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[Python-Dev] Preserving the definition order of class namespaces.

Mark Shannon mark at hotpy.org
Sun May 24 11:44:45 CEST 2015 
On 24/05/15 10:35, Nick Coghlan wrote:
> On 24 May 2015 at 15:53, Eric Snow <ericsnowcurrently at gmail.com> wrote:
>>
>> On May 23, 2015 10:47 PM, "Guido van Rossum" <guido at python.org> wrote:
>>>
>>> How will __definition_order__ be set in the case where __prepare__ doesn't
>>> return an OrderedDict? Or where a custom metaclass's __new__ calls its
>>> superclass's __new__ with a plain dict? (I just wrote some code that does
>>> that. :-)
>>
>> I was planning on setting it to None if the order is not available.  At the
>> moment that's just a check for OrderedDict.
>
> Is it specifically necessary to save the order by default? Metaclasses
> would be able to access the ordered namespace in their __new__ method
> regardless, and for 3.6, I still like the __init_subclass__ hook idea
> proposed in PEP 487, which includes passing the original namespace to
> the new hook.
>
> So while I'm sold on the value of making class execution namespaces
> ordered by default, I'm not yet sold on the idea of *remembering* that
> order without opting in to doing so in the metaclass.
>
> If we leave __definition_order__ out for the time being then, for the
> vast majority of code, the fact that the ephemeral namespace used to
> evaluate the class body switched from being a basic dictionary to an
> ordered one would be a hidden implementation detail, rather than
> making all type objects a little bigger.
and a little slower.

Cheers,
Mark.
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