inheritance on class attributes (python)

Question

Is there a way to accomplish something like this? I work in Python, but I am not sure if there is a way to do it in any programming language...

class Parent():
    class_attribute = "parent"

    @staticmethod
    def class_method():
        print __class__.class_attribute

class Child(Parent):
    class_attribute = "child"

I know I can't call __class__ directly. Its just an example, because I would want something like reference to the class itself, because I want the child class to act differently based on its class_attribute.

And then supposed output should be like this:

> Parent.class_method()
"parent"
> Child.class_method()
"child"

I know the same technique can be accomplish through the instances. But I don't want to create instances, because sometimes the code within the __init__ method could be long and demanding and if I would want to call class_method often, I would have to create plenty of instances used just for this one method call. And because class_attribute and class_method are static and won't be changed by instances.

are you talking about simply static functions?

Tigran
– Tigran

2012-02-06 20:09:17 +00:00
Commented Feb 6, 2012 at 20:09 — Tigran
– Tigran, Commented Feb 6, 2012 at 20:09

Daniel Roseman · Accepted Answer · 2012-02-06 20:20:41Z

10

Er, sounds like you want a classmethod, which not surprisingly is done with the classmethod decorator:

class Parent(object):
    class_attribute = "parent"

    @classmethod
    def class_method(cls):
        print cls.class_attribute

class Child(Parent):
    class_attribute = "child"


>>> Parent.class_method()
parent
>>> Child.class_method()
child

Or, as bgporter points out, you can do it directly with the attributes, with no need for the methods at all.

edited Feb 6, 2012 at 20:20

answered Feb 6, 2012 at 20:12

Daniel Roseman

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4 Comments

Tigran Over a year ago

but considering the documentation: "if a class method is called for a derived class, the derived class object is passed as the implied first argument", does it mean that derived class object will be created any way? Because OP says that would like to avoid instance constructon.

davekr Over a year ago

I wanted to call the method, because the code in method might actually do something more complicated then just printing attributes. I though making an easier exmaple would make easier explanation of what I wanted... so @classmethod is a solution

Daniel Roseman Over a year ago

@Tigran no. A class is itself an object (an instance of its metaclass, actually).

Tigran Over a year ago

@DanielRoseman: ah ok, that was actually what I was thinking of. +1

bgporter · Accepted Answer · 2012-02-06 20:11:31Z

4

It just works in Python, with or without creating instances:

>>> class Parent(object):
...    attribute = "parent"
... 
>>> class Child(Parent):
...    attribute = "child"
... 
>>> p = Parent()
>>> p.attribute
'parent'
>>> c = Child()
>>> c.attribute
'child'
>>> Parent.attribute
'parent'
>>> Child.attribute
'child'
>>>

answered Feb 6, 2012 at 20:11

bgporter

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