[Python-Dev] Enumeration items: `type(EnumClass.item) is EnumClass` ?

Glenn Linderman v+python at g.nevcal.com
Mon Apr 29 20:14:45 CEST 2013

On 4/29/2013 10:01 AM, Guido van Rossum wrote:
> On Mon, Apr 29, 2013 at 9:12 AM, Ethan Furman <ethan at stoneleaf.us> wrote:
>> On 04/29/2013 08:39 AM, Guido van Rossum wrote:
>>> Indeed, the "type(Color.red) is Color" claim was meant for the
>>> situation where red is defined directly in Color, and I used type()
>>> instead of isinstance() because Barry was proposing to overload
>>> isinstance() to make this true without equating the classes. But for
>>> the subclass case, I want MoreColor.red is Color.red and
>>> isinstance(MoreColor.red, Color), but not isinstance(Color.red,
>>> MoreColor). If you can't live with that, don't subclass enums.
>> So if I understand:
>> --> class Color(Enum):
>> ...     red = 1
>> ...     green = 2
>> ...     blue = 3
>> --> class MoreColor(Color):
>> ...     cyan = 4
>> ...     magenta = 5
>> ...     yellow = 6
>> --> type(MoreColor.red) is Color
>> --> type(MoreColor.red) is not MoreColor
>> In other words, while `red` is accessible in MoreColor, it's actually a
>> Color instance?
> Oh dear, this is actually a mess. I don't want MoreColor.red and
> Color.red to be distinct objects, but then the isinstance() checks
> will become confusing. If we don't override isinstance(), we'll get
>    not isinstance(Color.red, MoreColor)
>    isinstance(MoreColor.yellow, Color)
> This would be pretty backwards.
> I Ggoogled "enum subclassing" and found this StackOverflow article
> explaining why you can't subclass enums in Java:
> http://stackoverflow.com/questions/4604978/subclassing-an-enum which
> refers to this more elaborate answer:
> http://stackoverflow.com/questions/3427947/enumeration-inheritence-in-java/3428050#3428050
> I think this conclusively shows that it's better to disallow
> subclassing enums. (It also brings enum in line with bool, which is
> also a "final" class.)
> Adding Eli to the thread explicitly, because this needs to be
> explained in the PEP.

The only thing that needs to be disallowed is defining additional named 
elements, for Liskov Substitution to not be violated.

So in an inheritance tree derived from Enum, it would be a requirement 
that all members of the enumeration be enumerated in one class.

It would be extremely nice if:

1) Enum could be subclassed to provide different, sharable, types of 
behaviors, then further subclassed to provide a number of distinct sets 
of values with those behaviors.

2) Enum could be subclassed to provide one set of values, and then 
further subclassed to provide a number a distinct sets of behaviors for 
those sets of values.

(I see a lot more benefit to #1, than #2, if a choice must be made.)
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