Overriding Method Used in Base Class Python

Question

I have the following:

class Parent:

    @classmethod
    def add_2(cls, number):

        plus_1 = cls.add_1(number)
        plus_2 = cls.add_1(number)
        return plus_2

    def add_1(cls, number):

        return number + 1

class Child(Parent):

    @classmethod
    def add_1(cls, number):

        return number + 2

Child.add_1(5)
Child.add_2(5)

I want Child.add_2(5) to return 9, but it returns 7. Can anyone explain why this behavior is occurring? Its easy enough to fix it by overriding add_2 as well in the child class, but it seems overly clunky.

Is the lack of a @classmethod decorator for Parent.add_1 intentional? — Bryan Oakley
– Bryan Oakley, Commented Apr 3, 2018 at 15:35

CristiFati · Accepted Answer · 2018-04-03 15:49:36Z

2

I assume that you're expecting Child.add_2(5) to return 9 based on the fact that:

Child.add_1 adds 2 to the argument it receives (initially 5)
Child.add_2 (inherited from Parent) calls Child.add_1 twice (so it would make sense that 5 + 2 + 2 = 9)

The problem is in Parent.add_2:

plus_1 = cls.add_1(number)
plus_2 = cls.add_1(number)

cls.add_1 is called twice, but the result of the 1^st call (plus_1) is ignored, and the 2^nd call starts from number which is 5, resulting 7.

To correct the problem (which I think it's a typo due to copy/paste), modify cls.add_1 2^nd call to:

plus_2 = cls.add_1(plus_1)

to take the result of the 1^st call into account.

answered Apr 3, 2018 at 15:49

CristiFati

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1 Comment

rondo9 Over a year ago

Yes, it was a typo. Second call should be plus_2 = cls.add_1(plus_1). Purely the result of a typo! Whoops

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