Inheritance – extending classes

What you'll learn

Let one class inherit from another
Override methods and extend them with super()
Check what an object is with isinstance()

Don’t write everything twice

Imagine you have a class Robot with a name, a battery and a status display. Now you need a delivery robot: it can do everything a robot can – and carry parcels too. Instead of writing everything again, you let the new class inherit:

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

    def introduce(self):
        return f"I am {self.name}."

class Dog(Animal):
    def bark(self):
        return "Woof!"

bello = Dog("Bello")
print(bello.introduce())
print(bello.bark())

class Dog(Animal): means: a dog is an animal. It gets all the methods of Animal for free – and can add its own.

Overriding and super()

A child class may override an inherited method, i.e. write its own version. With super() you call the parent class’s version:

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

    def introduce(self):
        return f"I am {self.name}."

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

    def introduce(self):
        return super().introduce() + f" I love {self.favourite_food}."

print(Cat("Luna", "tuna").introduce())

Press Steps: you can see Python jumping from Cat.introduce into Animal.introduce and back.

What is what?

isinstance() checks whether an object belongs to a class – across inheritance too:

class Vehicle: pass
class Car(Vehicle): pass
class Bike(Vehicle): pass

things = [Car(), Bike(), "ball"]
for thing in things:
    print(type(thing).__name__, isinstance(thing, Vehicle))

Remember: Inheritance fits when you can say “an X is a Y” – a dog is an animal, a delivery robot is a robot. A robot “has” a motor, on the other hand – you solve that with an attribute, not with inheritance.

Quick check

Before you start the task: what does this program print? Type your guess – then Python works it out.

class Animal:
    def sound(self):
        return "..."


class Cat(Animal):
    def sound(self):
        return "Meow"


class Fish(Animal):
    pass


print(Cat().sound(), Fish().sound())

Your task

Your turn

The class Robot is finished. Write a class DeliveryRobot that inherits from Robot. It also gets a list parcels (empty at the start) and the method load(parcel). A delivery robot carries at most 3 parcels – if it's full, load returns False, otherwise True. Override status() so that , 2 parcels is added at the end (with the real number).

class Robot:
    def __init__(self, name):
        self.name = name
        self.battery = 100

    def status(self):
        return f"{self.name}: {self.battery} %"


# Your class DeliveryRobot


delivery = DeliveryRobot("Parcelo")
delivery.load("Book")
delivery.load("Ball")
print(delivery.status())

Something to think about
Write class DeliveryRobot(Robot): and call super().__init__(name) first inside __init__.
Which command helps?
In load, first check whether there are already 3 parcels in the list and return False then.
Use super().status() and add f", {len(self.parcels)} parcels".
Don't change the class Robot itself – everything new belongs in DeliveryRobot.

Gap code – fill in the ___

                        class Robot:
    def __init__(self, name):
        self.name = name
        self.battery = 100

    def status(self):
        return f"{self.name}: {self.battery} %"


class DeliveryRobot(Robot):
    def __init__(self, name):
        super(___)
        self.parcels = ___

    def load(self, parcel):
        if ___:
            return ___
        self.parcels.append(___)
        return ___

    def status(self):
        return ___


delivery = DeliveryRobot("Parcelo")
delivery.load("Book")
delivery.load("Ball")
print(delivery.status())

Solve it without hints to earn a ⭐ on your progress page.

Show solution

Try it yourself first – mistakes teach you the most. Here is one possible solution:

class Robot:
    def __init__(self, name):
        self.name = name
        self.battery = 100

    def status(self):
        return f"{self.name}: {self.battery} %"


class DeliveryRobot(Robot):
    def __init__(self, name):
        super().__init__(name)
        self.parcels = []

    def load(self, parcel):
        if len(self.parcels) >= 3:
            return False
        self.parcels.append(parcel)
        return True

    def status(self):
        return super().status() + f", {len(self.parcels)} parcels"


delivery = DeliveryRobot("Parcelo")
delivery.load("Book")
delivery.load("Ball")
print(delivery.status())