CODEWITHSUNDEEP
pythoninheritance
David Basalla
David Basalla

Reputation: 3106

Python parent class accessing class variable of child class

I'm currently trying to implement some inheritance in my Python project and have hit a road block. I'm trying to create a baseParentClass that will handle the basic functionality for a lot of child classes. In this specific example I am trying to initialize an instance with a number of attributes (set to 0), stored as a class variable list (called ATTRS) in the Child. I am unsure of how to use this ATTRS in the parent class. 

class Parent(object):
    def __init__():
        for attr in ATTRS:
            setattr(self, attr, 0)

class Child(Parent):
    #class variable
    ATTRS = [attr1, attr2, attr3]

    def __init__():
        super(Child, self).__init__()

I could store ATTRS as self.ATTRS in Child, and then use self.ATTRS in Parent successfully, but it seems preferable to me to have them stored as a class variable.

Alternatively I could pass ATTRS as a parameter like so:

class Child(Parent):
    #class variable
    ATTRS = [attr1, attr2, attr3]

    def __init__():
        super(Child, self).__init__(ATTRS)

but I'm wondering whether this somehow defeats the point of using inheritance in the first place?

I'd be grateful for any ideas, hints or feedback whether I'm barking up the wrong tree completely!

Thanks

Upvotes: 12

Views: 7641

Answers (2)

user2126456
user2126456

Reputation: 11

Adding to the above answer,

Although ATTR is a class variable in the child class, the instance variable of the child class is able to access the class variable ATTR because, Python does the following using namespaces

  1. Checks the namespace of the instance (self.dict) for the ATTR variable.
  2. If the above fails then it checks the namespace of its class. So it is able to get the value of the class variable although it's accessed using the instance variable.

Upvotes: 1

shx2
shx2

Reputation: 64298

You were close. This works:

class Parent(object):
    def __init__(self):
        for attr in self.ATTRS:
            setattr(self, attr, 0)

class Child(Parent):
    #class variable
    ATTRS = ['attr1', 'attr2', 'attr3']

    def __init__(self):
        super(Child, self).__init__()

Here, you set the ATTRS list at the class level, and conveniently access it in self.ATTRS at baseclass's __init__.

Upvotes: 7

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