If your application is threaded and you're getting a "KeyError" while using the non-decorator version of cachetool's LRUCache, then you need to put whatever is manipulating the cache object inside of a lock. Also, since LRUCache is modified when values are gotten from it, you will also need to make sure you're locking when you get values from cache too. If you can use the decorator version of LRUCache, that's preferred since it has built-in locking.
Here's an example of the error:
And an example of the fix: https://bitbucket.org/zzzeek/dogpile.cache/pull-requests/32/add-a-cachetools-lru-lfu-in-memory-backend/diff#comment-22242704