[ollysb]

This seems a bit leaky. You're exposing the caching mechanism in the method signature(yeah, ok, in practice it's unlikely to be a problem).

[anthonyb]

The other option is to create your own cache object and pass that in (and around, if it's a recursive function). Of course, in Python pretty much every cache object follows the dictionary interface anyway, so it doesn't really matter. One of the benefits of duck typing :)