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

Guido van Rossum guido at python.org
Wed May 1 19:48:45 CEST 2013

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.

One consequence of the way it is now (IntEnum, StrEnum, etc., are allowed) is that one can put methods and other non-Enum item in a base class and then inherit from that for actual implemented Enum classes.

--> class StrEnum(str, Enum): ... def describe(self): ... print("Hi! I'm a %s widget!" % self.value) ... --> class Season(StrEnum): ... spring = 'green' ... summer = 'brown' ... autumn = 'red' ... winter = 'white' ... --> class Planet(StrEnum): ... mars = 'red' ... earth = 'blue' ... --> Season.summer.descripbe() Hi! I'm a brown widget! --> Planet.earth.describe() Hi! I'm a blue widget!

If the base class doesn't define any instances (and perhaps is marked specifically for this purpose) I'm fine with that.

-- --Guido van Rossum (python.org/~guido)

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