# Τεκμηρίωση του Django - 1.10.7

# Πηγαίος κώδικας για το django.utils.cache

    """
    This module contains helper functions for controlling caching. It does so by
    managing the "Vary" header of responses. It includes functions to patch the
    header of response objects directly and decorators that change functions to do
    that header-patching themselves.

    For information on the Vary header, see:

        https://tools.ietf.org/html/rfc7231#section-7.1.4

    Essentially, the "Vary" HTTP header defines which headers a cache should take
    into account when building its cache key. Requests with the same path but
    different header content for headers named in "Vary" need to get different
    cache keys to prevent delivery of wrong content.

    An example: i18n middleware would need to distinguish caches by the
    "Accept-language" header.
    """
    from __future__ import unicode_literals

    import hashlib
    import logging
    import re
    import time

    from django.conf import settings
    from django.core.cache import caches
    from django.http import HttpResponse, HttpResponseNotModified
    from django.utils.encoding import force_bytes, force_text, iri_to_uri
    from django.utils.http import (
        http_date, parse_etags, parse_http_date_safe, quote_etag,
    )
    from django.utils.timezone import get_current_timezone_name
    from django.utils.translation import get_language

    cc_delim_re = re.compile(r'\s*,\s*')

    logger = logging.getLogger('django.request')

    def patch_cache_control(response, **kwargs):
        """
        This function patches the Cache-Control header by adding all
        keyword arguments to it. The transformation is as follows:

        * All keyword parameter names are turned to lowercase, and underscores
          are converted to hyphens.
        * If the value of a parameter is True (exactly True, not just a
          true value), only the parameter name is added to the header.
        * All other parameters are added with their value, after applying
          str() to it.
        """
        def dictitem(s):
            t = s.split('=', 1)
            if len(t) > 1:
                return (t[0].lower(), t[1])
            else:
                return (t[0].lower(), True)

        def dictvalue(t):
            if t[1] is True:
                return t[0]
            else:
                return '%s=%s' % (t[0], t[1])

        if response.get('Cache-Control'):
            cc = cc_delim_re.split(response['Cache-Control'])
            cc = dict(dictitem(el) for el in cc)
        else:
            cc = {}

        # If there's already a max-age header but we're being asked to set a new
        # max-age, use the minimum of the two ages. In practice this happens when
        # a decorator and a piece of middleware both operate on a given view.
        if 'max-age' in cc and 'max_age' in kwargs:
            kwargs['max_age'] = min(int(cc['max-age']), kwargs['max_age'])

        # Allow overriding private caching and vice versa
        if 'private' in cc and 'public' in kwargs:
            del cc['private']
        elif 'public' in cc and 'private' in kwargs:
            del cc['public']

        for (k, v) in kwargs.items():
            cc[k.replace('_', '-')] = v
        cc = ', '.join(dictvalue(el) for el in cc.items())
        response['Cache-Control'] = cc

    def get_max_age(response):
        """
        Returns the max-age from the response Cache-Control header as an integer
        (or ``None`` if it wasn't found or wasn't an integer.
        """
        if not response.has_header('Cache-Control'):
            return
        cc = dict(_to_tuple(el) for el in cc_delim_re.split(response['Cache-Control']))
        if 'max-age' in cc:
            try:
                return int(cc['max-age'])
            except (ValueError, TypeError):
                pass

    def set_response_etag(response):
        if not response.streaming:
            response['ETag'] = quote_etag(hashlib.md5(response.content).hexdigest())
        return response

    def _precondition_failed(request):
        logger.warning(
            'Precondition Failed: %s', request.path,
            extra={
                'status_code': 412,
                'request': request,
            },
        )
        return HttpResponse(status=412)

    def _not_modified(request, response=None):
        if response:
            # We need to keep the cookies, see ticket #4994.
            cookies = response.cookies
            response = HttpResponseNotModified()
            response.cookies = cookies
            return response
        else:
            return HttpResponseNotModified()

    def get_conditional_response(request, etag=None, last_modified=None, response=None):
        # Get HTTP request headers
        if_modified_since = request.META.get('HTTP_IF_MODIFIED_SINCE')
        if if_modified_since:
            if_modified_since = parse_http_date_safe(if_modified_since)
        if_unmodified_since = request.META.get('HTTP_IF_UNMODIFIED_SINCE')
        if if_unmodified_since:
            if_unmodified_since = parse_http_date_safe(if_unmodified_since)
        if_none_match = request.META.get('HTTP_IF_NONE_MATCH')
        if_match = request.META.get('HTTP_IF_MATCH')
        etags = []
        if if_none_match or if_match:
            # There can be more than one ETag in the request, so we
            # consider the list of values.
            try:
                etags = parse_etags(if_none_match or if_match)
            except ValueError:
                # In case of an invalid ETag, ignore all ETag headers.
                # Apparently Opera sends invalidly quoted headers at times
                # (we should be returning a 400 response, but that's a
                # little extreme) -- this is bug #10681.
                if_none_match = None
                if_match = None

        # If-None-Match must be ignored if original result would be anything
        # other than a 2XX or 304 status. 304 status would result in no change.
        # http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.26
        if response and not (200 <= response.status_code < 300):
            if_none_match = None
            if_match = None

        # If-Modified-Since must be ignored if the original result was not a 200.
        # http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.25
        if response and response.status_code != 200:
            if_modified_since = None
            if_unmodified_since = None

        if not ((if_match and if_modified_since) or
                (if_none_match and if_unmodified_since) or
                (if_modified_since and if_unmodified_since) or
                (if_match and if_none_match)):
            # We only get here if no undefined combinations of headers are
            # specified.
            if ((if_none_match and (etag in etags or '*' in etags and etag)) and
                    (not if_modified_since or
                        (last_modified and if_modified_since and last_modified <= if_modified_since))):
                if request.method in ('GET', 'HEAD'):
                    return _not_modified(request, response)
                else:
                    return _precondition_failed(request)
            elif (if_match and (
                (not etag and '*' in etags) or (etag and etag not in etags) or
                (last_modified and if_unmodified_since and last_modified > if_unmodified_since)
            )):
                return _precondition_failed(request)
            elif (not if_none_match and request.method in ('GET', 'HEAD') and
                    last_modified and if_modified_since and
                    last_modified <= if_modified_since):
                return _not_modified(request, response)
            elif (not if_match and
                    last_modified and if_unmodified_since and
                    last_modified > if_unmodified_since):
                return _precondition_failed(request)

        return response

    def patch_response_headers(response, cache_timeout=None):
        """
        Adds some useful headers to the given HttpResponse object:
            ETag, Last-Modified, Expires and Cache-Control

        Each header is only added if it isn't already set.

        cache_timeout is in seconds. The CACHE_MIDDLEWARE_SECONDS setting is used
        by default.
        """
        if cache_timeout is None:
            cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS
        if cache_timeout < 0:
            cache_timeout = 0  # Can't have max-age negative
        if settings.USE_ETAGS and not response.has_header('ETag'):
            if hasattr(response, 'render') and callable(response.render):
                response.add_post_render_callback(set_response_etag)
            else:
                response = set_response_etag(response)
        if not response.has_header('Last-Modified'):
            response['Last-Modified'] = http_date()
        if not response.has_header('Expires'):
            response['Expires'] = http_date(time.time() + cache_timeout)
        patch_cache_control(response, max_age=cache_timeout)

    def add_never_cache_headers(response):
        """
        Adds headers to a response to indicate that a page should never be cached.
        """
        patch_response_headers(response, cache_timeout=-1)
        patch_cache_control(response, no_cache=True, no_store=True, must_revalidate=True)

    def patch_vary_headers(response, newheaders):
        """
        Adds (or updates) the "Vary" header in the given HttpResponse object.
        newheaders is a list of header names that should be in "Vary". Existing
        headers in "Vary" aren't removed.
        """
        # Note that we need to keep the original order intact, because cache
        # implementations may rely on the order of the Vary contents in, say,
        # computing an MD5 hash.
        if response.has_header('Vary'):
            vary_headers = cc_delim_re.split(response['Vary'])
        else:
            vary_headers = []
        # Use .lower() here so we treat headers as case-insensitive.
        existing_headers = set(header.lower() for header in vary_headers)
        additional_headers = [newheader for newheader in newheaders
                              if newheader.lower() not in existing_headers]
        response['Vary'] = ', '.join(vary_headers + additional_headers)

    def has_vary_header(response, header_query):
        """
        Checks to see if the response has a given header name in its Vary header.
        """
        if not response.has_header('Vary'):
            return False
        vary_headers = cc_delim_re.split(response['Vary'])
        existing_headers = set(header.lower() for header in vary_headers)
        return header_query.lower() in existing_headers

    def _i18n_cache_key_suffix(request, cache_key):
        """If necessary, adds the current locale or time zone to the cache key."""
        if settings.USE_I18N or settings.USE_L10N:
            # first check if LocaleMiddleware or another middleware added
            # LANGUAGE_CODE to request, then fall back to the active language
            # which in turn can also fall back to settings.LANGUAGE_CODE
            cache_key += '.%s' % getattr(request, 'LANGUAGE_CODE', get_language())
        if settings.USE_TZ:
            # The datetime module doesn't restrict the output of tzname().
            # Windows is known to use non-standard, locale-dependent names.
            # User-defined tzinfo classes may return absolutely anything.
            # Hence this paranoid conversion to create a valid cache key.
            tz_name = force_text(get_current_timezone_name(), errors='ignore')
            cache_key += '.%s' % tz_name.encode('ascii', 'ignore').decode('ascii').replace(' ', '_')
        return cache_key

    def _generate_cache_key(request, method, headerlist, key_prefix):
        """Returns a cache key from the headers given in the header list."""
        ctx = hashlib.md5()
        for header in headerlist:
            value = request.META.get(header)
            if value is not None:
                ctx.update(force_bytes(value))
        url = hashlib.md5(force_bytes(iri_to_uri(request.build_absolute_uri())))
        cache_key = 'views.decorators.cache.cache_page.%s.%s.%s.%s' % (
            key_prefix, method, url.hexdigest(), ctx.hexdigest())
        return _i18n_cache_key_suffix(request, cache_key)

    def _generate_cache_header_key(key_prefix, request):
        """Returns a cache key for the header cache."""
        url = hashlib.md5(force_bytes(iri_to_uri(request.build_absolute_uri())))
        cache_key = 'views.decorators.cache.cache_header.%s.%s' % (
            key_prefix, url.hexdigest())
        return _i18n_cache_key_suffix(request, cache_key)

    def get_cache_key(request, key_prefix=None, method='GET', cache=None):
        """
        Returns a cache key based on the request URL and query. It can be used
        in the request phase because it pulls the list of headers to take into
        account from the global URL registry and uses those to build a cache key
        to check against.

        If there is no headerlist stored, the page needs to be rebuilt, so this
        function returns None.
        """
        if key_prefix is None:
            key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX
        cache_key = _generate_cache_header_key(key_prefix, request)
        if cache is None:
            cache = caches[settings.CACHE_MIDDLEWARE_ALIAS]
        headerlist = cache.get(cache_key)
        if headerlist is not None:
            return _generate_cache_key(request, method, headerlist, key_prefix)
        else:
            return None

    def learn_cache_key(request, response, cache_timeout=None, key_prefix=None, cache=None):
        """
        Learns what headers to take into account for some request URL from the
        response object. It stores those headers in a global URL registry so that
        later access to that URL will know what headers to take into account
        without building the response object itself. The headers are named in the
        Vary header of the response, but we want to prevent response generation.

        The list of headers to use for cache key generation is stored in the same
        cache as the pages themselves. If the cache ages some data out of the
        cache, this just means that we have to build the response once to get at
        the Vary header and so at the list of headers to use for the cache key.
        """
        if key_prefix is None:
            key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX
        if cache_timeout is None:
            cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS
        cache_key = _generate_cache_header_key(key_prefix, request)
        if cache is None:
            cache = caches[settings.CACHE_MIDDLEWARE_ALIAS]
        if response.has_header('Vary'):
            is_accept_language_redundant = settings.USE_I18N or settings.USE_L10N
            # If i18n or l10n are used, the generated cache key will be suffixed
            # with the current locale. Adding the raw value of Accept-Language is
            # redundant in that case and would result in storing the same content
            # under multiple keys in the cache. See #18191 for details.
            headerlist = []
            for header in cc_delim_re.split(response['Vary']):
                header = header.upper().replace('-', '_')
                if header == 'ACCEPT_LANGUAGE' and is_accept_language_redundant:
                    continue
                headerlist.append('HTTP_' + header)
            headerlist.sort()
            cache.set(cache_key, headerlist, cache_timeout)
            return _generate_cache_key(request, request.method, headerlist, key_prefix)
        else:
            # if there is no Vary header, we still need a cache key
            # for the request.build_absolute_uri()
            cache.set(cache_key, [], cache_timeout)
            return _generate_cache_key(request, request.method, [], key_prefix)

    def _to_tuple(s):
        t = s.split('=', 1)
        if len(t) == 2:
            return t[0].lower(), t[1]
        return t[0].lower(), True