how do I properly inherit from a superclass that has a __new__ method?

Question

Let\'s assume we have a class \'Parent\' , that for some reason has __new__ defined and a class \'Child\' that inherits from it. (In my case I\'m trying to inh

Answer 1

Firstly, it is not considered best practice to override __new__ exactly to avoid these problems... But it is not your fault, I know. For such cases, the best practice on overriding __new__ is to make it accept optional parameters...

class Parent(object):
    def __new__(cls, value, *args, **kwargs):
        print 'my value is', value
        return object.__new__(cls, *args, **kwargs)

...so children can receive their own:

class Child(Parent):
    def __init__(self, for_parent, my_stuff):
        self.my_stuff = my_stuff

Then, it would work:

>>> c = Child(2, "Child name is Juju")
my value is 2
>>> c.my_stuff
'Child name is Juju'

However, the author of your parent class was not that sensible and gave you this problem:

class Parent(object):
    def __new__(cls, value):
        print 'my value is', value
        return object.__new__(cls)

In this case, just override __new__ in the child, making it accept optional parameters, and call the parent's __new__ there:

class Child(Parent):
    def __new__(cls, value, *args, **kwargs):
        return Parent.__new__(cls, value)
    def __init__(self, for_parent, my_stuff):
        self.my_stuff = my_stuff