Why is Python's OrderedDict ordered?

Posted by misonic 10/30/2025

Why is Python's OrderedDict ordered?(www.piglei.com)

13 points | 17 comments

addaon 10/30/2025|

Title of the article is "How…", not "Why…", and "why" is not discussed. (My understanding is that the "why" is "because the implementation acted that way without an official guarantee, and folks depended on the implementation detail, so it became guaranteed.)

recursivecaveat 11/4/2025||

I was under that impression as well, but it was added as an implementation in 3.6 (https://bugs.python.org/issue27350), so I don't know that too many people would have become dependent on it before it was official in 3.7: https://docs.python.org/3/whatsnew/3.7.html

I think the "how/why" thing is just the automatic title de-clickbait-ifer going a little haywire?

DemocracyFTW2 10/30/2025|||

I don't think this is the "why". What you're depicting is what happened in JavaScript. Dictionary keys in Python always had that (to me) annoying property that they preserved insertion order until they don't. I'd frankly much prefer if they'd always be iterated in random order each time they're traversed.

not_kurt_godel 11/4/2025||

I'd love to see the results of mandating a random order dict impl at an actual company/org (but hate to be forced to participate). Hopefully you hired developers who really like to write sorting algos.

nielsbot 11/4/2025|||

Swift (heavily used by Apple) has randomly ordered dictionaries for security:

> In particular, random seeding enables better protection against (accidental or deliberate) hash-flooding attacks

https://forums.swift.org/t/psa-the-stdlib-now-uses-randomly-...

not_kurt_godel 11/4/2025||

Perhaps not unrelated to why Python is the #1 most popular language while Swift is #22 https://www.tiobe.com/tiobe-index/

nielsbot 11/4/2025||

Swift isn’t popular because its Dictionary type uses randomly ordered keys?

not_kurt_godel 11/4/2025|||

It certainly could be a reason among many. Just look at the thread GP shared, containing multiple years' worth of users voicing frustration at the introduction of this behavior.

cyanydeez 11/4/2025|||

Probably the inference is YAGNI .

yxhuvud 11/4/2025|||

Well, that is how hash tables in go works, so you'd not have to look that far.

tasty_freeze 11/4/2025|||

Perl since 5.8.something has had the option of perturbing the hash function, so it is different from run to run. You can also set the set to a given value in order to lock in the sequence.

In any case, it is not ordered. If you want that, you have to explicitly sort the keys of the hash.

not_kurt_godel 11/4/2025|||

Great. Maybe GP will go a step farther and also mandate arrays that return elements in random order too. Relying on insertion order for any reason is for weaklings.

snthpy 11/4/2025|||

Thanks, How makes much more sense. The post title is dumb.

Why is Hackernews news for hackers?

bradchoate 11/4/2025||

The URL for the post includes "why" (en-why-is-python-ordereddict-ordered), so I suppose the title of the article was updated after the HN post was created. The site's native language is Chinese and I'm guessing the post was translated via automation. In fact, if you run the article through an automated translation service (Google Translate), it reproduces the "Why" title.

snthpy 11/4/2025||

Thanks, makes sense.

more_corn 11/4/2025||

^ came here to say exactly this

DemocracyFTW2 10/30/2025|

Wrong title, article has "How Does Python’s OrderedDict Maintain Order?"