Is it possible to pass kwargs to customised python enum

Question

You can customise an enum so the members have extra attributes. There are plenty of examples of that to be found. They all look something like this:

class EnumWithAttrs(Enum):
    def __new__(cls, *args, **kwargs):
        value = len(cls.__members__) + 1
        obj = object.__new__(cls)
        obj._value_ = value
        return obj
    def __init__(self, a, b):
        self.a = a
        self.b = b
    
    GREEN = 'a', 'b'
    BLUE = 'c', 'd'

this example is from https://stackoverflow.com/a/19300424/6439229

The majority of the examples I found show __new__ to accept kwargs, but is it actually syntactically possible to pass kwargs to __new__?

This gives SyntaxErrors:

class EnumWithAttrs(Enum):
    GREEN = 'a', foo='b'
    BLUE = 'c', **{'bar':  'd'}

So does this functional syntax:

Color = EnumWithAttrs('Color', [('GREEN', 'a', foo='b'), ('BLUE', 'c', **{'bar': 'd'})])

Do people put the **kwargs in the function def of __new__ just out of habit, or is there really a way to use them?

EDIT: I'm not looking for a workaround to using kwargs, like passing a dict as a positional argument.

score 1 · Accepted Answer · answered Apr 04 '23 at 14:33

Currently, there is no way to pass keyword args to an enum's __new__ or __init__, although there may be one in the future.

The third-party library aenum¹ does allow such arguments using its custom auto.

¹ Disclosure: I am the author of the Python stdlib Enum, the enum34 backport, and the Advanced Enumeration (aenum) library.

score 0 · Answer 2 · answered Apr 04 '23 at 11:43

0

You could do

from enum import Enum


class EnumWithAttrs(Enum):
    def __new__(cls, *args, **kwargs):
        value = len(cls.__members__) + 1
        obj = object.__new__(cls)
        obj._value_ = value
        return obj

    def __init__(self, a, attrs):
        self.a = a
        self.attrs = attrs or {}


class AttributeyColor(EnumWithAttrs):
    GREEN = 'a', {'bar': 'b'}
    BLUE = 'c', {'glar': 'blarb'}


print(AttributeyColor.GREEN.attrs)

but this feels like abuse of Enums to me...

answered Apr 04 '23 at 11:43

AKX

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Off course you can pass a dict as a positional argument to achieve the same thing as using kwargs, but it doesn't answer the question. I'd like to know if the syntax exists to use kwargs. – mahkitah Apr 04 '23 at 12:24
The syntax is then `dict(bar="b", ...)`, there is no free-standing syntax for how keyword arguments are generally passed. – AKX Apr 04 '23 at 12:25

Is it possible to pass kwargs to customised python enum

2 Answers2