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

Eric Snow ericsnowcurrently at gmail.com
Tue Jun 7 14:51:34 EDT 2016

On Tue, Jun 7, 2016 at 11:36 AM, Émanuel Barry <vgr255 at live.ca> wrote:

From: Eric Snow * "dunder" attributes (e.g. _init_, _module_) are ignored What does this imply? If I define some dunder methods, will they simply not be present in definitionorder? What if I want to keep the order of those? While keeping the order of these might be meaningless in most cases, I don't think there's really a huge problem in doing so. Maybe I'm overthinking it.

"dunder" names (not just methods) will not be present in definition_order. I'll add an explanation to the PEP. The gist of it is that they are reserved for use by the interpreter and will always clutter up definition_order. Since needing dunder names included in definition_order would be rather exceptional, and there are other options available, leaving them out by default is a matter of practicality.

* _definitionorder_ is a tuple * _definitionorder_ is a read-only attribute * _definitionorder_ is always set:

* if _definitionorder_ is defined in the class body then it is used * types that do not have a class definition (e.g. builtins) have their _definitionorder_ set to None * types for which `_prepare_() returned something other than_ _OrderedDict(or a subclass) have their_definitionorder__ _set to None_ _I would probably like a type.definitionordermethod, for which the_ _return value is bound to _definitionorder_ when the class is created_ _(much like the link betweentype.mroandcls._mro_``.

What is the value of type.definition_order()? If you need a mutable copy then pass definition_order to list().

Additionally I'm not sure if setting the attribute to None is a good idea; I'd have it as an empty tuple. Then again I tend to overthink a lot.

None indicates that there is no order. An empty tuple indicates that there were no attributes.

definitionorder = tuple(k for k in locals() if (!k.startswith('') or_ !k.endswith('')))_ Mixing up C and Python syntax here.

nice catch :)

I'm +1 on the whole idea (one of my common uses of metaclasses was to keep the definition order somewhere). Thank you for doing that!

:)

-eric

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