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

Ethan Furman ethan at stoneleaf.us
Mon Sep 23 18:12:27 CEST 2013

On 09/23/2013 08:16 AM, Steven D'Aprano wrote:

I would expect instance.blue to work, and I'm completely at a loss as to how Enum has managed to prevent it.

[A peek behind the curtains...]

Currently, Enum members do not live in the class dict. So when, for example, blue is searched for in red, it is not found in the instance dict, and it is not found in the class dict. As you know, getattr will then be invoked -- but the Enum class does not have its own getattr, nor its own getattribute, and so we get an AttributeError.

Well, you may ask, if blue does not live in the class dict, how does Color.blue work? I'm glad you asked. ;)

Color is of type EnumMeta, and EnumMeta /does/ have getattr, so a failed /class/ lookup will invoke the metaclass getattr, which will search in the right place, find, and return, Color.blue.

-- Ethan

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