Hacker News Reader

Any hashmap will have problems with storing mutable objects, where the hash value itself can change after the object has been stored, causing some very unwanted paradoxes.

And that is the problem with Python dicts as keys.

View on HN

I don't think mutability is a concern if your goal is to get a bidi map between strings and (mutable) _instances_; i.e. if you just want to use the reference to the dict itself as the hash rather than the contents of the dict itself.

Yeah, I don't actually know if Python computes hashes for dicts by content or "identity".

I just assumed it was the former, based on the fact that dicts are disallowed as keys.

It’s been a while, but I’m pretty sure the reason why a dictionary can’t have a dictionary as it’s key is because the dictionary class doesn’t implement the __hash__ method that would compute the hashcode.

This is definitely the proximate reason.

The reason for that choice is because the object is mutable.

Sure, but as others have mentioned, you can make your own choice by subclassing dict and defining your own hash function, as long as that function is defined in a way that follows the appropriate rules.

Reply on news.ycombinator.com