detect if a web page is changed

In my python application I have to read many web pages to collect data. To decrease the http calls I would like to fetch only changed pages. My problem is that my code always tells me that the pages have been changed (code 200) but in reality it is not.

This is my code:

from models import mytab
import re
import urllib2
from wsgiref.handlers import format_date_time
from datetime import datetime
from time import mktime

def url_change():
    urls = mytab.objects.all()
    # this is some urls:
    # http://www.venere.com/it/pensioni/venezia/pensione-palazzo-guardi/#reviews
    # http://www.zoover.it/italia/sardegna/cala-gonone/san-francisco/hotel
    # http://www.orbitz.com/hotel/Italy/Venice/Palazzo_Guardi.h161844/#reviews
    # http://it.hotels.com/ho292636/casa-del-miele-susegana-italia/
    # http://www.expedia.it/Venezia-Hotel-Palazzo-Guardi.h1040663.Hotel-Information#reviews
    # ...

    for url in urls:
        request = urllib2.Request(url.url)
        if url.last_date == None:
            now = datetime.now()
            stamp = mktime(now.timetuple())
            url.last_date = format_date_time(stamp)
            url.save()

        request.add_header("If-Modified-Since", url.last_date)

        try:
            response = urllib2.urlopen(request) # Make the request
            # some actions
            now = datetime.now()
            stamp = mktime(now.timetuple())
            url.last_date = format_date_time(stamp)
            url.save()
        except urllib2.HTTPError, err:
            if err.code == 304:
                print "nothing...."
            else:
                print "Error code:", err.code 
                pass

I do not understand what has gone wrong. Can anyone help me?

Web servers aren't required to send a 304 header as the response when you send an 'If-Modified-Since' header. They're free to send a HTTP 200 and send the entire page again.

Sending a 'If-Modified-Since' or 'If-None-Since' alerts the server that you'd like a cached response if available. It's like sending an 'Accept-Encoding: gzip, deflate' header -- you're just telling the server you'll accept something, not requiring it.