Inheritance in Python: example with super()

With inheritance, a new class (the child) starts with everything from an existing class (the parent). The child can then add new methods or replace, or override, the ones it inherited.

The program

python

class Animal:
    def __init__(self, name):
        self.name = name

    def speak(self):
        return f"{self.name} makes a sound"

    def describe(self):
        return f"{self.name} is a {type(self).__name__.lower()}"


class Cat(Animal):
    def speak(self):
        return f"{self.name} says: Meow!"


class Dog(Animal):
    def __init__(self, name, tricks):
        super().__init__(name)
        self.tricks = tricks

    def speak(self):
        return f"{self.name} says: Woof! (knows {len(self.tricks)} tricks)"


class Fish(Animal):
    pass


for animal in [Cat("Whiskers"), Dog("Rex", ["sit", "stay"]), Fish("Nemo")]:
    print(animal.speak(), "|", animal.describe())

output

Whiskers says: Meow! | Whiskers is a cat
Rex says: Woof! (knows 2 tricks) | Rex is a dog
Nemo makes a sound | Nemo is a fish

How it works

class Cat(Animal) means "Cat inherits from Animal". Cat gets __init__, speak and describe without writing them again.
Cat and Dog define their own speak, which overrides the parent's. Fish defines nothing, so it uses Animal's speak.
Dog needs an extra attribute, so it has its own __init__. super().__init__(name) runs Animal's __init__ first, which sets self.name. Without that line, a Dog would have no name.
The loop calls speak() on each animal without checking its type. Each object runs its own version. This is called polymorphism.

Checking the family

isinstance() is True for the object's class and for every parent class above it:

python

class Animal:
    pass

class Dog(Animal):
    pass

rex = Dog()
print(isinstance(rex, Dog))
print(isinstance(rex, Animal))
print(issubclass(Dog, Animal))
print(isinstance(Animal(), Dog))

output

True
True
True
False