Arguments and subclassing

Question

Consider a rectangle and a square. If we treat them as objects it is clearly evident that a square can inherit most or all of its properties from a rectangle, since a square is a special case of a rectangle. The only restriction is that a square must have similar side lengths.

Take a look at this very basic implementation.

class Rectangle(object):

    def __init__(self, x, y):
        self.x = x
        self.y = y

    def get_area(self):
        return self.x * self.y


class Square(Rectangle):

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        if self.x != self.y:
            raise ValueError("x and y must be equal.")

Type-checking is avoided for simplicity. I used args and kwargs inorder to make sure that the subclass(es) will be future proof. I am, however, not sure if I am doing it right. Rectangle class' __init__ might add more arguments in the future.

The question is, if x and y must be equal, why ask for both of them in the first place? Shouldn't there be only one of them? The current inheritance scheme has this restriction - the user is forced to enter x and y. How can this problem be solved. Turning x and y in to keyword arguments is not a solution, since no defaults (or None) are allowed.

Attempting to define Square as a subclass of Rectangle is a classic violation of the Liskov Substitution Principle precisely because of this issue. — chepner
– chepner, Commented Jul 31, 2015 at 20:11
@chepner: Although, as the article notes, that depends on what the API of the class is. If modifying the dimensions after creation is not a supported operation, then there's no problem. — BrenBarn
– BrenBarn, Commented Jul 31, 2015 at 20:13

BrenBarn · Accepted Answer · 2015-07-31 20:09:38Z

7

You can do this:

class Square(Rectangle):

    def __init__(self, x):
        super().__init__(x, x)
        if self.x != self.y:
            raise ValueError("x and y must be equal.")

If you want to add *args and **kwargs you can. However, as an aside, this will not make anything "future proof" if you pass the *args and **kwargs to Rectangle, because Rectangle does not accept them.

answered Jul 31, 2015 at 20:09

BrenBarn

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3 Comments

Reut Sharabani Over a year ago

Probably no need to check if you're setting it yourself :-)

Kevin Over a year ago

@ReutSharabani: The check might still make sense as an assert, since if it fails, something has gone fundamentally wrong.

BrenBarn Over a year ago

@ReutSharabani: The original code is presumably doing the check to make sure the superclass did the right thing.

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