python - Django: Client-side caching using etags -

more questions work through lightweight django. in chapter 2, book talks client-side caching using django.views.decorators.http.etag decorator. wrote the following view:

import hashlib import pil.imagedraw imagedraw django.core.cache import cache django.http import httpresponse, httpresponsebadrequest django import forms io import bytesio pil import image django.views.decorators.http import etag  class imageform(forms.form):     """form vlaidate requested placeholder image."""     height = forms.integerfield(min_value=1, max_value=2000)     width = forms.integerfield(min_value=1, max_value=2000)      def generate(self, image_format="png"):         """generate image of given format , return raw bytes"""         height = self.cleaned_data["height"]         width = self.cleaned_data["width"]         image = image.new("rgb", (width, height)) # color optional          # create key cached content         cache_key = "{} x {} x {}".format(height, width, image_format)          # add text around image being served         draw = imagedraw.draw(image)         text = "{} x {}".format(width, height)          textwidth, textheight = draw.textsize(text)  # text size of text         if textwidth < width , textheight < height:             texttop = (height - textheight) // 2             textleft = (width - textwidth)  // 2             draw.text((textleft, texttop), text, fill=(255, 255, 255))          content = bytesio()            image.save(content, image_format)         content.seek(0)         cache.set(cache_key, content, 60 * 60)         return(content)  def etag_placeholder(request, width, height):     content_to_hash = "placeholder: {} x {}".format(width, height)     return(hashlib.sha1(content_to_hash.encode("utf-8")).hexdigest())  @etag(etag_placeholder) def placeholder(request, width, height):     form = imageform({'height': height, "width": width})     if form.is_valid():         image = form.generate()         return(httpresponse(image, content_type="image/png"))     else:         return(httpresponsebadrequest("incorrect image dimensions requested.")) 

however, view refuses cache, , continue 200 responses -- expecting see 304s. there setting missing?