404 errors

[John J. Lee]

Ivan Karajas <my_full_name_concatenated at myrealbox.com> writes:

> On Tue, 27 Apr 2004 10:46:47 +0200, Tut wrote:
> 
> > Tue, 27 Apr 2004 11:00:57 +0800, Derek Fountain wrote:
> > 
> >> Some servers respond with a nicely formatted bit of HTML explaining the
> >> problem, which is fine for a human, but not for a script. Is there some
> >> flag or something definitive on the response which says "this is a 404
> >> error"?
> > 
> > Maybe catch the urllib2.HTTPError?
> 
> This kind of answers the question. urllib will let you read whatever it
> receives, regardless of the HTTP status; you need to use urllib2 if you
> want to find out the status code when a request results in an error (any
> HTTP status beginning with a 4 or 5). This can be done like so:

FWIW, note that urllib2's own idea of an error (ie. something for
which it throws a response object as an HTTPError exception rather
than returning it) is: 'anything other than 200 is an error'.  The
only exceptions are where some responses happen to be handled by
urllib2 handlers (eg. 302), or at a lower level by httplib (eg. 100).


> import urllib2
> try:
>     asock = urllib2.urlopen("http://www.foo.com/qwerty.html")
> except urllib2.HTTPError, e:
>     print e.code
> 
> The value in urllib2.HTTPError.code comes from the first line of the web
> server's HTTP response, just before the headers begin, e.g. "HTTP/1.1 200
> OK", or "HTTP/1.1 404 Not Found".
> 
> One thing you need to be aware of is that some web sites don't behave as
> you would expect them to; e.g. responding with a redirection rather than a
> 404 error when you when you request a page that doesn't exist. In these
> cases you might still have to rely on some clever scripting.

The following kind of functionality is in urllib2 in Python 2.4 (there
are some loose ends, which I will tie up soon).  It's slightly simpler
in 2.4 than in my ClientCookie clone of that module, but (UNTESTED):

import ClientCookie
from ClientCookie._Util import response_seek_wrapper

class BadResponseProcessor(ClientCookie.BaseProcessor):
    # Convert apparently-successful 200 OK or 30x redirection responses to 404s
    # iff they contain tell-tale text that indicates failure.

    def __init__(self, diagnostic_text):
        self.diagnostic_text = diagnostic_text

    def http_response(self, request, response):
        if not hasattr(response, "seek"):
            response = response_seek_wrapper(response)

        if response.code in [200, 301, 302, 303, 307]:
            ct = response.info().getheaders("content-type")
            if ct and ct[0].startswith("text/html"):
                try:
                    data = response.read(4096)
                    if self.diagnostic_text in data:
                        response.code = 404
                finally:
                    response.seek(0)
        return response

    https_response = http_response

brp = BadResponseProcessor("Whoops, an error occurred.")
opener = ClientCookie.build_opener(brp)

r = opener.open("http://nonstandard.com/bad/url")
assert r.code == 404


Hmm, looking at that, I suppose it would be better done *after*
redirection (which is quite possible, with the modifications I've
made, without needing any heavy subclassing or other hacks -- use the
processor_order attribute).  You'd then just check for 200 rather than
200 or 30x in the code above.

A similar problem: as I mention above, by default, urllib2 only
returns 200 responses, and always raises an exception for other HTTP
response codes.  Occasionally, it's much more convenient to have an
OpenerDirector that behaves differently:

class HTTPErrorProcessor(ClientCookie.HTTPErrorProcessor):
    # return most error responses rather than raising an exception

    def http_response(self, request, response):
        code, msg, hdrs = response.code, response.msg, response.info()

        category = divmod(code, 100)[0]  # eg. 200 --> 2
        if category not in [2, 4, 5] or code in [401, 407]:
            response = self.parent.error(
                'http', request, response, code, msg, hdrs)

        return response

    https_response = http_response


John