又名余弦相似度，代码如下：

import math

def cos_dist(a, b):
    if len(a) != len(b):
        return None
    part_up = 0.0
    a_sq = 0.0
    b_sq = 0.0
    for a1, b1 in zip(a,b):
        part_up += a1*b1
        a_sq += a1**2
        b_sq += b1**2
    part_down = math.sqrt(a_sq*b_sq)
    if part_down == 0.0:
        return None
    else:
        return part_up / part_down

if __name__ == "__main__":
    #d1 = (0.5, 0.8, 0.3)
    #d2 = (0.9, 0.4, 0.2)
    #q = (1.5, 1.0, 0)
    d1 = (0.1, 0, 0.3)
    d2 = (0, 0.9, 0.4)
    q = (0, 0.8, 0.5)
    print cos_dist(d1, q)
    print cos_dist(d2, q)