Enum Composition in Python

Question

I've had a hard time finding examples of OO Enum composition in Python. So I wanted to ask if the following example is correct or more pythonic ways are advisable?

I prefer class syntax in declaring Enum but it seems for composition Functional API is advisable. Any way to do this using class syntax?

from enum import Enum


class Vertical(Enum):

    Tall = 1
    Short = 2


class Horizontal(Enum):

    Slim = 1
    Spacious = 2


composition = list(Vertical.__members__)
composition.extend(Horizontal.__members__)

Body = Enum('Body', composition)

Are you aware that with the above code: Vertical.Tall is Body.Tall is False? — Ethan Furman
– Ethan Furman, Commented Jan 4, 2019 at 21:20
Yes I am aware sir. But composition is cognitively so intuitive I will rather figure out a workaround for the truth test than give up on object composition. More so, when modeling natural discreet data focused on domain analyses, because a bottom-up approach would always start with Enum. — bad_coder
– bad_coder, Commented Jan 4, 2019 at 21:38

Patrick Artner · Accepted Answer · 2019-01-04 22:32:04Z

1

You cannot derive enums, they are "sealed":

class Body(Vertical): pass

leads to TypeError: Cannot extend enumerations.

If you want your composed enums to compare equal you can use IntEnums:

from enum import IntEnum 

class Vertical(IntEnum ):
    Tall = 1
    Short = 2 

class Horizontal(IntEnum):  # distinct int's
    Slim = 3
    Spacious = 4 

composition = list(Vertical.__members__)
composition.extend(Horizontal.__members__)

Body = IntEnum('Body', composition)

Usage:

print(Body.Tall == Vertical.Tall)  # True
print(Body.Tall == 1)              # Also True

Essentially it boils down to : your enums are now also int's. You need to take care to not give the same integer to different concepts though:

class Sizes(IntEnum):
    Tiny = 1

print(Sizes.Tiny == Vertical.Tall)  # True - but not really?

edited Jan 4, 2019 at 22:32

answered Jan 4, 2019 at 22:24

Patrick Artner

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1 Comment

bad_coder Over a year ago

Ontologically, this is correct. Man.Tall is Building.Tall is False. But Man.Tall == Building.Tall is True. (They are not the same, but both are Tall.) And your solution provides both == syntax comparison, and is difference.

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