python设置urllib的urlopen超时

python设置urllib的urlopen超时
就是在调用urlopen之前调用socket.setdefaulttimeout(2)
如：

#coding:utf-8
import os, urllib, socket

socket.setdefaulttimeout(2)
try:
    output = urllib.urlopen("")
    print "interface ok."
except:
    print "error"