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)
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