[Python-Dev] Enum: subclassing?
g.brandl at gmx.net
Wed May 1 23:38:35 CEST 2013
Am 01.05.2013 22:09, schrieb Eli Bendersky:
> On Wed, May 1, 2013 at 11:47 AM, Georg Brandl <g.brandl at gmx.net
> <mailto:g.brandl at gmx.net>> wrote:
> 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
> > 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 [value=2]
>>>> SocketFamily.AF_INET == 2
> <Enum 'SocketFamily'>
>>>> isinstance(SocketFamily.AF_INET, SocketFamily)
> 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.)
If __int__ stays where it is, a separate IntEnum is not necessary, but that
doesn't mean that enum baseclasses aren't useful for other use cases (and
they aren't hard to support, as ref435 shows.)
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