I just realized a bug in the code I wrote for hash code caching (#426). Because the hash code cache field gets serialized and deserialized by Pickle, when you deserialize a cache_hash=True attrs object, the hashcode will be the hashcode the object had at serialization-time. However, if your object has fields with hash codes which are not deterministic between interpreter runs, then on a new interpreter run your deserialized object will have a hash code which differs from a newly created identical object.

We can fix this for pickle by recomputing the hash code in __setstate__. Other serialization libraries which don't respect __setstate__ will still have a problem, but I don't think we can do anything about that. If the __setstate__ solution sounds acceptable I will implement it next week.