[Python-Dev] PEP 435: initial values must be specified? Yes

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 <enum metaclass>.__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 _enum_names 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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