You are too verbose. I have already said what I needed to say.
On Mon, Apr 29, 2013 at 11:24 AM, Steven D'Aprano <steve at pearwood.info> wrote:
> On 30/04/13 03:01, Guido van Rossum wrote:
>>> 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.
>>> Why is that backwards? MoreColor is a subclass of Color, so instances of
> MoreColor are instances of Color, but instances of Color are not instances
> of MoreColor. That's normal behaviour for subclasses. (All cats are mammals,
> but not all mammals are cats.)
>> The only "funny" thing about this is that enumeration values of a Enum are
> only instances of that Enum if they are declared inside that Enum, and as
> far as I'm concerned that's not especially funny at all. So:
>> Color declares red, so Color.red is a Color.
>> MoreColor does not declare red, it merely inherits it from Color, so
> MoreColor.red is Color.red which is not a MoreColor.
>>> This leads to a simple rule:
>> Enum values are either instances of the enum they are defined in, or
> instances of the parent class they are inherited from.
>> If the user doesn't like that, they're free to not subclass Enums :-)
>>>>>> 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>>> The first link says:
>> "And generally, a proper subclass is a specialization of its superclass."
>> and gives the Liskov Substitution Principle as the reason for prohibiting
> subclassing. LSP is a very useful principle, but it is not the only model
> for subclassing. That's why it's called a *principle* and not the Liskov
> Substitution *Law*.
>> (Yes, I stole that from Raymond Hettinger's talk on subclassing at PyCon.)
>> I think that the ability to extend enums with new ones, without duplicating
> code, is more important for enums than satisfying LSP. The common use-cases
> for enums do not lend itself to subtyping in the Liskov sense.
>> E.g. if I have a function that expects a Color red/blue/green enum, and I
> subclass it to give MoreColor yellow, I can't expect the function to
> suddenly recognise yellow. So I'm not going to use subclassing in this case,
> because it can't work.
>> But I will use subclassing to reuse values: if I have a function that
> expects red/blue/green/yellow, and red/blue/green are already defined in
> Color, I'll want to reuse them rather than duplicate them. Hence I will
> subclass Color specifically to extend it, not to specialise it.
>>> The second link quotes:
>> "For the most part, extensibility of enums turns out to be a bad idea. It is
> confusing that elements of an extension type are instances of the base type
> and not vice versa."
>> Confusing for who, and why?
>> You're going to confuse *somebody* no matter what you do. Either:
>> - confuse people who try to subclass enums, and can't; or
>> - confuse people who try to subclass enums, and can, but then get confused
> by the result.
>>>>>> --
> Steven
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--
--Guido van Rossum (python.org/~guido)
