combination.py

#!/usr/bin/env python3
# coding:utf-8
import sys

def combi(n, r):
    assert r >= 0, 'r must be larger than zero'

    if n < r:
        return 0
    if r == 0:
        return 1
    if r == 1:
        return n
    return combi(n-1, r) + combi(n-1, r-1)

if __name__ == '__main__':
    if len(sys.argv) != 3:
        print('Usage: {} n r'.format(sys.argv[0]))
        sys.exit(1)

    try:
        n = int(sys.argv[1])
        r = int(sys.argv[2])
    except ValueError:
        print('n or r must be a integer')
        sys.exit(1)

    assert n >= r, 'n must be larger or equals to r'

    print(combi(n, r))