From d23dad5778b3610a5f870b4757ba628780924dd1 Mon Sep 17 00:00:00 2001 From: Nick Pope Date: Thu, 17 Dec 2020 07:26:30 +0100 Subject: [PATCH] Refs #30181 -- Corrected note about storing None in the cache. --- docs/topics/cache.txt | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-) diff --git a/docs/topics/cache.txt b/docs/topics/cache.txt index 66551fc8fe..afab1cb61e 100644 --- a/docs/topics/cache.txt +++ b/docs/topics/cache.txt @@ -839,9 +839,21 @@ If the object doesn't exist in the cache, ``cache.get()`` returns ``None``:: >>> cache.get('my_key') None -We advise against storing the literal value ``None`` in the cache, because you -won't be able to distinguish between your stored ``None`` value and a cache -miss signified by a return value of ``None``. +If you need to determine whether the object exists in the cache and you have +stored a literal value ``None``, use a sentinel object as the default:: + + >>> sentinel = object() + >>> cache.get('my_key', sentinel) is sentinel + False + >>> # Wait 30 seconds for 'my_key' to expire... + >>> cache.get('my_key', sentinel) is sentinel + True + +.. admonition:: ``MemcachedCache`` + + Due to a ``python-memcached`` limitation, it's not possible to distinguish + between stored ``None`` value and a cache miss signified by a return value + of ``None`` on the deprecated ``MemcachedCache`` backend. ``cache.get()`` can take a ``default`` argument. This specifies which value to return if the object doesn't exist in the cache::