(IncPy author here ...)
yup agreed, but the programmer needs to figure out:
1.) when it's safe to memoize
2.) when it's worthwhile to memoize
also, your memoization decorator doesn't save data to disk. if you wrote a persistent memoizer, then you would need to also track all dependencies for the data you memoized, so that you can know when it's safe to invalidate on-disk cache entries.
IncPy takes care of all of this automatically ;)