Python boolean expression always true (when it shouldn't be)

I have a method that is intended to grab all img elements from some html and to add a css style to ensure the image is resized if it is large. It works great until the final test : largest_size < img_size - I have tried all manner of different ways to express this simple thing, and yet it always evaluates to true - which means all images are resized regardless of their original size.

The code:

    def adjust_html(self, html_text):
    # pull image links and adjust those larger than 30k
    # to be width=100%
    html = 开发者_如何学运维etree.HTML(html_text)
    r = html.xpath('.//img')
    changed_text = False
    for elem in r:
        for tag, value in elem.attrib.iteritems():
            if tag == 'src':
                largest_size = 30720
                img_size = 0
                img_url = value
                if self.bad_urls.has_key(img_url):
                    break
                try:
                    usock = urllib2.urlopen(img_url)
                    img_size =  usock.info().get('Content-Length')
                except:
                    self.log.debug("***** 406 for " + img_url)
                    self.bad_urls[img_url] = True
                    break
                if img_size is None:
                    break
                else:
                    **if (largest_size < img_size):**
                        self.log.debug("*** " + img_url + " ***")
                        self.log.debug("********** img size = " + str(img_size) + " **********")
                        elem.set("style","width:100%")
                        changed_text = True
                break

    if changed_text == True:
        html_text = etree.tostring(html)

    return html_text

I know there has to be something simple wrong here - I just don't see it :)

int is always less than str. Turn your header value into an int first. And remember, debug with repr(), not str().

img_size is a string not an int:

>>> 30720 < '0'
True