How to create a self-referential Python 3 Enum?

Question

Can I create an enum class RockPaperScissors such that ROCK.value == "rock" and ROCK.beats == SCISSORS, where ROCK and SCISSORS are both constants in RockPaperScissors?

Answer 1

Having Enum members refer to each other during class creation is a bit tricky; the trick is knowing that each member is created and initialized before it is added to the Enum itself. This means you can examine the state of the Enum you are creating and make adjustments to both the member-being-created as well as members-already-created.

The basic problem here is making circular references between the members, and we can solve that by modifying the circle with each new member:

class RPS(Enum):

    Rock = "rock"
    Paper = "paper"
    Scissors = "scissors"

    def __init__(self, value):
        if len(self.__class__):
            # make links
            all = list(self.__class__)
            first, previous = all[0], all[-1]
            first.beats = self
            self.beats = previous

and in use:

>>> print(RPS.Rock.beats)
RPS.Scissors

>>> print(RPS.Paper.beats)
RPS.Rock

>>> print(RPS.Scissors.beats)
RPS.Paper

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Disclosure: I am the author of the Python stdlib Enum, the enum34 backport, and the Advanced Enumeration (aenum) library.

Answer 2

By picking the order of the members carefully, each member can simply be described as beating the previous with a property.

from enum import Enum

class RPS(Enum):
    Rock = 0
    Paper = 1
    Scissor = 2

    @property
    def beats(self):
        return list(RPS)[self.value - 1]

for v in RPS:
    print(v.name, 'beats', v.beats.name)

Output

Rock beats Scissor
Paper beats Rock
Scissor beats Paper