[Python-Dev] Enum: subclassing?

[Antoine Pitrou]

On Wed, 1 May 2013 13:43:22 -0700
Eli Bendersky <eliben at gmail.com> wrote:

> On Wed, May 1, 2013 at 1:33 PM, Antoine Pitrou <solipsis at pitrou.net> wrote:
> 
> > On Wed, 1 May 2013 13:05:53 -0700
> > Eli Bendersky <eliben at gmail.com> wrote:
> > > On Wed, May 1, 2013 at 11:59 AM, Georg Brandl <g.brandl at gmx.net> wrote:
> > >
> > > > Am 01.05.2013 20:44, schrieb Antoine Pitrou:
> > > > > On Wed, 01 May 2013 10:21:30 -0700
> > > > > Ethan Furman <ethan at stoneleaf.us> wrote:
> > > > >> We may not want to /completely/ disallow subclassing.  Consider:
> > > > >>
> > > > >> --> class StrEnum(str, Enum):
> > > > >> ...    '''string enums for Business Basic variable names'''
> > > > >> ...
> > > > >> --> class Vendors(StrEnum):
> > > > >> EnumError: subclassing not allowed
> > > > >
> > > > > I don't see the point of disallowing subclassing. It sounds like
> > > > > a pointless restriction.
> > > > >
> > > > > However, perhaps the constructor should forbid the returning of a
> > base
> > > > > type, e.g.:
> > > > >
> > > > > class Season(Enum):
> > > > >     spring = 1
> > > > >
> > > > > class MySeason(Season):
> > > > >     """I look nicer than Season"""
> > > > >
> > > > > MySeason('spring')
> > > > > ...
> > > > > ValueError: Season.spring is not a MySeason instance
> > > > >
> > > > > (what this means is perhaps the subclassing of non-empty enum classes
> > > > > should be forbidden)
> > > >
> > > > That's exactly what's implemented in the ref435 code at the moment.
> > > >
> > > >
> > > It can't be because __call__ is by-value lookup, not by-name lookup.
> >
> > Ok, I've mixed up the example. But, still, since Season(1) should
> > return the Season.spring singleton, I don't see any reasonable thing
> > for MySeason(1) to return. Hence the request to raise an exception.
> >
> 
> What do you need MySeason for, though? IIUC, you don't ask to allow adding
> enum values in it, so it only leaves adding extra functionality (methods)?
> What are the use cases?

I was talking in the context where subclassing is allowed. I don't
think there's a use-case for subclassing of non-empty enums. On the
other hand, empty enums should probably allow subclassing (they are
"abstract base enums", in a way).

Regards

Antoine.