Fixed #19810 -- MemcachedCache now uses pickle.HIGHEST_PROTOCOL

django/core/cache/backends/memcached.py

 "Memcached cache backend"
 
 import time
+import pickle
 from threading import local
 
 from django.core.cache.backends.base import BaseCache, InvalidCacheBackendError
@@ -146,6 +147,12 @@ class MemcachedCache(BaseMemcachedCache):
                                              library=memcache,
                                              value_not_found_exception=ValueError)
 
+    @property
+    def _cache(self):
+        if getattr(self, '_client', None) is None:
+            self._client = self._lib.Client(self._servers, pickleProtocol=pickle.HIGHEST_PROTOCOL)
+        return self._client
+
 class PyLibMCCache(BaseMemcachedCache):
     "An implementation of a cache binding using pylibmc"
     def __init__(self, server, params):

docs/releases/1.6.txt

@@ -89,6 +89,9 @@ Minor features
   :class:`~django.http.HttpResponsePermanentRedirect` now provide an ``url``
   attribute (equivalent to the URL the response will redirect to).
 
+* The ``MemcachedCache`` cache backend now uses the latest :mod:`pickle`
+  protocol available.
+
 * Added the :attr:`django.db.models.ForeignKey.db_constraint`
   option.
 

tests/regressiontests/cache/tests.py

@@ -12,6 +12,7 @@ import string
 import tempfile
 import time
 import warnings
+import pickle
 
 from django.conf import settings
 from django.core import management
@@ -984,6 +985,18 @@ class MemcachedCacheTests(unittest.TestCase, BaseCacheTests):
         # memcached limits key length to 250
         self.assertRaises(Exception, self.cache.set, 'a' * 251, 'value')
 
+    # Explicitly display a skipped test if no configured cache uses MemcachedCache
+    @unittest.skipUnless(
+        any(cache['BACKEND'] == 'django.core.cache.backends.memcached.MemcachedCache'
+            for cache in settings.CACHES.values()),
+        "cache with python-memcached library not available")
+    def test_memcached_uses_highest_pickle_version(self):
+        # Regression test for #19810
+        for cache_key, cache in settings.CACHES.items():
+            if cache['BACKEND'] == 'django.core.cache.backends.memcached.MemcachedCache':
+                self.assertEqual(get_cache(cache_key)._cache.pickleProtocol,
+                                 pickle.HIGHEST_PROTOCOL)
+
 
 class FileBasedCacheTests(unittest.TestCase, BaseCacheTests):
     """