[Python-Dev] More compact dictionaries with faster iteration (original) (raw)

Andrew Svetlov andrew.svetlov at gmail.com
Mon Dec 10 23:14:10 CET 2012

On Mon, Dec 10, 2012 at 11:29 PM, Tim Delaney <tim.delaney at aptare.com> wrote:

On 11 December 2012 05:01, Armin Rigo <arigo at tunes.org> wrote:

Hi Philip, On Mon, Dec 10, 2012 at 5:16 PM, PJ Eby <pje at telecommunity.com> wrote: > On the other hand, this would also make a fast ordered dictionary > subclass possible, just by not using the free list for additions, > combined with periodic compaction before adds or after deletes. Technically, I could see Python switching to ordered dictionaries everywhere. Raymond's insight suddenly makes it easy for CPython and PyPy, and at least Jython could use the LinkedHashMap class (although this would need checking with Jython guys). I'd vaguely argue that dictionary orders are one of the few non-reproducible factors in a Python program, so it might be a good thing. But only vaguely --- maybe I got far too used to random orders over time... Whilst I think Python should not move to ordered dictionaries everywhere, I would say there is an argument (no pun intended) for making **kwargs a dictionary that maintains insertion order if there are no deletions. It sounds like we could get that for free with this implementation, although from another post IronPython might not have something suitable. Please, no. dict and kwargs should be unordered without any assumptions.

I think there are real advantages to doing so - a trivial one being the ability to easily initialise an ordered dictionary from another ordered dictionary. It can be done with adding short-circuit for OrderedDict class to accept another OrderedDict instance.

I could also see an argument for having this property for all dicts. There are many dictionaries that are never deleted from (probably most dict literals) and it would be nice to have an iteration order for them that matched the source code.

However if deletions occur all bets would be off. If you need to maintain insertion order in the face of deletions, use an explicit ordereddict. Tim Delaney

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-- Thanks, Andrew Svetlov

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