[Python-Dev] Enum: subclassing?

[Eli Bendersky]

> >     Am 01.05.2013 20:04, schrieb Eli Bendersky:
> >
> >     > Actually, in flufl.enum, IntEnum had to define a magic
> __value_factory__
> >     > attribute, but in the current ref435 implementation this isn't
> needed, so
> >     > IntEnum is just:
> >     >
> >     > class IntEnum(int, Enum):
> >     >     '''
> >     >     Class where every instance is a subclass of int.
> >     >     '''
> >     >
> >     > So why don't we just drop IntEnum from the API and tell users they
> should
> >     do the
> >     > above explicitly, i.e.:
> >     >
> >     > class SocketFamily(int, Enum):
> >     >   AF_UNIX = 1
> >     >   AF_INET = 2
> >     >
> >     > As opposed to having an IntEnum explicitly, this just saves 2
> characters
> >     > (comma+space), but is more explicit (zen!) and helps us avoid the
> >     special-casing
> >     > the subclass restriction implementation.
> >
> >     Wait a moment... it might not be immediately useful for IntEnums
> (however,
> >     that's because base Enum currently defines __int__ which I find
> questionable),
> >     but with  current ref435 you *can* create your own enum base classes
> with your
> >     own methods, and derive concrete enums from that.  It also lets you
> have a
> >     base class for enums and use it in isinstance().
> >
> >     If you forbid subclassing completely that will be impossible.
> >
> >
> > I'm not sure what you mean, Georg, could you clarify?
> > This works:
> >
> >>>> from ref435 import Enum
> >>>> class SocketFamily(int, Enum):
> > ...   AF_UNIX = 1
> > ...   AF_INET = 2
> > ...
> >>>> SocketFamily.AF_INET
> > SocketFamily.AF_INET [value=2]
> >>>> SocketFamily.AF_INET == 2
> > True
> >>>> type(SocketFamily.AF_INET)
> > <Enum 'SocketFamily'>
> >>>> isinstance(SocketFamily.AF_INET, SocketFamily)
> > True
> >
> > Now, with the way things are currently implemented, class IntEnum is just
> > syntactic sugar for above. Guido decided against allowing any kind of
> > subclassing, but as an implementation need we should keep some
> restricted form
> > to implement IntEnum. But is IntEnum really needed if the above explicit
> > multiple-inheritance of int and Enum is possible?
>
> Well, my point is that you currently don't have to inherit from int (or
> IntEnum)
> to get an __int__ method on your Enum, which is what I find questionable.
>  IMO
> conversion to integers should only be defined for IntEnums.  (But I haven't
> followed all of the discussion and this may already have been decided.)
>

Good point. I think this may be just an artifact of the implementation -
PEP 435 prohibits implicit conversion to integers for non-IntEnum enums.
Since IntEnum came into existence, there's no real need for int-opearbility
of other enums, and their values can be arbitrary anyway.

Ethan - unless I'm missing something, __int__ should probably be removed.

Eli
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