python why the enum has this Attribute

Question

this is my code

#coding:utf8 

from enum import Enum


class Color(Enum):
    red=1
    green=2
    black=3


Color.red.price=30.0
Color.black.price=40.0
Color.green.price=40.0

Color.green.xxx = True

why the color.red has this attribute ,such as price or xxx or anything? Thanks for you answer

I would expect this to cause an error. It shouldn't have those attributes. — Carcigenicate, Aug 02 '17 at 04:26
@cᴏʟᴅsᴘᴇᴇᴅ I tried it, but QPython on my phone isn't running a new enough version to have the module. I looked over the docs and can't find a price field. Does python allow attributes to be added on the fly, JS-style with a special tweak? — Carcigenicate, Aug 02 '17 at 04:29
@Carcigenicate Yes, it would seem there is no protection against this sort of thing. — cs95, Aug 02 '17 at 04:31
@cᴏʟᴅsᴘᴇᴇᴅ The docs suggest that Enums are special. "Even though we use the class syntax to create Enums, Enums are not normal Python classes. See How are Enums different? for more details." My Python is too rusty to understand wtf it's talking about after that though. — Carcigenicate, Aug 02 '17 at 04:32
Wait, my python must be really rusty. I just created a new bare class and was able to do this. This doesn't seem to have anything to do with Enums. Never knew Python allows attributes to be added at runtime. — Carcigenicate, Aug 02 '17 at 04:36
https://stackoverflow.com/questions/2827623/python-create-object-and-add-attributes-to-it — Carcigenicate, Aug 02 '17 at 04:40
The type of `Color.red` is an ``, which is an object, why does adding an attribute seem fancy? @Carcigenicate python is a dynamic language, you can add attributes to any object even functions: `def f(): pass; f.a = 10` is perfectly legal. — AChampion, Aug 02 '17 at 04:41
@AChampion Wow. Not sure how I missed that during the year that I wrote Python. Good to know! — Carcigenicate, Aug 02 '17 at 04:42

Ethan Furman · Answer 1 · 2019-06-27T17:46:25.587

1

Color.red has a price attribute because you gave it one. The only (mostly) immutable parts of an Enum member are its name and value.

Note that Color.red does not have an xxx attribute as you only assigned that one to Color.green.

If you really want to disallow adding extra attributes at run-time, please see Python enum prevent invalid attribute assignment.

edited Jun 27 '19 at 17:46

answered Aug 02 '17 at 04:46

Ethan Furman

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score 0 · Answer 2 · answered Aug 02 '17 at 04:37

In this example, the Color class objects don't have such attribute.

What's happening here is that you are creating those attribute, which could then be used. Apparently, python being dynamic and stuff allows such a thing.

If we see objects as arrays of values and references, what you'd be doing here is appending another value (or reference) to the current object.

I did a quick and dirty example using this online interpreter :

from enum import Enum

class names(Enum):
    Paul=1
    Robert=2,
    Jean=3,
    Charles=4

names.Paul.price = 25
print(names.Paul.price)  # works, as we just added the price poperty.
print(isinstance(names.Paul, names))  # returns true, we don't lose property.
print(names.Robert.price)  # AttributeError, we didn't add it.

Omg, it was getting late and I realised I had described behavior without explaining much. Will probably edit later on. — Alceste_, Aug 02 '17 at 12:11

score 0 · Answer 3 · answered Aug 02 '17 at 05:59

This is just like any other object. You can add new attributes to an class object(those attributes belongs only to the object)

class A:
  foo1="bar"

a=A()
a.foo2="bar2"
print(a.foo2)
print(A.foo1) 
print(A.foo2) # gives an attribute error

In case of enum

 from enum import Enum
   class A(Enum): 
..   a="enum" 
..

each member is an Enum object

   dir(A.a)
=> ['__class__', '__doc__', '__module__', 'name', 'value']

   dir(Enum)
=> ['__class__', '__doc__', '__members__', '__module__']

Hence just like any other class object you can add attribute to the object

A.a.foo1="bar1"

python why the enum has this Attribute

3 Answers3