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

[Eric Snow]

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.

-eric

>
> On Sat, May 23, 2015 at 7:38 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>>
>> On 24 May 2015 at 12:04, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> > On 24 May 2015 at 11:15, Eric Snow <ericsnowcurrently at gmail.com> wrote:
>> >> tl;dr Are there any objections to making making the default
>> >> cls.__prepare__ return OrderedDict instead of dict (and preserve that
>> >> order in a list on the class)?
>> >>
>> >> A couple years ago [1][2] I proposed making class definition
>> >> namespaces use OrderedDict by default.  Said Guido [3]:
>> >>
>> >>     I'm fine with doing this by default for a class namespace; the
type of
>> >>     cls.__dict__ is already a non-dict (it's a proxy) and it's
unlikely to
>> >>     have 100,000 entries.
>> >>
>> >> It turns out making cls.__dict__ an OrderedDict isn't reasonably
>> >> tractable (due to the concrete API v. subclasses), but really that
>> >> isn't what I was looking for anyway.
>> >>
>> >> Regardless, since it's been a while I just want to run the proposal by
>> >> the group again.  I'm hopeful about landing my C implementation of
>> >> OrderedDict [4] in the next few days.  Also, I have a patch up [5]
>> >> that implements using OrderedDict for class definitions.  So mostly I
>> >> just want to double check that I'm still good to go.
>> >
>> > While it isn't controversial (since you already have the +1 from
>> > Guido), it's worth writing up the change as a PEP for 3.6 anyway,
>> > since that then provides clearer guidance to alternate implementations
>> > that they're going to need to change the way their class namespace
>> > evaluation works for 3.6.
>>
>> Eric clarified for me that Larry was considering granting a feature
>> freeze exemption to defer landing this to beta 2 while Eric tracked
>> down a segfault bug in the current patch that provides a C
>> implementation of OrderedDict. That sounds like a nicer approach than
>> what I did for PEP 489 (where I checked in an initial version that I
>> knew still had a refleak bug in it), so +1 from me for going down that
>> path.
>>
>> A top level section in the What's New would cover my concerns
>> regarding making sure folks are suitably aware of the change (as I
>> believe leaving it out of the original 2.6 What's New document was the
>> real problem with making people aware of the addition of zip archive
>> and directory execution support).
>>
>> Regards,
>> Nick.
>>
>> --
>> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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>
>
>
>
> --
> --Guido van Rossum (python.org/~guido)
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