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

Nick Coghlan ncoghlan at gmail.com
Thu Jun 9 17:39:40 EDT 2016

On 7 June 2016 at 20:17, Eric Snow <ericsnowcurrently at gmail.com> wrote:
> On Tue, Jun 7, 2016 at 6:20 PM, Ethan Furman <ethan at stoneleaf.us> wrote:
>> If __definition_order__ is supposed to be immutable as well as read-only
>> then we should convert non-tuples to tuples.  No point in letting that
>> user bug slip through.
> Do you mean if a class explicitly defines __definition_order__?  If
> so, I'm not clear on how that would work.  It could be set to
> anything, including None or a value that does not iterate into a
> definition order.  If someone explicitly set __definition_order__ then
> I think it should be used as-is.

I'm guessing Ethan is suggesting defining it as:

    __definition_order__ = tuple(ns["__definition_order__"])

When the attribute is present in the method body.

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


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

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