'WSGIRequest' object has no attribute 'session'

Question

I get this error sometimes in custom Middleware in process_response method. I have the following list of middlewares:

MIDDLEWARE_CLASSES = [
'django.middleware.common.CommonMiddleware',
'django.contrib.sessions.middleware.SessionMiddleware',
'django.contrib.auth.middleware.AuthenticationMiddleware',
'django.middleware.doc.XViewMiddleware',
'django.contrib.flatpages.middleware.FlatpageFallbackMiddleware',
'django.contrib.messages.middleware.MessageMiddleware',
'django.middleware.csrf.CsrfViewMiddleware',
'application.middleware.LastCampaignSessionMiddleware'

]

So session middleware is placed before my middleware. I don't have any del request.session expressions in source code. This is process_response method:

    def process_response(self, request, response):
        if 'last_campaign_id' in request.session and request.session['last_campaign_id']:
            if request.COOKIES['last_campaign_id'] != request.session['last_campaign_id']:
                response.set_cookie('last_campaign_id', request.session['last_campaign_id'])
        return response

Not sure why it could happen at all.

EDIT 03-08-2012 12-30

It looks like browser requesting favicon:

[03/Aug/2012 10:26:42] "GET /favicon.ico/ HTTP/1.1" 404 6701

Is there no default behavior in django to resolve this url? Because I didn't explicitly specify view which should handle this request. And I don't use favicon in page source code. So I guess it's browser who requests /favicon.ico. I guess in case of 404 error HttpRequest wouldn't construct properly so no wonder I have no session in request object. But it's just my assumptions.

Also if it necessary I am using django dev server while getting this error.

EDIT 13-00

I have fixed this problem with favicon but still getting error. Why session may not exist in request?

Answer 1

The problem was in middlewares order.

CommonMiddleware returns HttpResponsePermanentRedirect in cases when to request url have been added 'www' or trailing '/' (APPEND_SLASH and PREPEND_WWW in settings). In such case django stops looking through middleware list for process_request methods and begins to run process_response methods.

It's bad there is no information about such behavior for standard django middlewares (i.e. middleware could return in some cases HttpResponse object).