Method overriding in python class multiple inheritance

Question

I have a Base class that is inherited by two classes Adder and subtractor in which I have overridden the method called test_func() to adapt to respective class properties.

class Base():
    def __init__(self, K):
        self.K = K
    def test_func(self):
        'Must be overridden since used by another_func()'
        return self.K
    def another_func(self):
        return self.K * self.test_func()

class Adder(Base):
    def __init__(self, K, M):
        Base.__init__(self, K)
        self.M = M
    def test_func(self):
        return self.M + self.K

class Subtractor(Base):
    def __init__(self, K, N):
        Base.__init__(self, K)
        self.N = N
    def test_func(self):
        return self.N - self.K

I am writing a superclass called Algebra that inherits both Adder and Subtractor.

class Algebra(Adder, Subtractor):
    def __init__(self, K, M, N):
        Adder.__init__(self, K, M)
        Subtractor.__init__(self, K, N)

Now I want to use the test_func() method selectively from Adder or Subtractor in the superclass Algebra. Is there a way in which I can do this? I tried

G = Algebra(K=2, M=5, N=7)
G.test_func()
>> 7

but it only takes method from Adder class. Is ther a way I can tell the superclass which class to evaluate the method from? For example,

G.test_func('Adder')

user2357112 · Accepted Answer · 2020-01-08 13:26:24Z

3

The only way to get this result would be to redefine Algebra.test_func() to make it explicitly work that way, ie:

class Algebra(Adder, Subtractor):
    def __init__(self, K, M, N):
        Adder.__init__(self, K, M)
        Subtractor.__init__(self, K, N)

    def test_func(self, which):
        if which == "Adder"):
            return Adder.test_func(self)
        elif which == "Subtractor":
            return Subtractor.test_func(self)
        else:
            raise ValueError("don't know %s" % which)

BUT this is a complete mess and a perfect example of inheritance abuse. Inheritance implies a "is a" relationship, and this means that any subclass of Base should keep a compatible interface (the same client code must be able to work the same with instances of whatever Base subclass) - which the above doesn't.

Now ask yourself: is an Algebra an Adder and a Subtractor ? If yes, then your design issue is with test_func() itself. Else (an Algebra has an Adder and a Subtractor), do not inherit from Adder and Subtractor but use composition/delegation instead.

edited Jan 8, 2020 at 13:26

user2357112

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answered Jan 8, 2020 at 13:18

bruno desthuilliers

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1 Comment

Pranjal Dhole Over a year ago

Thanks for the trick. Yes, it seems I am abusing inheritance for my particular problem. I my code, the Base class is a solver that takes test_funct() as particular function that is specific to the child class. The solver then uses this function to compute other functions. Therefore it was not possible for me to just have separate definitions of test_func but had to override it in child class. This is a good trick to get me going; but I'll refactor my code and see if I can avoid using this weird construction.

chepner · Accepted Answer · 2020-01-08 13:27:40Z

3

Don't override test_func; provide two methods, each of which calls the desired parent class's test_func.

class Algebra(Adder, Subtractor):
    def __init__(self, K, M, N):
        Adder.__init__(self, K, M)
        Subtractor.__init__(self, K, N)

    def test_adder_func(self):
        return Adder.test_func(self)

    def test_subtractor_func(self):
        return Subtractor.test_func(self)

I am ignoring any deeper question, like why you chose multiple inheritance here.

answered Jan 8, 2020 at 13:27

chepner

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1 Comment

Pranjal Dhole Over a year ago

Thanks for raising the deeper issue! I need to override the function since it is used by other methods in the Base class. I've updated this in my question.

Ganesh Satpute · Accepted Answer · 2020-01-08 13:15:26Z

0

The way inheritance works is

Most of the time you will have reference wiht parent class
Instantiation with child class

In your case, instead of

G = Algebra(K=2, M=5, N=7)

You should say

G = Adder(K=2, M=5, N=7)

And then call the respective function by G.test_func()

In your example, the runtime would never know how to call the child method because you have instantiated a parent object.

answered Jan 8, 2020 at 13:15

Ganesh Satpute

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