python: set read-only attribute for class objects

Question

I've created a class object called 'Node'. I then created two subclasses 'Beetle' and 'Dakota'. You'll notice there is an attr called 'superclass' which is part of the baseclass. I want this attribute to be set for each subclass and once it's set it should never be changed. It's a read-only attribute. I was wondering how to set this attribute up correctly in order to be a read only attribute?

class Node(object):
    def __init__(self, name, superclass, attributes, children):
        self.name = name
        self.superclass = superclass
        self.attributes = attributes if attributes is not None else {}
        self.children = children if children is not None else []


    class Beetle(Node):
        def __init__(self, name="", superclass="Cars", attributes=None, children=None, enabled=True):
            super(Beetle, self).__init__(name=name, superclass=superclass, attributes=attributes, children=children)
            self.enabled = enabled


    class Dakota(Node):
        def __init__(self, name="", superclass="Trucks", attributes=None, children=None, enabled=True):
            super(Dakota, self).__init__(name=name, superclass=superclass, attributes=attributes, children=children)
            self.enabled = enabled

The answers to [this question](http://stackoverflow.com/questions/14594120/python-read-only-property) seem relevant. — Matt Hall, Nov 23 '15 at 13:57

score 2 · Answer 1 · edited May 23 '17 at 12:23

2

To have a 'read-only' property in a class you can make use of the @property decoration.

An example:

class Dakota(object):
def __init__(self, superclass):
    self.__superclass = superclass

    @property
    def superclass(self):
        return self.__superclass

Usage:

>>> a = Dakota('lol')
>>> a.superclass
'lol'
>>> a.superclass = 'hah'
AttributeError...    

AttributeError: can't set attribute

You can refer to this other answer thread

edited May 23 '17 at 12:23

Community

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1

answered Nov 23 '15 at 14:05

mattgathu

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2 ways to set the attribute: 1 (set the attribute) `a._Dakota__superclass = None`; 2 (add a setter to `Dakota.superclass.fset`) `Dakota.superclass.__init__( Dakota.superclass.fget, lambda self,value: setattr(self,'_%s__superclass'%(self.__class__.__name__),value) )` (it's funny how `slice()` has more security than `property()` haha) – Tcll Jul 08 '23 at 22:32

score 1 · Accepted Answer · answered Nov 23 '15 at 14:08

1

Rename you variable to add __ to the begining

self.__superclass = superclass

you can't access self.__superclass with something like Dakota().__superclass

if you want to get value of __superclass add a function in Node class to return it.

def getsuperclass(self):
    return self.__superclass
...

Dakota().getsuperclass()

answered Nov 23 '15 at 14:08

Kenly

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what if i prefixed it with just a single _ would i then be able to get it by Dakota()._superclass – JokerMartini Nov 23 '15 at 14:17
Yes.it will possible to get it by `Dakota()._superclass`. – Kenly Nov 23 '15 at 14:20

python: set read-only attribute for class objects

2 Answers2