为什么这个值没有变动?

Question

class W():

headers = {"user-agent": UserAgent().random,}

def a(self):
    print(self.headers)
    pass

def b(self):
    print(self.headers)
    print(self.headers)
    pass
pass

aa = W();
aa.b()
aa.b()

结果
{'user-agent': 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2227.0 Safari/537.36'}
{'user-agent': 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2227.0 Safari/537.36'}
{'user-agent': 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2227.0 Safari/537.36'}
{'user-agent': 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2227.0 Safari/537.36'}
为什么结果 不是随机的????