
On Sun, May 5, 2013 at 3:55 PM, Eli Bendersky <eliben@gmail.com> wrote:
On Sun, May 5, 2013 at 3:34 PM, Tim Delaney <timothy.c.delaney@gmail.com> wrote:
On 6 May 2013 08:00, Guido van Rossum <guido@python.org> wrote:
On Sun, May 5, 2013 at 2:55 PM, Tim Delaney <timothy.c.delaney@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?
Hm. Either you should argue much more strongly against Tim's solution, or you should expose the implementation detail he needs. Recommending that he should just use an internal detail of the implementation and hope it never changes sounds like encouraging a bad habit. It also seems you're contradicting yourself by saying that the code is unlikely to change and at the same time wanting to reserve the right to change it. Also note that the future malleability of a stdlib module is affected even by 3rd party use that goes beyond the documented API -- it all depends on a pragmatic weighing of how important a proposed change is against how likely it is to break existing use, and there are plenty of examples in the past where we have resisted changing an implementation detail because it would break too much code. If you really don't want to guarantee this part of the implementation, you should recommend that Tim just copy all of ref435. TBH I don't see what deriving AutoNumberEnum from the stdlib Enum class buy him except that he has to maintain less code. I don't expect there to be a lot of opportunities anywhere for writing isinstance(x, Enum). -- --Guido van Rossum (python.org/~guido)