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 inherit from a 3rd party class that I cannot modify)

class Parent:
    def __new__(cls, arg):
        # ... something important is done here with arg

My attempt was:

class Child(Parent):
    def __init__(self, myArg, argForSuperclass):
         Parent.__new__(argForSuperclass)
         self.field = myArg

But while

p = Parent("argForSuperclass")

works as expected

c = Child("myArg", "argForSuperclass")

fails, because ‘Child’ tries to call the __new__ method it inherits from ‘Parent’ instead of its own __init__ method.

What do I have to change in ‘Child’ to get the expected behavior?

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