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I'm trying to get back to Python but I don't get why the following code doesn't work as intended.

class Cat:
    age = 0


class Dog(Cat):
    pass


Dog.age = 1
Cat.age = 2

print(Dog.age, Cat.age)

My output is:

1 2

But why doesn't Dog.age equals 2?

Dog is a subclass of Cat and modifying the class variable of the superclass Cat would normally affect every subclass that inherits the variable as well.

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3
  • modifying the class variable of the supperclass Cat would normally affect every subclass that inherits the variable as well No it wouldn't. Where did you read that? Commented Nov 5, 2022 at 3:36
  • Do you mean print(Dog.age, Cat.age)? Commented Nov 5, 2022 at 3:40
  • @JohnGordon I'm thinking in Java, at least that's how my code works if I try running it there. Commented Nov 5, 2022 at 3:57

2 Answers 2

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3

Any property of Dog will override a property inherited from Cat. You can re-define a value in Cat, but it won't matter because it has already been overridden by the child. For example:

class Cat:
    age = 0  # Cat.age = 0


class Dog(Cat):
    pass  # Dog.age = Cat.age = 0


Dog.age=1  # Dog.age = 1, and Dog.age no longer points to Cat.age
Cat.age=2  # Cat.age = 2

print(Dog.age, Cat.age)  # Dog.age is no longer Cat.age. They are completely different

Contrast that with this:

class Cat:
    age = 0  # Cat.age = 0


class Dog(Cat):
    pass  # Dog.age = Cat.age = 0

Cat.age = 10  # Cat.age = 10

print(Dog.age, Cat.age)  # Dog.age points to Cat.age, so Dog.age resolves to 10
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2 Comments

Just to clarify, in your first codeblock Dog.age isn't a reference to Cat.age anymore as soon as I assign a value to it? So, my modified Dog class is now temporarily : class Dog(Cat) : Dog.age =2 ? (With the correct formatting of course). Because in Java Dog.age would still point to Cat.age and therefore changes both values.
@Mio You are correct, but inheritance doesn't make references to inherited properties. Instead, Python searches for the properties of superclasses if they aren't found in the class itself. Therefore, it "points to" the superclass property because the child class doesn't have its own property, not because the child class contains a reference to the superclass.
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Inheritance refers to defining a new class with little or no modification to an existing class. The new class is called derived (or child) class and the one from which it inherits is called the base (or parent) class. The class inheritance mechanism allows multiple base classes, a derived class can override any methods of its base class(es), and a method can call the method of a base class with the same name.

class Cat:
    def __init__(self):
        self.age = 2
        self.sound = "meow"


class Dog(Cat):
    def __init__(self):
        super().__init__()
        self.sound = "bark"


cat = Cat()
dog = Dog()

print(f"The cat's age is {cat.age}, and the dog's age is {dog.age}.")
print(f"Cats {cat.sound}, and dogs {dog.sound}.")

The cat's age is 2, and the dog's age is 2.

Cats meow, and dogs bark.

So, you can see the dog.age can inherit from class Cat. Notice the sound part, the method in the derived class overrides that in the base class. This is to say, we wrote the dog sound, it gets preference over the class Cat sound.

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1 Comment

Hi@Mio, this way can make your code work.