On Fri, Jul 29, 2011 at 3:00 PM, Ben Finney <ben+python@benfinney.id.au> wrote:
Guido van Rossum <guido@python.org> writes:
These could be two separate features. Named values could be a building block for a built-in Enum type -- the named value would take care of the str() and repr(), while the Enum type would only have to take care of the nice syntax ("class Color(Enum): red = 1; ...") and just create a bunch of named values in the class scope.
+1.
As for EnumValue instances having a defined order, thanks to my reading of <URL:http://en.wikipedia.org/wiki/Enumerated_type> I can now say that some of us want an enumerated type that is not ordinal, and others want an ordinal type.
And non-ordinal enumerated types, built-in, could then further be used as the basis for a custom ordinal type.
Hence:
* a built-in “named value” feature
* a built-in, or standard-library, non-ordinal enumerated type feature (using the “named value” feature)
* a standard-library ordinal type feature (using the enumerated type feature)
Would that make us all happy?
I like the tower, but I'd like to see us approach this incrementally. 1. Design and provide a standard "named value" feature (CPython unfortunately makes it difficult to do this efficiently - we need to add state to hold the name, but doing that in a way that is compatible with the C layout of tuples and similar objects means the state has to be stored in __dict__) 2. Perhaps provide a non-ordinal NamedGroup feature (avoiding the Enum naming with its 'enumerated' ordinal implications) Explicitly naming several values is likely to be clumsy enough that a metaclass based solution may be a beneficial convenience API, but Named Groups wouldn't enforce any additional behaviour on their members. I'm not completely persuaded on this point as yet, but I can see the merit of the argument. As an initial step, I thinks that's the absolute limit of how far we should go. I see it as similar to the handling of function annotations - we can provide a standard infrastructure for naming instances without otherwise affecting their behaviour, and then allow third parties to build on that and see where they take it. Cheers, Nick. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia