Yet, I disagree ;) Whether a weakref is the correct thing to use or not depends entirely on the purpose of the cache. I often find myself using caches were weakref would not be very useful, because it would cool the cache a lot.

It's hard to tell the difference between a real memory leak and Python objects being accumulated infinitely in memory - at least if we rely only on the memory use of a process. That's why we need to use either gc or objgraph as a first step.