With the most recent release (1.5.3, last week) App Engine's Python API for Memcache has added a new feature, Compare-And-Set. This feature (with a different API) was already available in Java; it has also been available in the non-App-Engine pure-Python memcache client. In fact, I designed the App Engine Python API for this feature to be compatible with the latter, since most of the rest of the App Engine Python API also strives to be at least a superset of that package. But what is it? There seems to be little information on how to use Compare-And-Set with memcache. It is also sometimes (incorrectly) referred to as Compare-And-Swap -- incorrect, because the cas() operation does not actually "swap" anything.