1 files changed, 27 insertions, 1 deletions
diff --git a/django/core/cache/backends/base.py b/django/core/cache/backends/base.py
index e58267a2e9..83dd461804 100644
--- a/django/core/cache/backends/base.py
+++ b/django/core/cache/backends/base.py
@@ -1,10 +1,18 @@
 "Base Cache class."
 
-from django.core.exceptions import ImproperlyConfigured
+import warnings
+
+from django.core.exceptions import ImproperlyConfigured, DjangoRuntimeWarning
 
 class InvalidCacheBackendError(ImproperlyConfigured):
     pass
 
+class CacheKeyWarning(DjangoRuntimeWarning):
+    pass
+
+# Memcached does not accept keys longer than this.
+MEMCACHE_MAX_KEY_LENGTH = 250
+
 class BaseCache(object):
     def __init__(self, params):
         timeout = params.get('timeout', 300)
@@ -116,3 +124,21 @@ class BaseCache(object):
     def clear(self):
         """Remove *all* values from the cache at once."""
         raise NotImplementedError
+
+    def validate_key(self, key):
+        """
+        Warn about keys that would not be portable to the memcached
+        backend. This encourages (but does not force) writing backend-portable
+        cache code.
+
+        """
+        if len(key) > MEMCACHE_MAX_KEY_LENGTH:
+            warnings.warn('Cache key will cause errors if used with memcached: '
+                    '%s (longer than %s)' % (key, MEMCACHE_MAX_KEY_LENGTH),
+                    CacheKeyWarning)
+        for char in key:
+            if ord(char) < 33 or ord(char) == 127:
+                warnings.warn('Cache key contains characters that will cause '
+                        'errors if used with memcached: %r' % key,
+                              CacheKeyWarning)
+