[Python-Dev] The current dict is not an "OrderedDict"

[Yury Selivanov]

On Tue, Nov 7, 2017 at 9:32 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:
> On Wed, 8 Nov 2017 00:01:04 +1000
> Nick Coghlan <ncoghlan at gmail.com> wrote:
>
>> On 7 November 2017 at 23:48, Stefan Krah <stefan at bytereef.org> wrote:
>> >
>> >
>> > This is just a reminder that the current dict is not an "OrderedDict":
>> >
>> >>>> from collections import OrderedDict
>> >>>> OrderedDict(a=0, b=1) == OrderedDict(b=1, a=0)
>> > False
>> >>>> dict(a=0, b=1) == dict(b=1, a=0)
>> > True
>> >
>> > The recent proposal was primarily about guaranteeing the insertion order of
>> > dict literals.
>> >
>> > If further guarantees are proposed, perhaps it would be a good idea to
>> > open a new thread and state what exactly is being proposed.
>>
>> "Insertion ordered until the first key removal" is the only guarantee
>> that's being proposed.
>
> Is it?  It seems to me that many arguments being made are only relevant
> under the hypothesis that insertion is ordered even after the first key
> removal.  For example the user-friendliness argument, for I don't
> think it's very user-friendly to have a guarantee that disappears
> forever on the first __del__.

One common pattern that I see frequently is this:

     def foo(**kwargs):
          kwargs.pop('somekey', None)
          bar(**kwargs)

With ordering breaking on first pop/delete we essentially have no
guarantee about kwargs order, or at least it's very easy to break.  It
would make writing wrappers like this extremely tedious -- we are
essentially forcing people to use OrderedDict to just pop an item from
kwargs.  Not to mention that this isn't cheap in terms of performance.

Is there a *real* motivation for saying that pop/delete can break the order?

Yury