Python dicts are now ordered • Hacker News Board

Re: Python dicts are now ordered

#241

Post by anthonypasq » Fri, Feb 07, 2020, 6:48 PM UTC

jnwatson wrote: post #220

alayne wrote: post #164

Earlier quoted context omitted.

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

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

Re: Python dicts are now ordered

#242

Post by skrebbel » Fri, Feb 07, 2020, 6:48 PM UTC

svnpenn wrote: post #226

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…

> 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…

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 code pretending that it has arrays when really, it does not. The gotchas are way bigger than the gotchas in Python <3.7's unordered maps. It's a mess.

Re: Python dicts are now ordered

#243

Post by ehsankia » Fri, Feb 07, 2020, 6:49 PM UTC

takeda wrote: post #234

biddlesby wrote: post #26

Earlier quoted context omitted.

It doesn’t break existing code. Code written for Python 3.7 might break on older versions of Python

Well... full support for 3.6 ended in December of 2018 (now it only has security fixes), the older versions are already unsupported. Also, this change was implemented 3.6, but in 3.7 they officially documented it as a language feature (i.e. that all other Python implementations also need to preserve the order).

Furthermore, there's a lot more stuff that are not backward compatible after 3.6 or 3.7, and if you're writing a library that targets other versions, I would hope that you have tests for all said versions.

Re: Python dicts are now ordered

#244

Post by styfle » Fri, Feb 07, 2020, 6:50 PM UTC

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…

Also don’t forget about JS Map which is ordered

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

Re: Python dicts are now ordered

#245

Post by masklinn » Fri, Feb 07, 2020, 6:50 PM UTC

dukoid wrote: post #174

alayne wrote: post #164

Earlier quoted context omitted.

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

Java added LinkedHashMap a while ago. I tend to default to it except for small temporary use cases where order absolutely won't matter or have an outside impact...

LinkedHashMap is not Java's "standard maps" though. If you tend to use it by default it's really a personal idiosyncrasy, especially as LLHM tend to be larger and slower than regular hashmaps due to having to maintain a doubly linked list.

Python has had one such in the standard library for a decade or so.

Re: Python dicts are now ordered

#246

Post by lostcolony » Fri, Feb 07, 2020, 6:51 PM UTC

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…

>There's so many cases where it's a benefit for map entries to retain order (and none where it's a problem)'

Don't know if this could actually be the case in practice, but theoretically the ordering could allow for a timing attack to glean some bit of information when performing a linear scan of the map (size of the map, relative location of the data, etc).

Just a contrarian thought given the definitive statement of "none where it's a problem". I'm generally of the same opinion as you though; mostly if not entirely harmless, potentially helpful in certain applications.

Re: Python dicts are now ordered

#247

Post by rossmohax » Fri, Feb 07, 2020, 6:52 PM UTC

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.

Re: Python dicts are now ordered

#248

Post by slim » Fri, Feb 07, 2020, 6:53 PM UTC

svnpenn wrote: post #226

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…

> 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 ?

Re: Python dicts are now ordered

#249

Post by gowld » Fri, Feb 07, 2020, 6:53 PM UTC

_whiteCaps_ wrote: post #196

alayne wrote: post #164

Earlier quoted context omitted.

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

Golang map iteration is returned in random order specifically to make sure that people don't rely on the order. I think this feature says a lot about the philosophy of Python vs Go.

What does it say? That Python is willing to spend more computation time for simplicity, without waiting for the programmer to ask for it? That Python will make semantic changes in minor version bumps of the language?

Re: Python dicts are now ordered

#250

Post by pavanky » Fri, Feb 07, 2020, 6:54 PM UTC

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.