当たりが1つしかないくじ引きの確率

gistfile1.py

def f(num, max_num):
    """
    num: くじを引く回数
    max_num: くじの総数
    """
    p = 1.0
    for i in xrange(num):
        p = p * (float(max_num - i - 1) / (max_num - i))
    return p
    
def g(num, max_num):
    q = (1 - f(num, max_num)) * 100
    return "{0:.2f}%".format(q)