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

Antoine Pitrou solipsis at pitrou.net
Wed May 1 20:44:39 CEST 2013

On Wed, 01 May 2013 10:21:30 -0700 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

I don't see the point of disallowing subclassing. It sounds like a pointless restriction.

However, perhaps the constructor should forbid the returning of a base type, e.g.:

class Season(Enum): spring = 1

class MySeason(Season): """I look nicer than Season"""

MySeason('spring') ... ValueError: Season.spring is not a MySeason instance

(what this means is perhaps the subclassing of non-empty enum classes should be forbidden)

Regards

Antoine.

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. 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. 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! -- Ethan

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