是否可以在枚举类型中添加名为“None”的值？

Question

can I add a value named 'None' to a enum? for example

from enum import Enum
class Color(Enum):
    None=0 #represent no color at all
    red = 1
    green = 2
    blue = 3

color=Color.None

if (color==Color.None):
    #don't fill the rect
else:
    #fill the rect with the color

This question is related to my previous question How to set a variable's subproperty?

Of course, I understand the above None in enum doesn't work. but from the vendor's code, I do see something like this: bird.eye.Color=bird.eye.Color.enum.None I checked the type(bird.eye.Color) it is a <class 'flufl.enum._enum.IntEnumValue'> so a flufl.enum is used. I suppose it should not be very different to use a flufl.enum or a Enum . Thanks a lot!

Answer 1

You can do this using the Enum constructor rather than creating a subclass

>>> from enum import Enum
>>> 
>>> Color = Enum('Color', {'None': 0, 'Red': 1, 'Green': 2, 'Blue': 3})
>>> Color.None
<Color.None: 0

EDIT: This works using the enum34 backport for python 2. In python 3, you will be able to create the Enum with the None attribute, but you won't be able to access using dot notation.

>>> Color.None
SyntaxError: invalid syntax

Oddly, you can still access it with getattr

>>> getattr(Color, 'None')
<Color.None: 0>

Answer 2

You can not do this directly because it is a syntax error to assign to None .

Neither should you set an attribute on your enum class dynamically, because this will interfere with the metaclass logic that Enum uses to prepare your class.

You should just use a lowercase name none to avoid the name collision with python's None singleton. For the use-case you have described, there is no disadvantage to this approach.

Answer 3

Not quite the way you tried, but you can do this:

# After defining the class Color as normal, but excluding the part for None...
setattr(Color, 'None', 0)

color = Color.None
if color == Color.None:
    ...

Note: I did this in Python 2. Not sure if you want this in Python 2 or 3 because you didn't specify, and I don't have a copy of Python 3 installed on this machine to test with.