from django.utils.cache import cc_delim_re, get_conditional_response, set_response_etag from django.utils.deprecation import MiddlewareMixin from django.utils.http import parse_http_date_safe class ConditionalGetMiddleware(MiddlewareMixin): """ Handle conditional GET operations. If the response has an ETag or Last-Modified header and the request has If-None-Match or If-Modified-Since, replace the response with HttpNotModified. Add an ETag header if needed. """ def process_response(self, request, response): # It's too late to prevent an unsafe request with a 412 response, and # for a HEAD request, the response body is always empty so computing # an accurate ETag isn't possible. if request.method != "GET": return response if self.needs_etag(response) and not response.has_header("ETag"): set_response_etag(response) etag = response.get("ETag") last_modified = response.get("Last-Modified") last_modified = last_modified and parse_http_date_safe(last_modified) if etag or last_modified: return get_conditional_response( request, etag=etag, last_modified=last_modified, response=response, ) return response def needs_etag(self, response): """Return True if an ETag header should be added to response.""" cache_control_headers = cc_delim_re.split(response.get("Cache-Control", "")) return all(header.lower() != "no-store" for header in cache_control_headers)