[Python-Dev] PEP 520: Ordered Class Definition Namespace

[Eric Snow]

On Thu, Jun 9, 2016 at 2:39 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> I'm guessing Ethan is suggesting defining it as:
>
>     __definition_order__ = tuple(ns["__definition_order__"])
>
> When the attribute is present in the method body.

Ah.  I'd rather stick to "consenting adults" in the case that
__definition_order__ is explicitly set.  We'll strongly recommend
setting it to None or a tuple of identifier strings.

>
> That restriction would be comparable to what we do with __slots__ today:
>
>     >>> class C:
>     ...     __slots__ = 1
>     ...
>     Traceback (most recent call last):
>      File "<stdin>", line 1, in <module>
>     TypeError: 'int' object is not iterable

Are you suggesting that we require it be a tuple of identifiers (or
None) and raise TypeError otherwise, similar to __slots__?  The
difference is that __slots__ has specific type requirements that do
not apply to __definition_order__, as well as a different purpose.
__definition_order__ is about preserving definition-type info that we
are currently throwing away.

-eric