#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!')