urllib.error

 #捕获异常
 import urllib.request
 import urllib.error

 try:
     response = urllib.request.urlopen('http://sasd.com')
 except urllib.error.URLError as e:
     print('挂掉的情形是：',e.reason)

 import urllib.request
 import urllib.error
 try:
     response = urllib.request.urlopen('http://www.asdddwf.com')
 except urllib.error.HTTPError as e:
     print(e.reason,e.code,e.headers,sep='\n')
 except urllib.error.URLError as e:
     print(e.reason)
 else:
     print('Requset Successfully')
 #urllib.error的异常分类
 #URlError，下面有个reason
 #URLError的子类是HTTPError，下面有code,reason,headers
 #捕捉异常要把子类开始捕捉，因为一旦捕捉第一个，后面的异常判断就不会继续执行了

 #验证异常的具体原因
 import socket
 import urllib.request
 import urllib.error

 try:
     response = urllib.request.urlopen('http://www.baidu.com',timeout=0.01)
 except urllib.error.URLError as e:
     print(e.reason)
     if isinstance(e.reason,socket.timeout):
         print('TIME OUT!')