django/middleware/http.py
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)