How do define an attribute in Python 3 enum class that is NOT an enum value? [duplicate]

Question

How to define an attribute in a Python 3 enum class that is NOT an enum value?

class Color(Enum):
    red = 0
    blue = 1
    violet = 2
    foo = 'this is a regular attribute'
    bar = 55  # this is also a regular attribute

But this seems to fail for me. It seems that Color tries to include foo and bar as part of its enum values.

EDIT: Lest you think I'm not using Enum in a way that's intended... For example, take the official Python documentation's example enum class Planet (docs.python.org/3/library/enum.html#planet). Note that they define the gravitational constant G within the surface_gravity() method. But this is weird code. A normal programmer would say, set that constant G once, outside the function. But if I try to move G out (but not to global scope, just class scope), then I run into the issue I'm asking about here.

Answer 1

The point of the Enum type is to define enum values, so non-enum values are theoretically out of scope for this type. For constants, you should consider moving them out of the type anyway: They are likely not directly related to the enum values (but rather some logic that builds on those), and also should be a mutable property of the type. Usually, you would just create a constant at module level.

If you really need something on the type, then you could add it as a class method:

class Color(Enum):
    red = 0
    blue = 1
    violet = 2
    bar = 55

    @classmethod
    def foo (cls):
        return 'this is a not really an attribute…'

And using the classproperty descriptor from this answer, you can also turn this into a property at class level which you can access as if it was a normal attribute:

class Color(enum.Enum):
    red = 0
    blue = 1
    violet = 2
    bar = 55

    @classproperty
    def foo (cls):
        return 'this is a almost a real attribute'

>>> Color.foo
'this is a almost a real attribute'
>>> list(Color)
[<Color.red: 0>, <Color.blue: 1>, <Color.violet: 2>, <Color.bar: 55>]

Answer 2

When building an enum.Enum class, all regular attributes become members of the enumeration. A different type of value does not make a difference.

By regular attributes I mean all objects that are not descriptors (like functions are) and excluded names (using single underscore names, see the Allowed members and attributes of enumerations section).

If you need additional attributes on the final enum.Enum object, add attributes afterwards:

class Color(Enum):
    red = 0
    blue = 1
    violet = 2

Color.foo = 'this is a regular attribute'
Color.bar = 55

Demo:

>>> from enum import Enum
>>> class Color(Enum):
...     red = 0
...     blue = 1
...     violet = 2
... 
>>> Color.foo = 'this is a regular attribute'
>>> Color.bar = 55
>>> Color.foo
'this is a regular attribute'
>>> Color.bar
55
>>> Color.red
<Color.red: 0>
>>> list(Color)
[<Color.red: 0>, <Color.blue: 1>, <Color.violet: 2>]