PEP 354: Enumerations in Python

[Carl Banks]

Stefan Rank wrote:
> on 28.02.2006 07:50 Carl Banks said the following:
> > Ben Finney wrote:
> >> This PEP specifies an enumeration data type for Python.
> >
> [snip]
> >
> > Here's why I think it's not too useful to begin with: the benefits of
> > the enum you describe here are pretty weak.
>
> I need to disagree heavily here :)

"the benefits of the enum you describe here [beyond my example which I
claimed enum was only a minor improvement over] are pretty weak."


> > It's a pretty weak case to have a dedicated builtin to prevent
> > duplicates in something that changes maybe once a month, as enums tend
> > to change rather slowly.  (At least, that's the way enums in other
> > languages are used, and the design you present here seems to suggest
> > you intend to use them that way as well.)  And frankly, a unit test or
> > assertion could check this.
> [snip]
>
> I don't understand what you mean by 'change rather slowly'?

Construct data structure on-the-fly from an XML file edited by multiple
people every day  = changes rather quickly

Construct data structure from a Python file that was last edited a year
and a half ago = changes rather slowly

Typically, enums fall into the latter category.  You set the enum
values, and then pretty much leave them alone, adding new values only
occasionally.  (Come on, how often do the days of the week change?)
All I was saying is, changes to the enum values are infrequent enough
that having a special type just to make sure there are no duplicates is
a waste.  The only justification for a built-in enum is the other stuff
you mentioned.


> One thing that is probably missing to allow this, is a enum-set-creation
> with the | operator::
>
>    Weekdays = enum('mon', 'tue', 'wed', 'thu', 'fri', 'sat', 'sun')
>    daysihate = Weekdays.mon | Weekdays.thu
>
> (and this discussion needs to move to python-dev ?)

What's wrong with set((Weekdays.mon,Weekdays.thu))?  Explicit is better
than implicit.


> As for the metaclass versions: For myself, the above version feels more
> natural and straightforward (in the same way as the PEP author describes
> it), though I understand the subclassing ideas.
>
> But are there use cases for subclassing, that aren't better served with
> a new enum or something homegrown?
> Can C++/Pascal/Java enums be subclassed?

In C++, enum is a type but not a class.  Same thing with Ada.  Java
didn't have enums last time I checked.  Don't know about Pascal.  I
didn't care too much about subclassing; I just thought different enum
constant that couldn't (or, rather, oughtn't) be compared probably
should be instances of a separate class.  It doesn't matter much,
though.

Should something like this work:

day = Weekdays.mon
isinstance(day,Weekdays)

?


Carl Banks