> When the returned mutable object is modified, the cache is modified as well. In my opinion, functools.lru_cache should store a deep copy of the returned object.

It would be inefficient to deep copy the mutable result and can defeat the purpose of the cache...

I would rather to add a note to the documentation to explain to either not return mutable objects or to not modify them :-)
https://docs.python.org/dev/library/functools.html#functools.lru_cache