Adding Attributes And Starting Value To Python enum.IntEnum

Question

How can I define a Python enum class that somehow derives from int, has a custom starting value, and adds custom attributes? I know how to derive from int using enum.IntEnum and set the starting value,

Goo = enum.IntEnum("Goo", "MOO FOO LOO", start = 42)

and how to add custom attributes to the base enum type,

class Goo(enum.Enum):
    MOO = (42, "forty-two")
    FOO = (43, "forty-three")
    LOO = (44, "forty-four")

    def __init__(self, value, alias):
        self._value = value   #self.value gives AttributeError, as expected, grrr...
        self.alias = alias

but how do I do all three? I've tried all manner of __new__() too,

class Goo(int, enum.Enum):
    MOO = (42, "forty-two")
    FOO = (43, "forty-three")
    LOO = (44, "forty-four")

    def __new__(cls, value, alias):
        self = super().__new__(cls, value)
        self.alias = alias

        return self

mostly with odd errors,

TypeError: int() takes at most 2 arguments (3 given)

in this case. Thanks.

Jim

Ethan Furman · Accepted Answer · 2020-09-28 19:21:15Z

5

The behavior you want isn't already available in the stdlib Enum. You could use itertools.count() to get the numbers:

from enum import IntEnum
from itertools import count

_goo_count = count(42)
class Goo(IntEnum):
    #
    MOO = "forty-two"
    FOO = "forty-three"
    LOO = "forty-four"
    #
    def __new__(cls, alias):
        value = next(_goo_count)
        member = int.__new__(cls, value)
        member._value_ = value
        member.alias = alias
        return member

or you can use aenum:

from aenum import IntEnum

class Goo(IntEnum):
    _start_ = 42
    _init_ = 'alias'
    #
    MOO = "forty-two"
    FOO = "forty-three"
    LOO = "forty-four"

either way, in use it looks like:

>>> Goo.MOO
<Goo.MOO: 42>

>>> Goo.MOO.alias
'forty-two'

>>> Goo.MOO == 42
True

A couple stylistic notes:

you don't need parentheses to define a tuple
- except for an empty tuple
- or some place (like a function call), where the command would be interpreted as not a tuple
white space is good ;-)

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

edited Sep 28, 2020 at 19:21

answered Sep 24, 2020 at 6:33

Ethan Furman

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3 Comments

Jim Over a year ago

Mine does have a __new__(), and not an __init__(). The only real difference is that I called super().__new__(), and you called int.__new__(). That gets rid of the error, but still doesn't let me specify a starting value. I'd rather avoid listing the value of every enum, as I'm able to do in my first example. Thanks for the reply.

Jim Over a year ago

You also named the integer _value_. I didn't, but it's required by the metaclass.

Ethan Furman Over a year ago

@Jim: Redid answer to get all three points: derives from int, has custom starting value, and has custom attributes.

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