How can I set override default kwargs in a parent class?

Question

Let's say I have the following parent and child classes:

class A(object):
    def __init__(self, *args, **kwargs):
        self.a = kwargs.get('a', 'default_A')
        self.b = kwargs.get('b', 'default_B')

class B(A):

    a = "override_A"

    def __init__(self, *args, **kwargs):
        super(B, self).__init__(**kwargs)

b = B()

print b.b  # this is "default_B", as expected
print b.a  # I expected this to be "override_A"

What am I doing wrong here? I've tried to understand how inheritance works via answers like this one but haven't found something that describes this specific requirement.

@jonrsharpe I just added the semantic print statements. If you run the exact code you'll quickly realize that the issue is not a printing error. — YPCrumble
– YPCrumble, Commented Jun 23, 2016 at 8:14
Yep, I've edited the question to hopefully make that clearer. You hadn't distinguished which outputs were expected and which weren't. — jonrsharpe
– jonrsharpe, Commented Jun 23, 2016 at 8:16

Ilja Everilä · Accepted Answer · 2016-06-23 08:18:14Z

8

You're mixing class and instance variables. B.a is a class variable, which is shadowed by the instance variable set in A.__init__().

You could for example use dict.setdefault():

class B(A):

    def __init__(self, *args, **kwargs):
        # If the key 'a' exists, this'll be effectively no-operation.
        # If not, then 'a' is set to 'override_A'.
        kwargs.setdefault('a', 'override_A')
        super(B, self).__init__(**kwargs)

edited Jun 23, 2016 at 8:18

answered Jun 23, 2016 at 8:12

Ilja Everilä

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1 Comment

Mahi Over a year ago

Why the (ab) though? This is a perfectly fine use of setdefault().

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