How to inherit function from parent class?

Question

Lets say I have the following

 class cls1():   
    def __init__(self, num):  
        self.num = num  
    def square(self):  
        return self.num*self.num

Then I create a subclass.

class cls2(cls1):
    k1 = super().square(self)

Basically I want to use the square function of the parent class. But this is giving an error RuntimeError: super(): no arguments
If I directly use the square function as following

class cls2(cls1):
        k1 = square(self)

This gives an error NameError: name 'square' is not defined .
I don't understand what is the problem. This is what inheritance means right? cls2 should automatically take all the data and procedural attributes from cls1. So why is this not working?

You said the answer cls2 automatically inherits everything you don't need to add manually or if you want to use square with a different name you can use `k1 = self.square`. — Attila Bognár, Dec 11 '18 at 08:41
should be `self.num` in the `square` method of `cls1`. And likewise your last snippet is failing because it should be `self.square`. (There's no need to call `super` here, it's just basic inheritance.) — Robin Zigmond, Dec 11 '18 at 08:41
Is this `k1 = ...` line at `class` level or inside a function? — user2390182, Dec 11 '18 at 08:43
@schwobaseggl This is at the class level. I am just trying to understand how to use the function from the super class in a sub class — idpd15, Dec 11 '18 at 08:44
The main problem is that you're trying to call the *instance method* `square` without having an *instance* (you're still in the *class definition* stage there, you don't have an instance of the class yet). Why do you need to call `square` *there*? — deceze, Dec 11 '18 at 08:47
@deceze What is the problem with using square? Let's say I want to save a class variable k1 which stores the square of the num . How can I do that? — idpd15, Dec 11 '18 at 08:51
Again, there's a difference between *instance methods*, which need an instance like `cls1().square()`, and *class methods*, which can be called on the class directly like `cls1.square()`. Since `square` **depends** on `self.num`, it's an instance method, and you cannot call it without an instance. It's unclear how you *expect* this to work at that point. — deceze, Dec 11 '18 at 08:52
@deceze Thanks a lot for your patience in explaining. Seems like asking a dumb question(for others, not for me! - for me it's a genuine question. I am still learning the basics) gets you all the downvotes! — idpd15, Dec 11 '18 at 08:59
The downvotes are likely because it's unclear how you expect this to work, and it's therefore [not a super useful question for others to find in the future.](https://meta.stackoverflow.com/a/253230/476) — deceze, Dec 11 '18 at 09:01

score 1 · Answer 1 · answered Dec 11 '18 at 08:45

You need to do nothing.
Super() is just needed to execute a parent's class method, although your definition of cls2 was not correct:

class cls1():   
    def __init__(self, num):  
        self.num = num  
    def square(self):  
        return self.num * self.num


class cls2(cls1):
    pass


c1 = cls1(5)
print ('C1:', c1.square())

c2 = cls2(8)
print ('C2:', c2.square())

Prints:

('C1:', 25)
('C2:', 64)

score 1 · Answer 2 · edited Dec 11 '18 at 09:02

1

As has been stated, you don't need to do anything if you just want the square method to be present, it is inherited automatically. If you want a method k1 that does the square action, you can do:

class cls2(cls1):
    k1 = cls1.square

edited Dec 11 '18 at 09:02

Maurice Meyer

17,279
4
30
47

answered Dec 11 '18 at 08:57

user2390182

72,016
6
67
89

score 0 · Answer 3 · answered Dec 11 '18 at 08:46

0

This could be what you looking for :

class cls1():
    def __init__(self, num):
        self.num = num

    def square(self):
        return self.num * self.num


class cls2(cls1):
    def __init__(self, num):
        super(cls2, self).__init__(num)

    def square(self):
        return super(cls2, self).square()


cls = cls2(3)
print(cls.square())

i think the code is self explainatory . hope that helps

answered Dec 11 '18 at 08:46

Midhun Mohan

552
5
18

You should not be passing any arguments to `super`, just `super().foo()` will do just fine. – deceze Dec 11 '18 at 08:49
Why do I need to define a square method in cls2? Can't I directly use the method in cls1. You are defining a method in cls2 using the method in cls1 – idpd15 Dec 11 '18 at 08:53
@idpd15 Yes, this is redundant and superfluous here. I think it's just supposed to illustrate when you might use `super` (when you want to access the parent's implementation of a method). – deceze Dec 11 '18 at 08:54
@Midhun Yes, you *can*, but you don't need to and therefore shouldn't: https://stackoverflow.com/a/10483204/476 – deceze Dec 11 '18 at 08:56
it is just a illustration super method usage – Midhun Mohan Dec 11 '18 at 08:56

How to inherit function from parent class?

3 Answers3