[Python-Dev] Enum Eccentricities (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Tue Sep 24 01:25:10 CEST 2013

On 24 Sep 2013 08:09, "Greg Ewing" <greg.ewing at canterbury.ac.nz> wrote:

Steven D'Aprano wrote:

It might not be a rule, but it's certainly the norm. I reckon that class attributes that aren't accessible from the instance are significantly more surprising than Color.red.blue. There are really two different kinds of things that we refer to as "class attributes". One is things that really are attributes of the class itself, and the other is things that are meant to serve as default or shared instance attributes. The confusion comes in because we use the same terminology for both, and because Python doesn't provide any straightforward way of creating user-defined class-only attributes,

Using @property in a metaclass definition isn't that complicated :)

(says the guy who helps maintain the type system)

so shared attributes tend to get abused for that purpose. Then when someone comes along and creates a true class-only attribute, people get all surprised and complain about it.

One of the interesting aspects of adding Enum has been the subtle descriptor handling bugs it has uncovered in the inspect module :)

They shouldn't, IMO.

There's a helper for Enum's descriptors that will probably be exposed through the types module in the next 3.4 alpha (tentative name is types.DynamicClassAttribute). It's the inverse, though - it throws AttributeError when looked up on the class in order to trigger getattr on the metaclass.

Cheers, Nick.

-- Greg

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