On Sun, Sep 11, 2016 at 11:05 AM, Nick Coghlan <ncogh...@gmail.com> wrote:

> On 11 September 2016 at 07:26, Guido van Rossum <gu...@python.org> wrote:
> > On Sat, Sep 10, 2016 at 10:57 AM, Nick Coghlan <ncogh...@gmail.com>
> wrote:
> >> On 11 September 2016 at 03:08, Guido van Rossum <gu...@python.org>
> wrote:
> >>> So I'm happy to continue thinking about this, but I expect this is not
> >>> such a big deal as you fear. Anyway, let's see if someone comes up
> >>> with a more convincing argument by beta 2!
> >>
> >> For CPython specifically, I don't have anything more convincing than
> >> Ethan's Enum example (where the way the metaclass works means most of
> >> the interesting attributes don't live directly in the class dict, they
> >> live in private data structures stored in the class dict, making
> >> "list(MyEnum.__dict__)" inherently uninteresting, regardless of
> >> whether it's ordered or not).
> >
> > But that would only matter if we also defined a helper utility that
> > used __definition_order__. I expect that the implementation of Enum
> > could be simplified somewhat in Python 3.6 since it can trust that the
> > namespace passed into __new__ is ordered (so it doesn't have to switch
> > it to an OrderedDict in __prepare__, perhaps).
> >
> > In any case the most likely way to use __definition_order__ in general
> > was always to filter its contents through some other condition (e.g.
> > "isn't a method and doesn't start with underscore") -- you can do the
> > same with keys(). Classes that want to provide a custom list of
> > "interesting" attributes can provide that using whatever class method
> > or attribute they want -- it's just easier to keep those attributes
> > ordered because the namespace is always ordered.
> For example,it's already possible to expose order information via
> __dir__, consumers of the information just have to bypass the implicit
> sorting applied by the dir() builtin:
>   >>> class Example:
>   ...     def __dir__(self):
>   ...         return "first second third fourth".split()
>   ...
>   >>> dir(Example())
>   ['first', 'fourth', 'second', 'third']
>   >>> Example().__dir__()
>   ['first', 'second', 'third', 'fourth']
> You've persuaded me that omitting __definition_order__ is the right
> thing to do for now, so the last thing I'm going to do is to
> explicitly double check with the creators of a few interesting
> alternate implementations (MicroPython, VOC for JVM environments,
> Batavia for JavaScript environments) to see if this may cause them
> problems in officially implementing 3.6 (we know PyPy will be OK,
> since they did it first).
> VOC & Batavia *should* be OK (worst case, they return
> collections.OrderedDict from __prepare__ and also use it for __dict__
> attributes), but I'm less certain about MicroPython (since I don't
> know enough about how its current dict implementation works to know
> whether or not they'll be able to make the same change PyPy and
> CPython did)

>From the perspective of VOC and Batavia: As Nick notes, there may be some
changes needed to use OrderDict (or a native analog) in a couple of places,
but other than that, it doesn’t strike me as a change that will pose any
significant difficulty.

Russ Magee %-)
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