digit(99999999999)
    9
    """
    if number < 10:
        return number

    ones = (number % 10)
    tens = (number / 10)

    return digit(tens + ones)

def digit_no_recursion(number):
    """Add digits as long as there is more than one digit

    >>> digit_no_recursion(2)
    2
    >>> digit_no_recursion(12)
    3
    >>> digit_no_recursion(99)
    9
    >>> digit_no_recursion(83928)
    3
    >>> digit_no_recursion(123456789)
    9
    >>> digit_no_recursion(99999999999)
    9
    """
    while number > 10:
        number = (number / 10) + (number % 10)

    return number

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