text: from django.utils.http import http_date, parse_http_date_safe
  
  
  class ConditionalGetMiddleware(object):
      """
      Handles conditional GET operations. If the response has a ETag or
      Last-Modified header, and the request has If-None-Match or
      If-Modified-Since, the response is replaced by an HttpNotModified.
  
      Also sets the Date and Content-Length response-headers.
      """
      def process_response(self, request, response):
          response['Date'] = http_date()
          if not response.streaming and not response.has_header('Content-Length'):
              response['Content-Length'] = str(len(response.content))
  
          if response.has_header('ETag'):
              if_none_match = request.META.get('HTTP_IF_NONE_MATCH')
              if if_none_match == response['ETag']:
                  # Setting the status is enough here. The response handling path
                  # automatically removes content for this status code (in
                  # http.conditional_content_removal()).
                  response.status_code = 304
  
          if response.has_header('Last-Modified'):
              if_modified_since = request.META.get('HTTP_IF_MODIFIED_SINCE')
              if if_modified_since is not None:
                  if_modified_since = parse_http_date_safe(if_modified_since)
              if if_modified_since is not None:
                  last_modified = parse_http_date_safe(response['Last-Modified'])
                  if last_modified is not None and last_modified <= if_modified_since:
                      # Setting the status code is enough here (same reasons as
                      # above).
                      response.status_code = 304
  
          return response
  