[Python-Dev] Enum: subclassing? (original) (raw)

Eli Bendersky eliben at gmail.com
Wed May 1 20:04:32 CEST 2013

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.

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