[Python-Dev] PEP 435: initial values must be specified? Yes (original) (raw)

Eli Bendersky eliben at gmail.com
Mon May 6 00:55:46 CEST 2013

On Sun, May 5, 2013 at 3:34 PM, Tim Delaney <timothy.c.delaney at gmail.com>wrote:

On 6 May 2013 08:00, Guido van Rossum <guido at python.org> wrote:

On Sun, May 5, 2013 at 2:55 PM, Tim Delaney <timothy.c.delaney at gmail.com> wrote: > So long as I can get one of the requirements documented to implement an > auto-number syntax I'll be happy enough with stdlib enums I think.

Specifically what do you want the PEP to promise? It was mentioned in the other threads, but the requirement is either: 1. That the dictionary returned from .prepare provide a way to obtain the enum instance names once it's been populated (e.g. once it's been passed as the classdict to new). The reference implementation provides a enumnames list attribute. The enum names need to be available to a metaclass subclass before calling the base metaclass new. OR 2. A way for subclasses of Enum to modify the value before it's assigned to the actual enum - see the PEP 435 reference implementation - discussion thread where I modified the reference implementation to give enum instances 2-phase construction, passing the value to Enum.init. This way is more limited, as you need to use an appropriate mix-in type which puts certain constraints on the behaviour of the enum instances (e.g. they have to be int instances for auto-numbering). The implementation is also more complex, and as noted in that thread, init might not be appropriate for an Enum.

So your preferred solution is (1), which requires exposing the metaclass and an attribute publicly? I have to ask - to what end? What is the goal of this? To have an AutoNumberedEnum which is guaranteed to be compatible with stdlib's Enum?

IMHO this goal is not important enough, and I'm not aware of other stdlib modules that go to such lengths exposing implementation details publicly (but I'd be happy to be educated on this!)

Assuming ref435 goes as-is into stdlib in 3.4, can't you just assume its implementation? And then change yours if it changes? Python's stdlib doesn't change that often, but if we do want to change the implementation at some point, this documented piece of internals is surely going to be in the way. Why should the future malleability of a stdlib module be sacrificed for the sake of this extension?

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