python如何批量把时间戳转换成北京时间

#! /usr/bin/env python
#coding=utf-8


import time

kaishi = time.time()
for y in open("time.txt","r"):
    x = time.localtime(int(y.strip()[:10]))
    ok = time.strftime('%Y-%m-%d %H:%M:%S',x)
    f = open("timeok.txt","a")
    f.write(ok+"
")
    f.close()
print time.time()-kaishi