# documentación de Django - 1.10.7

# Código fuente para django.middleware.cache

    """
    Cache middleware. If enabled, each Django-powered page will be cached based on
    URL. The canonical way to enable cache middleware is to set
    ``UpdateCacheMiddleware`` as your first piece of middleware, and
    ``FetchFromCacheMiddleware`` as the last::

        MIDDLEWARE = [
            'django.middleware.cache.UpdateCacheMiddleware',
            ...
            'django.middleware.cache.FetchFromCacheMiddleware'
        ]

    This is counter-intuitive, but correct: ``UpdateCacheMiddleware`` needs to run
    last during the response phase, which processes middleware bottom-up;
    ``FetchFromCacheMiddleware`` needs to run last during the request phase, which
    processes middleware top-down.

    The single-class ``CacheMiddleware`` can be used for some simple sites.
    However, if any other piece of middleware needs to affect the cache key, you'll
    need to use the two-part ``UpdateCacheMiddleware`` and
    ``FetchFromCacheMiddleware``. This'll most often happen when you're using
    Django's ``LocaleMiddleware``.

    More details about how the caching works:

    * Only GET or HEAD-requests with status code 200 are cached.

    * The number of seconds each page is stored for is set by the "max-age" section
      of the response's "Cache-Control" header, falling back to the
      CACHE_MIDDLEWARE_SECONDS setting if the section was not found.

    * This middleware expects that a HEAD request is answered with the same response
      headers exactly like the corresponding GET request.

    * When a hit occurs, a shallow copy of the original response object is returned
      from process_request.

    * Pages will be cached based on the contents of the request headers listed in
      the response's "Vary" header.

    * This middleware also sets ETag, Last-Modified, Expires and Cache-Control
      headers on the response object.

    """

    from django.conf import settings
    from django.core.cache import DEFAULT_CACHE_ALIAS, caches
    from django.utils.cache import (
        get_cache_key, get_max_age, has_vary_header, learn_cache_key,
        patch_response_headers,
    )
    from django.utils.deprecation import MiddlewareMixin

    class UpdateCacheMiddleware(MiddlewareMixin):
        """
        Response-phase cache middleware that updates the cache if the response is
        cacheable.

        Must be used as part of the two-part update/fetch cache middleware.
        UpdateCacheMiddleware must be the first piece of middleware in MIDDLEWARE
        so that it'll get called last during the response phase.
        """
        def __init__(self, get_response=None):
            self.cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS
            self.key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX
            self.cache_alias = settings.CACHE_MIDDLEWARE_ALIAS
            self.cache = caches[self.cache_alias]
            self.get_response = get_response

        def _should_update_cache(self, request, response):
            return hasattr(request, '_cache_update_cache') and request._cache_update_cache

        def process_response(self, request, response):
            """Sets the cache, if needed."""
            if not self._should_update_cache(request, response):
                # We don't need to update the cache, just return.
                return response

            if response.streaming or response.status_code != 200:
                return response

            # Don't cache responses that set a user-specific (and maybe security
            # sensitive) cookie in response to a cookie-less request.
            if not request.COOKIES and response.cookies and has_vary_header(response, 'Cookie'):
                return response

            # Try to get the timeout from the "max-age" section of the "Cache-
            # Control" header before reverting to using the default cache_timeout
            # length.
            timeout = get_max_age(response)
            if timeout is None:
                timeout = self.cache_timeout
            elif timeout == 0:
                # max-age was set to 0, don't bother caching.
                return response
            patch_response_headers(response, timeout)
            if timeout:
                cache_key = learn_cache_key(request, response, timeout, self.key_prefix, cache=self.cache)
                if hasattr(response, 'render') and callable(response.render):
                    response.add_post_render_callback(
                        lambda r: self.cache.set(cache_key, r, timeout)
                    )
                else:
                    self.cache.set(cache_key, response, timeout)
            return response

    class FetchFromCacheMiddleware(MiddlewareMixin):
        """
        Request-phase cache middleware that fetches a page from the cache.

        Must be used as part of the two-part update/fetch cache middleware.
        FetchFromCacheMiddleware must be the last piece of middleware in MIDDLEWARE
        so that it'll get called last during the request phase.
        """
        def __init__(self, get_response=None):
            self.key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX
            self.cache_alias = settings.CACHE_MIDDLEWARE_ALIAS
            self.cache = caches[self.cache_alias]
            self.get_response = get_response

        def process_request(self, request):
            """
            Checks whether the page is already cached and returns the cached
            version if available.
            """
            if request.method not in ('GET', 'HEAD'):
                request._cache_update_cache = False
                return None  # Don't bother checking the cache.

            # try and get the cached GET response
            cache_key = get_cache_key(request, self.key_prefix, 'GET', cache=self.cache)
            if cache_key is None:
                request._cache_update_cache = True
                return None  # No cache information available, need to rebuild.
            response = self.cache.get(cache_key)
            # if it wasn't found and we are looking for a HEAD, try looking just for that
            if response is None and request.method == 'HEAD':
                cache_key = get_cache_key(request, self.key_prefix, 'HEAD', cache=self.cache)
                response = self.cache.get(cache_key)

            if response is None:
                request._cache_update_cache = True
                return None  # No cache information available, need to rebuild.

            # hit, return cached response
            request._cache_update_cache = False
            return response

    class CacheMiddleware(UpdateCacheMiddleware, FetchFromCacheMiddleware):
        """
        Cache middleware that provides basic behavior for many simple sites.

        Also used as the hook point for the cache decorator, which is generated
        using the decorator-from-middleware utility.
        """
        def __init__(self, get_response=None, cache_timeout=None, **kwargs):
            self.get_response = get_response
            # We need to differentiate between "provided, but using default value",
            # and "not provided". If the value is provided using a default, then
            # we fall back to system defaults. If it is not provided at all,
            # we need to use middleware defaults.

            try:
                key_prefix = kwargs['key_prefix']
                if key_prefix is None:
                    key_prefix = ''
            except KeyError:
                key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX
            self.key_prefix = key_prefix

            try:
                cache_alias = kwargs['cache_alias']
                if cache_alias is None:
                    cache_alias = DEFAULT_CACHE_ALIAS
            except KeyError:
                cache_alias = settings.CACHE_MIDDLEWARE_ALIAS
            self.cache_alias = cache_alias

            if cache_timeout is None:
                cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS
            self.cache_timeout = cache_timeout
            self.cache = caches[self.cache_alias]