python - 检测网页是否被更改

Question

在我的 python 应用程序中，我必须阅读许多网页来收集数据。为了减少 http 调用，我只想获取更改的页面。我的问题是我的代码总是告诉我页面已更改(代码 200)，但实际上并没有。

这是我的代码:

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

我不明白哪里出了问题。谁能帮帮我？

Answer 1

当您发送“If-Modified-Since” header 时，Web 服务器不需要发送 304 header 作为响应。他们可以随意发送 HTTP 200 并再次发送整个页面。

发送“If-Modified-Since”或“If-None-Since”会提醒服务器您希望缓存响应(如果可用)。这就像发送“Accept-Encoding: gzip, deflate” header ——您只是告诉服务器您将接受某些内容，而不是要求它。