Inheritance

2 min read 4 runnable examples Quiz included

Inheritance lets a new class build on an existing one. The new class (the child or subclass) gets everything from the parent (base class), and can add or change behaviour.

Python

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

    def speak(self):
        return "..."

    def introduce(self):
        return f"I'm {self.name} and I say {self.speak()}"

class Dog(Animal):          # Dog inherits from Animal
    def speak(self):
        return "Woof!"

class Cat(Animal):
    def speak(self):
        return "Meow!"

for pet in [Dog("Rex"), Cat("Luna"), Animal("Mystery")]:
    print(pet.introduce())

Dog and Cat never define __init__ or introduce; they inherit them. They override speak with their own version.

super(): extending the parent

To add to a parent's method rather than replace it, call the parent's version with super():

Python

class Employee:
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    def describe(self):
        return f"{self.name} earns {self.salary:,}"

class Manager(Employee):
    def __init__(self, name, salary, team_size):
        super().__init__(name, salary)      # let Employee set name and salary
        self.team_size = team_size

    def describe(self):
        return super().describe() + f" and leads {self.team_size} people"

print(Employee("Ada", 138000).describe())
print(Manager("Grace", 210000, 12).describe())

isinstance and issubclass

Python

class Animal: pass
class Dog(Animal): pass

rex = Dog()
print(isinstance(rex, Dog))       # True
print(isinstance(rex, Animal))    # True: a Dog IS an Animal
print(issubclass(Dog, Animal))
print(isinstance(42, (int, float)))

Polymorphism

Code that works with the parent type works with every child, each behaving in its own way. This is polymorphism:

Python

class Shape:
    def area(self):
        raise NotImplementedError

class Circle(Shape):
    def __init__(self, r): self.r = r
    def area(self): return 3.14159 * self.r ** 2

class Square(Shape):
    def __init__(self, s): self.s = s
    def area(self): return self.s ** 2

shapes = [Circle(1), Square(3), Circle(2.5)]
print(round(sum(s.area() for s in shapes), 2))

The loop doesn't care which kind of shape each one is; it just calls area().

Finished this lesson?Mark it complete to earn +20 XP and move your progress on.