Fixed #22085 -- Added a feature for setting non-expiring keys as the default.

This feature allows the default `TIMEOUT` Cache argument to be set to `None`,
so that cache instances can set a non-expiring key as the default,
instead of using the default value of 5 minutes.

Previously, this was possible only by passing `None` as an argument to
the set() method of objects of type `BaseCache` (and subtypes).

docs/ref/settings.txt

@@ -255,6 +255,10 @@ Default: 300
 
 The number of seconds before a cache entry is considered stale.
 
+.. versionadded:: 1.7
+
+If the value of this settings is ``None``, cache entries will not expire.
+
 .. setting:: CACHES-VERSION
 
 VERSION

docs/releases/1.7.txt

@@ -441,6 +441,11 @@ Cache
   thread-safe any more, as :data:`django.core.cache.caches` now yields
   different instances per thread.
 
+* Defining the :setting:`TIMEOUT <CACHES-TIMEOUT>` argument of the
+  :setting:`CACHES` setting as ``None`` will set the cache keys as
+  "non-expiring" by default. Previously, it was only possible to pass
+  ``timeout=None` to the cache backend's ``set()`` method.
+
 Email
 ^^^^^
 

docs/topics/cache.txt

@@ -363,9 +363,14 @@ Each cache backend can be given additional arguments to control caching
 behavior. These arguments are provided as additional keys in the
 :setting:`CACHES` setting. Valid arguments are as follows:
 
+
 * :setting:`TIMEOUT <CACHES-TIMEOUT>`: The default timeout, in
-  seconds, to use for the cache. This argument defaults to ``300``
-  seconds (5 minutes).
+  seconds, to use for the cache. This argument defaults to ``300`` seconds (5 minutes).
+
+  .. versionadded:: 1.7
+
+  You can set ``TIMEOUT`` to ``None`` so that, by default, cache keys never
+  expire.
 
 * :setting:`OPTIONS <CACHES-OPTIONS>`: Any options that should be
   passed to the cache backend. The list of valid options will vary