[Python-Dev] PEP520 and absence of definition_order (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sat Sep 10 05:27:59 EDT 2016

On 10 September 2016 at 17:49, Ethan Furman <ethan at stoneleaf.us> wrote:

Per Victor's advice I'm posting this here.

PEP 520 has been accepted, but without the definitionorder attribute. The accompanying comment:

"Note: Since compact dict has landed in 3.6, definitionorder has been removed. cls.dict now mostly accomplishes the same thing instead." The "mostly" is what concerns me. Much like having a custom dir lets a class fine-tune what is of interest, a custom definitionorder allows a class to present a unified view of the class creation process. This could be important to classes that employ getattr (or getattribute) to provide virtual attributes, such as Enum or proxy classes.

+1

The reasoning for modifying the PEP post-acceptance is faulty - definition_order wasn't just there as a CPython implementation detail, it was there as a way to allow class and metaclass developers to hide their own irrelevant implementation details.

Since definition_order was already accepted, and the rationale for removing it is incorrect, could we please have it back for beta 1?

Regards, Nick.

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

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