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

Nick Coghlan ncoghlan at gmail.com
Sun May 24 04:38:34 CEST 2015

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

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).


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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