Check if the internet cannot be accessed in Python

I have an app that makes a HTTP GET request to a particular URL on the internet. But when the network is down (say, no public wifi - or my ISP is down, or some such thing), I get the following traceback at urllib2.urlopen:

70, in get
    u = urllib2.urlopen(req)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 126, in urlopen
    return _opener.open(url, data, timeout)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 391, in open
    response = self._open(req, data)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 409, in _open
    '_open', req)
  File "/Library开发者_运维问答/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 369, in _call_chain
    result = func(*args)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 1161, in http_open
    return self.do_open(httplib.HTTPConnection, req)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/urllib2.py", line 1136, in do_open
    raise URLError(err)
URLError: <urlopen error [Errno 8] nodename nor servname provided, or not known>

I want to print a friendly error to the user telling him that his network maybe down instead of this unfriendly "nodename nor servname provided" error message. Sure I can catch URLError, but that would catch every url error, not just the one related to network downtime.

I am not a purist, so even an error message like "The server example.com cannot be reached; either the server is indeed having problems or your network connection is down" would be nice. How do I go about selectively catching such errors? (For a start, if DNS resolution fails at urllib2.urlopen, that can be reasonably assumed as network inaccessibility? If so, how do I "catch" it in the except block?)

You should wrap the request in a try/except statement so that you catch the fault and then let them know.

try:
   u = urllib2.urlopen(req)
except HTTPError as e:
   #inform them of the specific error here (based off the error code)
except URLError as e:
   #inform them of the specific error here
except Exception as e:
   #inform them that a general error has occurred 

urllib2 - The Missing Manual has a good section on how to handle URLError and HTTPError exceptions and how to differentiate the conditions that caused them.

How about catching URLError, then testing the reason attribute? If the reason isn't one you're interested in, re-throw the URLError and handle it somewhere else.

Alternatively, you could try httplib2. Its ServerNotFoundError exception would probably suit your needs.