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Python dicts are now ordered

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Re: Python dicts are now ordered

#331

Post by takeda »

rplnt wrote: post #251
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.

That still was announced a while ago. When reading the title I though it was something new (maybe now Python decided to keep the keys sorted? ;)

Re: Python dicts are now ordered

#332

Post by gsnedders »

inimino wrote: post #312
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…

> JS objects are ordered maps So currently (now that there are Symbols in JS), the order is: array-indexed properties first (integers), then string-key properties, in insertion order, then Symbol-keyed properties in insertion order. The reason for the funny behavior in treating integer keys differently is that property keys are always treated as strings, so obj["3"] and obj[3] can't be distinguished, and arrays are a…

> The reason for the funny behavior in treating integer keys differently is that property keys are always treated as strings, so obj["3"] and obj[3] can't be distinguished, and arrays are also ordinary objects, so setting obj[3] was made to do the same thing on any object rather than special-casing arrays and non-array plain objects...

Note that special-casing arrays is precisely what implementations used to do (i.e., for non-Array objects, the order in most VMs was simply "properties in insertion order").

V8 was the first JS VM to stop special-casing Array (and this was done before Chrome went public, and was the behaviour in the first Chrome beta). It turned out that virtually no websites relied on "array index" properties appearing in enumeration order (there was some breakage, but it was relatively few and far between, and believed to be worthwhile for the gains in cache-hits when accessing properties), and this also allowed the more compact array representations to be used for them.

Re: Python dicts are now ordered

#333

Post by mattkrause »

kingdomcome50 wrote: post #278
skrebbel wrote: post #242

Earlier quoted context omitted.

Sure, PHP arrays are actually ordered maps, but it's super confusing when used as arrays. I mean, look at code like this: $arr = []; $arr[2] = "two"; $arr[0] = "zero"; $arr[1] = "one"; echo join($arr, ", "); // two, zero, one In any other language, the result of similar code would be "zero, one, two". You'd need to `ksort` this thing to get it to behave like a normal bog-standard boring array. Everybody's writing PHP…

I disagree. The behavior you exhibit makes more sense than the behavior you imagine. You start with an _empty_ array, then (somehow) set the third element of that array? That makes very little sense. Had you _initialized_ your example as an array with a length of 3, your desired behavior would, indeed, manifest.

In that situation, Matlab allocates an array of length 3, fills it with “empty” values (depending on the type), and then sets the third element to the value. That’s what I’d have expected to happen here too...

Re: Python dicts are now ordered

#334

Post by gshulegaard »

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.

I have lived in Python ~80% of my career (~50% now) currently), although a large chunk of my career was writing 2.7 compatible with 2.6 (thanks RHEL). For the last year and a half I have written 3.6+ exclusively and I didn't realize that regular dicts were now ordered.

I don't use dicts if I require ordering, but if I did I would probably still reach for collections.OrderedDict...

I wonder what the implications are of dict ordering with the "new" (new for me...) function args and *kwargs...from the hip it sounds pretty handy.

Re: Python dicts are now ordered

#336

Post by jrochkind1 »

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…

Ruby hashes are currently ordered and have been for quite a while now. Ruby 1.9 maybe?

But back in the day they indeed were not, for appropriate values of back in the day. :)

it did make a LOT of things more convenient and less buggy when they made ruby hashes ordered, I recall. I didn't expect it would matter much, by found myself loving it. I believe the ruby maintainers investigated and determined they could do it with very little performance hit. It turns out that having a repeatable and predictable order, that also matches insertion order, is what a lot of people end up assuming whether they realize it or not, just makes everything smoother when it is.

Re: Python dicts are now ordered

#337

Post by xapata »

yvdriess wrote: post #300
rplnt wrote: post #251

Earlier quoted context omitted.

> 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.

There is no language spec for Python, the CPython implementation defines the language.

The language reference is a specification for the language. The library reference is a specification for the standard library.

https://docs.python.org/3/reference/index.html

Re: Python dicts are now ordered

#339

Post by jrochkind1 »

zwp wrote: post #177
drngdds wrote: post #128

Earlier quoted context omitted.

I haven't done a ton of Python, but I can't really think of a situation where relying on a dict to be ordered is an easy mistake to make. Do you have an example?

I saw this in Perl a long time ago but it could just as easily have happened in Python. The dict (Perl hash) was a set of mappings for template replacement of "from" strings to "to" strings. "FOO" => "bar" # Replace FOO with bar The author had considered the case where one key (FOO) might be a left-substring of another (FOOBAR), and so reversed the output from keys() before iterating over the hash. This ensured that…

I definitely recall there being no fall out or or problems when ruby did it in I think 1.9. Semantically, I can't think of how there would be. Since previously order was unpredictable and not guaranteed, it could be anything at all; always using insertion order is, after all, one possible value of "anything at all".

Only possible downside might be performance (or memory). I recall the ruby maintainers ensuring they had an implementation with very little if any performance/memory implication.

Re: Python dicts are now ordered

#340

Post by coldtea »

cdelsolar wrote: post #329
skrebbel wrote: post #242

Earlier quoted context omitted.

Sure, PHP arrays are actually ordered maps, but it's super confusing when used as arrays. I mean, look at code like this: $arr = []; $arr[2] = "two"; $arr[0] = "zero"; $arr[1] = "one"; echo join($arr, ", "); // two, zero, one In any other language, the result of similar code would be "zero, one, two". You'd need to `ksort` this thing to get it to behave like a normal bog-standard boring array. Everybody's writing PHP…

I literally just learned "bog-standard" 10 minutes ago and now I see it here. And no, don't Baader-Meinhof me, I would have noticed this weird term in random text in the past.

>I would have noticed this weird term in random text in the past.

Well, no so weird, I'm not a native english speaker, and I've seen the term hundreds of times over the years...

It's also quite common on HN:

https://www.google.com/search?q=site%3Anews.ycombinator.com+...

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