# Django 2.2.9.dev20191202104323 文档

# django.middleware.http 源代码

    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)