保留日志文件最新的多少行(python)

 #!/usr/bin/env python
# coding:utf-8
import os,sys
filename="1.txt"
hang=0
with open(filename) as f:
    for i in f:
        hang+=1
hang=hang-100     #要保留最后多少行
num=0
with open(filename) as f:
    with open("new{}".format(filename),"w") as fw:
        for i in f:

            if num>hang:
                print num
                fw.write(i)
            num+=1
os.remove(filename)
os.rename("new{}".format(filename),filename)