[Python-Dev] PEP: Ordered Class Definition Namespace

Ethan Furman ethan at stoneleaf.us
Tue Jun 7 17:34:58 EDT 2016

On 06/07/2016 02:20 PM, Eric Snow wrote:
> On Tue, Jun 7, 2016 at 12:30 PM, Nick Coghlan wrote:
>> On 7 June 2016 at 10:51, Eric Snow wrote:

>>> * ``__definition_order__`` is a tuple
>>> * ``__definition_order__`` is a read-only attribute
>> Thinking about the class decorator use case, I think this may need to
>> be reconsidered, as class decorators may:
>> 1. Remove class attributes
>> 2. Add class attributes
>> This will then lead to __definition_order__ getting out of sync with
>> the current state of the class namespace.
> I'm not clear on your point.  Decorators are applied after the class
> has been created.  Hence they have no impact on the class's definition
> order.  I'd expect __definition_order__ to strictly represent what
> happened in the class body during definition, and not anything
> afterward.
> Certainly __definition_order__ might not align with __dict__ (or
> dir()); we don't have any way to guarantee that it would, do we?  If
> anything, the ability to diff __definition_order__ and __dict__ is a
> positive, since it allows you to see changes on the class since it was
> defined.
>> One option for dealing with that would be to make type.__setattr__ and
>> type.__delattr__ aware of __definition_order__, and have them replace
>> the tuple with a new one as needed. If we did that, then the main
>> question would be whether updating an existing attribute changed the
>> definition order, and I'd be inclined to say "No" (to minimise the
>> side effects of monkey-patching).
>> The main alternative would be to make __definition_order__ writable,
>> so the default behaviour would be for it to reflect the original class
>> body, but decorators would be free to update it to reflect their
>> changes, as well as to make other modifications (e.g. stripping out
>> all callables from the list).
> I think both of those make __definition_order__ more complicated and
> less useful.  As the PEP stands, folks can be confident in what
> __definition_order__ represents.  What would you consider to be the
> benefit of a mutable (or replaceable) __definition_order__ that
> outweighs the benefit of a simpler definition of what's in it.

I think the question is which is more useful?

- a definition order that lists items that are not in the class, as
   well as not having items that are in the class (set by the decorator)


- a definition order that is representative of the class state after
   all decorators have been applied

One argument for the latter is that, even though the class has been 
technically "defined" (class body executed, type.__new__ called, etc.), 
applying decorators feels like continued class definition.

One argument for the former is simplified implementation, and is 
definition order really important after the class body has been 
executed?  (okay, two arguments ;)

Perhaps the best thing is just to make it writeable -- after all, if 
__class__, __name__, etc., can all be changed, why should 
__definition_order__ be special?


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