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

Tim Delaney timothy.c.delaney at gmail.com
Mon May 6 00:34:54 CEST 2013

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__.


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.

Tim Delaney
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