[Python-Dev] PEP: Ordered Class Definition Namespace (original) (raw)

Eric Snow ericsnowcurrently at gmail.com
Tue Jun 7 17:20:02 EDT 2016

On Tue, Jun 7, 2016 at 12:30 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

On 7 June 2016 at 10:51, Eric Snow <ericsnowcurrently at gmail.com> wrote:

* _definitionorder_ is a tuple * _definitionorder_ 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 definitionorder 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 definitionorder, 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 definitionorder 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.

BTW, thanks for bringing this up. :)

-eric

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