Python dicts are now ordered

Post by wrsh07 » Sat, Feb 08, 2020, 1:44 AM UTC

lostcolony wrote: post #246

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…

Post by fctorial » Sat, Feb 08, 2020, 2:03 AM UTC

xapata wrote: post #66

rcfox wrote: post #42

I still see dict ordering as an implementation detail; not a technical one but a descriptive one. If you want to rely on insertion order, use collections.OrderedDict. It communicates your intention far better, and there should be no overhead.

OrderedDict is less efficient. It's best to replace usage with basic dict where possible to improve efficiency. There is one obscure feature of OrderedDict that isn't in the basic, so it can't always be swapped.

Post by ummonk » Sat, Feb 08, 2020, 2:11 AM UTC

Rapzid wrote: post #384

ummonk wrote: post #380

Wait, if it is held in a separate dense array, is removal of a key from a dictionary O(N)?

You can use memcpy to remove items from the middle of a memory segment. It's probably not a single memory segment either; I imagine it works more like a Golang slice.

Post by zeroimpl » Sat, Feb 08, 2020, 2:15 AM UTC

mattkrause wrote: post #333

kingdomcome50 wrote: post #278

Earlier quoted context omitted.

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...

    $id = 12835151;
    $arr[$id] = Get_thing_with_id( $id );

Post by zeroimpl » Sat, Feb 08, 2020, 2:19 AM UTC

cliffy wrote: post #306

kingdomcome50 wrote: post #278

Earlier quoted context omitted.

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.

The problem is PHP calling an ordered map an array. Array in pretty much every other language means a sequence indexed by integers. Ideally the maintainers of PHP would rename it and deprecate the use of `array()` over a long period of time.