Hashable mappings

Posted Dec 6, 2025 0:49 UTC (Sat) by AdamW (subscriber, #48457)
In reply to: Hashable mappings by iabervon
Parent article: A "frozen" dictionary for Python

Huh. I just read the PEP, and that's in interesting detail.

It says frozendicts will be ordered, but hashes and comparisons will not care about the order.

So frozendict({"a": "b", "c": "d"}) and frozendict({"c": "d", "a": "b"}) will have the same hash and compare as equal, but they're not really the same?

I don't know how I feel about that!

Hashable mappings

Posted Dec 6, 2025 5:23 UTC (Sat) by iabervon (subscriber, #722) [Link]

It's already the case that {"a": "b", "c": "d"} and {"c": "d", "a": "b"} compare as equal, but are distinguishable with respect to the order that iterators visit the items.

The history is that the iterator order used to be unpredictable, so the same object might give different orders when traversed multiple times and objects constructed by adding the items in different order might give the same order when traversed multiple times. However, a more recent implementation of dict started to traverse the items in the order the keys were first added, just because that was more convenient, and then the language changed to guarantee this. Of course, that meant that there was now something you could reliably determine about dicts that wasn't included in the equality rules that had always existed.