Python: RecursionError with Class Inheritance

Question

I have given a class A with methods f and g. In the implementation of A.f, the method self.g is called, i.e. A.g is used. (I am not able/willing to change this class A since it comes from a python package.)

Now I want to build a child class B of A and want to overwrite the methods f and g. In the implementation of B.f, I call super().f, i.e. A.f is used. In the implementation of B.g., I call self.f, i.e. B.f is used.

I would expect that everything is well defined: B.g calls B.f calls A.f calls A.g

In fact, I get a RecursionError as I am running into an infinite number of recursions: B.g calls B.f calls A.f calls B.g

What do I not understand correctly?

Here is a implementation:

class A:
    def f(self):
        print('Method A.f was called.')
        self.g()

    def g(self):
        print('Method A.g was called.')

class B(A):
    def f(self):
        print('Method B.f was called.')
        super().f()

    def g(self):
        print('Method B.g was called.')
        self.f()

b = B()
b.g()

For clarification: In my application, A is a scikit-learn class, f is fit and g is fit_transform. In this scikit-learn class, fit_transform is explicitly overwritten, and fit unfortunately uses fit_transform. To ensure that my child class B does not inherit fit_transform from A, I have to redefine it in the usual way, and this leads to infinite recursion as explained.

self.g() is actually b.g() as in your initial call.

Klaus D.
– Klaus D.

2020-05-16 15:41:14 +00:00
Commented May 16, 2020 at 15:41 — Klaus D.
– Klaus D., Commented May 16, 2020 at 15:41

Simon A · Accepted Answer · 2020-05-16 15:41:50Z

1

The g method of B overwrites the one of A as you said, meaning A.f will call B.g, if you change the name of the method B.g your code will run as you expect it to

answered May 16, 2020 at 15:41

Simon A

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3 Comments

Sebastian Thomas Over a year ago

Is there a solution without changing the name of B.g? A is a scikit-learn transformer, f is fit and g is fit_transform. As this is standard terminology throughout scikit-learn and beyond, I want to avoid changing it for my subclass.

Simon A Over a year ago

I think the solution given by rdas would work well in your case then

Sebastian Thomas Over a year ago

In this case, my subclass B does not inherit all the other attributes from A, I would have to access them in a more complicated way. I am (almost) sure that there is a more elegant solution to this. The critical point seems to super(), but I cannot find a solution.

Karl Knechtel · Accepted Answer · 2020-05-16 15:42:19Z

1

What do I not understand correctly?

Even though self.g() was called within A.f, the value that was passed as self is still an instance of B, so B.g is looked up by this operation rather than A.g.

It would help if you could explain what problem you are trying to solve with this structure. In particular, I don't understand why, for any real-world code, it would be useful within the B class for g to call upon f, but in the base class to do it the other way around.

answered May 16, 2020 at 15:42

Karl Knechtel

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Comments

rdas · Accepted Answer · 2020-05-16 15:43:17Z

0

self is bound to the B object on which .g() was called.

Try replacing the inheritance with composition instead:

class A:
    def f(self):
        print('Method A.f was called.')
        self.g()

    def g(self):
        print('Method A.g was called.')

class B:
    def __init__(self, a):
        self.a = a

    def f(self):
        print('Method B.f was called.')
        self.a.f()

    def g(self):
        print('Method B.g was called.')
        self.f()

a = A()
b = B(a)
b.g()

Output:

Method B.g was called.
Method B.f was called.
Method A.f was called.
Method A.g was called.

answered May 16, 2020 at 15:43

rdas

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