header If-Modified-Since does not give 304 code

Question

I am using below code to save an html file with a time stamp in its name:

import contextlib
import datetime
import urllib2
import lxml.html
import os
import os.path
timestamp=''
filename=''
for dirs, subdirs, files in os.walk("/home/test/Desktop/"):
    for f in files:
        if "_timestampedfile.html" in f.lower():
            timestamp=f.split('_')[0]
            filename=f
            break
if timestamp is '': 
    timestamp=datetime.datetime.now()

with contextlib.closing(urllib2.urlopen(urllib2.Request(
        "http://www.google.com",
        headers={"If-Modified-Since": timestamp}))) as u:
    if u.getcode() != 304:
        myfile="/home/test/Desktop/"+str(datetime.datetime.now())+"_timestampedfile.html"
        file(myfile, "w").write(urllib2.urlopen("http://www.google.com").read())
        if os.path.isfile("/home/test/Desktop/"+filename):
        os.remove("/home/test/Desktop/"+filename)
        html = lxml.html.parse(myfile)
    else:
        html = lxml.html.parse("/home/test/Desktop/"+timestamp+"_timestampedfile.html")

links=html.xpath("//a/@href")
print u.getcode()

When I run this code every time I get the code 200 from If-Modified-since header. Where am I doing mistake? My goal here is to save and use an html file and if it is modified after last time it is accessed, html file should be overwritten.

Answer 1

The problem is that If-Modified-Since is supposed to be a formatted date string:

If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

but you're passing in a datetime tuple.

Try something like this:

timestamp = time.time()
...
time.strftime('%a, %d %b %Y %H:%M:%S GMT', time.gmtime(timestamp))

---

The second reason your code isn't working as you expect:

http://www.google.com/ does not seem to honor If-modified-since. That's allowed per the RFC, and they may have various reasons for choosing that behavior.

  c) If the variant has not been modified since a valid If-
     Modified-Since date, the server SHOULD return a 304 (Not
     Modified) response.

If you try http://www.stackoverflow.com/, for example, you'll see a 304. (I just tried it.)