python - Using pickle.dumps to hash mutable objects

Question

I understand why putting mutable objects in a dictionary is dangerous. However, converting all lists/sets to tuples/frozensets is expensive; and for many types, there's no easily available immutable versions at all. So, sometimes it may be worth hashing mutable objects directly, with the appropriate precautions to ensure the objects in question are never modified.

Before I start implementing the very complicated custom hash functions for mutable objects, I wanted to check if there's any disadvantage to using pickle.dumps as the hash function - either in terms of performance or collisions or anything else.

Answer 1

The output from pickle is not guaranteed to be canonical for similar reasons to dict and set order being non-deterministic. Don't use pickle or pprint or repr for hashing.