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Python dicts are now ordered

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Re: Python dicts are now ordered

#251

Post by rplnt »

BiteCode_dev wrote: post #227

I live in the Python bubble, so I haven't realized until this post that so few people knew about that. This post is massively popular despite talking about a feature we had since Python 3.6, in 2016, that was posted on HN at the time and that is featured in most popular tutorials. A good reminder that most of the world doesn't revolve about my favorite language. And that information is not that fast to spread.

> about a feature we had since Python 3.6, in 2016

No, you had that feature in one implementation. Now it's in the language specification. That's vastly different because only now you can rely on it without fearing it can go away with the next release.

Re: Python dicts are now ordered

#252

Post by svnpenn »

rossmohax wrote: post #247
skrebbel wrote: post #223

This is awesome, because the ordered map is the best data structure out there for easy & predictable programming, possibly only barring the array. There's so many cases where it's a benefit for map entries to retain order (and none where it's a problem). PHP really got this one right (and immediately messed it up by mixing ordered maps with arrays into a big soup, but hey, PHP). And so did, JS, sorta-kinda-by-acciden…

Python dicts are not ordered maps, they are dicts preserving insertion order.

...thats the same thing. Maybe youre thinking about "sorted maps"?

Re: Python dicts are now ordered

#253

Post by skrebbel »

pavanky wrote: post #250
skrebbel wrote: post #223

This is awesome, because the ordered map is the best data structure out there for easy & predictable programming, possibly only barring the array. There's so many cases where it's a benefit for map entries to retain order (and none where it's a problem). PHP really got this one right (and immediately messed it up by mixing ordered maps with arrays into a big soup, but hey, PHP). And so did, JS, sorta-kinda-by-acciden…

> and immediately messed it up by mixing ordered maps with arrays into a big soup, but hey, PHP And here I thought Lua was the only language insane enough to do something like that. Just realized Lua tables aren't ordered, but it still mixes arrays and maps / tables into a single data structure.

In JS's defense, it really tried as well. Fortunately though they failed harder at it, which is why arrays and objects are probably more different from one another than Eich had intended there in that mad 2 week language design frenzy. Good for us!

Back when JS got popular and the majority of web programmers knew PHP, there were dozens of tutorials out there trying to convince you not to do stuff like this:

    var a = new Array();
    a["moo"] = "five";
    a["foo"] = "six";
After all, well, it works! Like in PHP! So what's the problem? :-) Most people felt the same way about these tutorials as people feel about monad tutorials nowadays.

Good times!

Re: Python dicts are now ordered

#254

Post by ehsankia »

zurtex wrote: post #109
rcfox wrote: post #70

Earlier quoted context omitted.

Really? I would have figured they'd just do something like: class OrderedDict(dict): pass (Plus a little bit of API shimming.)

No, OrderedDict has it's own C implementation which was created just before it was decided that dict would preserve order across iteration. Further there is a big difference, regular dict preserves order across iteration but OrderedDict treats order up to equality. I.e. this returns True: {1: 1, 2: 2} == {2: 2, 1: 1} Where as this returns False: OrderedDict({1: 1, 2: 2}) == OrderedDict({2: 2, 1: 1}) To make that diff…

For the former, I realize that's how it's done now, but there's nothing forcing it to stay that way.

For the latter issue, as explained above, can't they just implement a replacement __eq__ only for OrderedDict, and still re-use the new dict implementation?

Similarly, any subtle difference can be shimmed on top of the new implementation inside Python, no?

Re: Python dicts are now ordered

#255

Post by jwkane »

anthonypasq wrote: post #241
jnwatson wrote: post #220

Earlier quoted context omitted.

Users of JSON, probably the most common data interchange format on the planet, frequently have implicit requirements about key ordering. It is highly convenient to be able to parse a JSON string into a native Python data structure, add a field, emit it back, and preserve the ordering.

in what reasonable use case would the order of the properties on an object matter? I can't think of one

when you are diffing the serialized output?

Re: Python dicts are now ordered

#256

Post by fock »

hprotagonist wrote: post #175
abetlen wrote: post #91

Earlier quoted context omitted.

TIL I suppose Python doesn't even have backwards compatibility within the same major release as we saw with the addition of the async keyword in Python3.5. Many older Python 3 packages broke because they expected that to be a legal identifier for a variable name.

tensorflow didn't work on 3.7 for a solid 8 months because some people at google very unwisely decided that `async` and `await` were great choices for variable names, despite PEP492 landing in 2015.

that's because tensorflow is advertisement for Google and while it's technically open-source, it doesn't stand for any kind of community-project, it's all there to show off (and ingrain in its users) the way Google wants things to Go (just look at the byzantine Bazel-build-processes - tensorflow taking hours to build and pytorch about 10minutes...).

Re: Python dicts are now ordered

#257

Post by reidrac »

rossmohax wrote: post #247
skrebbel wrote: post #223

This is awesome, because the ordered map is the best data structure out there for easy & predictable programming, possibly only barring the array. There's so many cases where it's a benefit for map entries to retain order (and none where it's a problem). PHP really got this one right (and immediately messed it up by mixing ordered maps with arrays into a big soup, but hey, PHP). And so did, JS, sorta-kinda-by-acciden…

Python dicts are not ordered maps, they are dicts preserving insertion order.

The title of the post is misleading: it means "ordered by insertion order" (not sorted).

Previously it was unpredictable, and that's why collections have OrderedDict; which makes sense instead of trusting an implementation detail of CPython from 3.6.

Is not mentioned in the docs: https://docs.python.org/3/library/stdtypes.html#mapping-type...

EDIT: but the behaviour is documented in OrderedDict itself! https://docs.python.org/3/library/collections.html?highlight...

Interesting!

Re: Python dicts are now ordered

#258

Post by skrebbel »

slim wrote: post #248
svnpenn wrote: post #226

Earlier quoted context omitted.

> PHP really got this one right (and immediately messed it up by mixing it with arrays, but hey, PHP). No, it didnt. What PHP calls "array" is actually an Ordered Map, as you alluded to: https://yaml.org/type/omap it literally says that in the documentation, paragraph one, sentence one: > An array in PHP is actually an ordered map. https://php.net/types.array The issue, if one exists, is that PHP doesnt have a sequen…

So how can you access, say, the third element of your ordered map in PHP ?

    array_values($arr)[2];

Re: Python dicts are now ordered

#259

Post by masklinn »

Izkata wrote: post #237
alayne wrote: post #164

Earlier quoted context omitted.

Other languages don't generally have special order guarantees about standard maps. This seems very idiosyncratic.

In regards to sibling replies, does it feel to anyone else like everything (except golang) is converging towards PHP's array() ?

Interestingly enough here it was kinda but kinda not the other way around: historically PHP used a closed-addressing hash map and threaded a doubly linked list through it to maintain the insertion order.

But the dict TFA talks about doesn't use a doubly linked list, or closed addressing, its ordering is a side-effect of its implementation but not originally a core goal (memory saving and iteration speed were). It'd probably been proposed by others before but it came to wider attention after Raymond Hettinger (a core dev) proposed it a few years back[0]. PHP actually released it first[1], closely followed by pypy[2]. CPython only got around to it some time later[3]

[0] https://mail.python.org/pipermail/python-dev/2012-December/1...

[1] https://nikic.github.io/2014/12/22/PHPs-new-hashtable-implem...

[2] https://morepypy.blogspot.com/2015/01/faster-more-memory-eff...

[3] https://docs.python.org/3/whatsnew/3.6.html?highlight=3.6#wh...

Re: Python dicts are now ordered

#260

Post by wutbrodo »

shrimpx wrote: post #169
wutbrodo wrote: post #159

Earlier quoted context omitted.

> As for people criticizing a change to what use to be a non-deterministic ordering of a dict iteration; I don't know what to say to them, other than, are you serious? There are people out there who are working for us, they work for free and they did some heavy lifting to give us this. They might read what you wrote and think, "Why bother? Maybe I should spend my weekends playing with my kids instead." I don't agree…

As it’s widely known, more often than not “criticism” of open source software quickly devolves into hate and toxicity. Helpful criticism is great but be careful with defending the “criticism culture” around foss, it’s often angry unhappy people that want it all for free on a golden platter, and yesterday.

Sure, hate and toxicity themselves should be called out. But that's not what's happening here, and not what GP comment was referring to. It doesn't make any sense to throw out the baby of legitimate criticism with the bathwater of toxicity.
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