[Python-Dev] Enum: subclassing?

[Eli Bendersky]

On Wed, May 1, 2013 at 10:48 AM, Guido van Rossum <guido at python.org> wrote:

> On Wed, May 1, 2013 at 10:21 AM, 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
> >
> >
> > My point is that IntEnum, StrEnum, ListEnum, FloatEnum are all
> "subclasses"
> > of Enum.  To then have a subclass of
> > that, such as Season(StrEnum), is subclassing a subclass.
>
> True, and Enum itself also falls in this category. Maybe there could
> be a special marker that you have to set in the class body (or a
> keyword arg in the class statement) to flag that a class is meant as a
> "category of enums" rather than a specific enum type. Such categorical
> classes should not define any instances. (And maybe "defines no
> instances" is enough to flag an Enum class as subclassable.)
>
> > Now, if we do want to completely disallow it, we can ditch IntEnum and
> force
> > the user to always specify the mixin
> > type:
> >
> > --> class Season(str, Enum):
> >          .
> >          .
> >          .
> >
> > --> class Names(str, Enum):
> >          .
> >          .
> >          .
> >
> > But that's not very user friendly... although it's not too bad, either.
>
> Indeed, given that we mostly want IntEnum as a last-resort backward
> compatibility thing for os and socket, it may not be so bad.
>
>
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.

Eli
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