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[......]