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[Python-Dev] PEP 487: Simpler customization of class creation

Guido van Rossum guido at python.org
Tue Jun 21 13:18:50 EDT 2016 
On Tue, Jun 21, 2016 at 10:12 AM, Eric Snow <ericsnowcurrently at gmail.com>
wrote:

> On Mon, Jun 20, 2016 at 12:31 PM, Nikita Nemkin <nikita at nemkin.ru> wrote:
> > Right. Ordered by default is a very serious implementation constraint.
> > It's only superior in a sense that it completely subsumes/obsoletes
> > PEP 520.
>
> Just to be clear, PEP 520 is more than just OrderedDict-by-default.
> In fact, the key point is preserving the definition order, which the
> PEP now reflects better.  Raymond's compact dict would only provide
> the ordered-by-default part and does nothing to persist the definition
> order like the PEP specifies.
>

Judging from Inada's message there seems to be some confusion about how
well the compact dict preserves order (personally I think if it doesn't
guarantee order after deletions it's pretty useless).

Assuming it preserves order across deletions/compactions (like IIUC
OrderedDict does) isn't that good enough for any of the use cases
considered? It would require a delete+insert to change an item's order. If
we had had these semantics in the language from the start, there would have
been plenty uses of this order, and I suspect nobody would have considered
asking for __definition_order__.

-- 
--Guido van Rossum (python.org/~guido)
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