This is 9.

nine.py

#!/usr/bin/env python
# coding: UTF-8

import argparse

def nine(x):
    retval = x # <--- the given integer

    # side note: recursion can always be converted to a loop
    # particularly tail recursion like this would have been
    while retval > 10:
        # the sum of all the digits
        digit_total = sum([ int(i) for i in str(retval) ])

        # subtract the sum of the digits of the current return value
        retval -= digit_total

    return retval # <--- this is 9

if __name__ == '__main__':
    parser = argparse.ArgumentParser(description="this is 9")
    parser.add_argument("some_number", type=int)

    args = parser.parse_args()

    if int( args.some_number ) < 10:
        print "given number must be a positive integer greater than 9"
        exit(1)

    print nine(args.some_number)

~$ ~/code/python/nine.py 72
9
~$