[Python-Dev] Enum: subclassing?
ethan at stoneleaf.us
Thu May 2 00:11:20 CEST 2013
On 05/01/2013 02:48 PM, Eli Bendersky 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
> > 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.
The reason __int__ is there is because pure Enums should be using plain ints as their value 95% or more of the time, and
being able to easily convert to a real int for either database storage, wire transmission, or C functions is a Good Thing.
IntEnum is for when the enum item *must* be a real, bonafide int in its own right, and the use case here is backwards
compatibility with APIs that are already using real ints -- and this is really the *only* time IntEnum should be used).
The downside to IntEnum is you lose all Enum type protection; so if you don't need a real int, use a fake int, er, I
mean, Enum, which can easily be int'ified on demand due to its handy dandy __int__ method.
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