256 lines
8.0 KiB
Python
256 lines
8.0 KiB
Python
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"""
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Caching framework.
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This module defines set of cache backends that all conform to a simple API.
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In a nutshell, a cache is a set of values -- which can be any object that
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may be pickled -- identified by string keys. For the complete API, see
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the abstract Cache object, below.
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Client code should not access a cache backend directly; instead
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it should use the get_cache() function. This function will look at
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settings.CACHE_BACKEND and use that to create and load a cache object.
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The CACHE_BACKEND setting is a quasi-URI; examples are:
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memcached://127.0.0.1:11211/ A memcached backend; the server is running
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on localhost port 11211.
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pgsql://tablename/ A pgsql backend (the pgsql backend uses
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the same database/username as the rest of
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the CMS, so only a table name is needed.)
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file:///var/tmp/django.cache/ A file-based cache at /var/tmp/django.cache
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simple:/// A simple single-process memory cache; you
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probably don't want to use this except for
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testing. Note that this cache backend is
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NOT threadsafe!
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All caches may take arguments; these are given in query-string style. Valid
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arguments are:
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timeout
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Default timeout, in seconds, to use for the cache. Defaults
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to 5 minutes (300 seconds).
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max_entries
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For the simple, file, and database backends, the maximum number of
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entries allowed in the cache before it is cleaned. Defaults to
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300.
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cull_percentage
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The percentage of entries that are culled when max_entries is reached.
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The actual percentage is 1/cull_percentage, so set cull_percentage=3 to
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cull 1/3 of the entries when max_entries is reached.
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A value of 0 for cull_percentage means that the entire cache will be
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dumped when max_entries is reached. This makes culling *much* faster
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at the expense of more cache misses.
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For example:
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memcached://127.0.0.1:11211/?timeout=60
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pgsql://tablename/?timeout=120&max_entries=500&cull_percentage=4
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Invalid arguments are silently ignored, as are invalid values of known
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arguments.
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So far, only the memcached and simple backend have been implemented; backends
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using postgres, and file-system storage are planned.
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"""
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##############
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# Exceptions #
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##############
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class InvalidCacheBackendError(Exception):
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pass
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################################
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# Abstract base implementation #
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################################
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class _Cache:
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def __init__(self, params):
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timeout = params.get('timeout', 300)
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try:
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timeout = int(timeout)
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except (ValueError, TypeError):
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timeout = 300
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self.default_timeout = timeout
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def get(self, key, default=None):
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'''
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Fetch a given key from the cache. If the key does not exist, return
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default, which itself defaults to None.
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'''
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raise NotImplementedError
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def set(self, key, value, timeout=None):
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'''
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Set a value in the cache. If timeout is given, that timeout will be
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used for the key; otherwise the default cache timeout will be used.
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'''
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raise NotImplementedError
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def delete(self, key):
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'''
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Delete a key from the cache, failing silently.
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'''
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raise NotImplementedError
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def get_many(self, keys):
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'''
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Fetch a bunch of keys from the cache. For certain backends (memcached,
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pgsql) this can be *much* faster when fetching multiple values.
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Returns a dict mapping each key in keys to its value. If the given
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key is missing, it will be missing from the response dict.
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'''
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d = {}
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for k in keys:
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val = self.get(k)
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if val is not None:
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d[k] = val
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return d
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def has_key(self, key):
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'''
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Returns True if the key is in the cache and has not expired.
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'''
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return self.get(key) is not None
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###########################
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# memcached cache backend #
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###########################
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try:
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import memcache
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except ImportError:
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_MemcachedCache = None
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else:
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class _MemcachedCache(_Cache):
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"""Memcached cache backend."""
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def __init__(self, server, params):
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_Cache.__init__(self, params)
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self._cache = memcache.Client([server])
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def get(self, key, default=None):
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val = self._cache.get(key)
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if val is None:
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return default
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else:
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return val
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def set(self, key, value, timeout=0):
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self._cache.set(key, value, timeout)
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def delete(self, key):
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self._cache.delete(key)
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def get_many(self, keys):
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return self._cache.get_multi(keys)
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##################################
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# Single-process in-memory cache #
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##################################
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import time
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class _SimpleCache(_Cache):
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"""Simple single-process in-memory cache"""
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def __init__(self, host, params):
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_Cache.__init__(self, params)
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self._cache = {}
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self._expire_info = {}
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max_entries = params.get('max_entries', 300)
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try:
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self._max_entries = int(max_entries)
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except (ValueError, TypeError):
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self._max_entries = 300
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cull_frequency = params.get('cull_frequency', 3)
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try:
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self._cull_frequency = int(cull_frequency)
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except (ValueError, TypeError):
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self._cull_frequency = 3
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def get(self, key, default=None):
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now = time.time()
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exp = self._expire_info.get(key, now)
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if exp is not None and exp < now:
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del self._cache[key]
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del self._expire_info[key]
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return default
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else:
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return self._cache.get(key, default)
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def set(self, key, value, timeout=None):
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if len(self._cache) >= self._max_entries:
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self._cull()
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if timeout is None:
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timeout = self.default_timeout
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self._cache[key] = value
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self._expire_info[key] = time.time() + timeout
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def delete(self, key):
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try:
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del self._cache[key]
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except KeyError:
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pass
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try:
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del self._expire_info[key]
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except KeyError:
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pass
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def has_key(self, key):
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return self._cache.has_key(key)
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def _cull(self):
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if self._cull_frequency == 0:
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self._cache.clear()
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self._expire_info.clear()
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else:
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doomed = [k for (i, k) in enumerate(self._cache) if i % self._cull_frequency == 0]
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for k in doomed:
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self.delete(k)
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##########################################
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# Read settings and load a cache backend #
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##########################################
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from cgi import parse_qsl
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_BACKENDS = {
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'memcached' : _MemcachedCache,
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'simple' : _SimpleCache,
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}
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def get_cache(backend_uri):
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if backend_uri.find(':') == -1:
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raise InvalidCacheBackendError("Backend URI must start with scheme://")
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scheme, rest = backend_uri.split(':', 1)
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if not rest.startswith('//'):
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raise InvalidCacheBackendError("Backend URI must start with scheme://")
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if scheme not in _BACKENDS.keys():
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raise InvalidCacheBackendError("%r is not a valid cache backend" % scheme)
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host = rest[2:]
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qpos = rest.find('?')
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if qpos != -1:
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params = dict(parse_qsl(rest[qpos+1:]))
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host = rest[:qpos]
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else:
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params = {}
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if host.endswith('/'):
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host = host[:-1]
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return _BACKENDS[scheme](host, params)
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from django.conf.settings import CACHE_BACKEND
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cache = get_cache(CACHE_BACKEND)
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