python判断url资源是否存在

目前有一批图片地址，有些已经被删掉了，以下用python查找出还存在的图片资源

import urllib2
from urllib2 import URLError

f=open("images.txt","r")
img_exist=open("exist.txt","w+")
img_not_exist=open("notexist.txt","w+")

for line in f:
    try:
        response=urllib2.urlopen(line)
    except URLError, e:
        img_not_exist.write(line)
    else:
        img_exist.write(line)
        response.close()
    finally:
        pass

f.close()
img_exist.close()
img_not_exist.close()