How to change parent attribute in subclass python

Question

I have the following class

class Foo():
    data = "abc"

And i subclass it

class Bar(Foo):
    data +="def"

I am trying to edit a parent class attribute in subclass. I want my parent class to have some string, and my subclass should add some extra data to that string. How it should be done in Python? Am i wrong by design?

Robᵩ · Accepted Answer · 2015-11-18T16:40:42.100

30

You ask two questions:

How it should be done in Python?

class Bar(Foo):
    data = Foo.data + "def"

Am i wrong by design?

I generally don't use class variables in Python. A more typical paradigm is to initialize an instance variable:

>>> class Foo(object):
...  data = "abc"
... 
>>> class Bar(Foo):
...     def __init__(self):
...         super(Bar, self).__init__()
...         self.data += "def"
... 
>>> b = Bar()
>>> b.data
'abcdef'
>>>

edited Nov 18 '15 at 16:40

answered Nov 18 '15 at 16:23

Robᵩ

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Couldn't edit, because such a small change, but be sure to add a closing quote. – Mike Covington Nov 18 '15 at 16:27
Thank you! Btw is it a good practice to store data in classes this way? – George Nov 18 '15 at 16:28

score 7 · Answer 2 · answered Nov 18 '15 at 16:34

You can initialize the state of the Foo class from the Bar class and then append to the data like so:

class Foo(object):
    def __init__(self):
        self.data = "abc"

class Bar(Foo):
    def __init__(self):
        super(Bar, self).__init__()
        self.data += "def"

When you instantiate an instance of Bar, the value of data that results is:

abcdef

vaultah · Answer 3 · 2015-11-18T16:33:34.417

1

You can refer to class variables of Foo from Bar's namespace via regular attribute access:

class Bar(Foo):
    data = Foo.data + 'def'

I don't know if it's a good practice, but there's no reason for it to be bad.

edited Nov 18 '15 at 16:33

answered Nov 18 '15 at 16:24

vaultah

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How to change parent attribute in subclass python

3 Answers3