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

[Guido van Rossum]

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. :-)

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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