On 2 May 2013 02:18, Tres Seaver <tsea...@palladion.com> wrote:
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> On 05/01/2013 12:14 PM, Guido van Rossum wrote:
> > But we'd probably have to give up something else, e.g. adding methods
> > to enums, or any hope that the instance/class/subclass relationships
> > make any sense.
>
> I'd be glad to drop both of those in favor of subclassing: I think the
> emphasis on "class-ness" makes no sense, given the driving usecases for
> adopting enums into the stdlib in the first place. IOW, I would vote
> that real-world usecases trump hypothetical purity.
>
I have real-world use cases of enums (in java) that are essentially classes
and happen to use the enum portion purely to obtain a unique name without
explicitly supplying an ID.
In the particular use case I'm thinking of, the flow is basically like this:
1. An Enum where each instance describes the shape of a database query.
2. Wire protocol where the Enum instance name is passed.
3. At one end, the data for performing the DB query is populated.
4. At the other end, the data is extracted and the appropriate enum is used
to perform the query.
Why use an enum? By using the name in the wire protocol I'm guaranteed a
unique ID that won't change across versions (there is a requirement to only
add to the enum) but does not rely on people setting it manually - the
compiler will complain if there is a conflict, as opposed to setting
values. And having the behaviour be part of the class simplifies things
immensely.
Yes, I could do all of this without an enum (have class check that each
supplied ID is unique, etc) but the code is much clearer using the enum.
I am happy to give up subclassing of enums in order to have behaviour on
enum instances. I've always seen enums more as a container for their
instances. I do want to be able to find out what enum class a particular
enum belongs to (I've used this property in the past) and it's nice that
the enum instance is an instance of the defining class (although IMO not
required).
I see advantages to enums being subclassable, but also significant
disadvantages. For example, given the following:
class Color(Enum):
red = 1
class MoreColor(Color):
blue = 2
class DifferentMoreColor(Color):
green = 2
then the only reasonable way for it to work IMO is that MoreColor contains
both (red, blue) and DifferentMoreColor contains both (red, green) and that
red is not an instance of either MoreColor or DifferentMoreColor. If you
allow subclassing, at some point either something is going to be
intuitively backwards to some people (in the above that Color.red is not an
instance of MoreColor), or is going to result in a contravariance violation.
Tim Delaney
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