Python Inheritance

1Python Inheritance: overview

Inheritance lets a new class (the child) start with all the attributes and methods of an existing class (the parent) and then add to them or change them, so shared code is written only once.

Before this lesson: Python Functions, Python Classes and Objects

2Python Inheritance: diagram

Quick reference: Python Inheritance

• Python OOP Cheat Sheet: Classes, Inheritance, Dunder Methods — In Python OOP you define a `class` as a blueprint, build objects with it, store data in attributes set in `__…
• Python isinstance() Function — `isinstance(obj, classinfo)` returns `True` if `obj` is an instance of the class (or of any class in a tuple …

4Python Inheritance: syntax

class Parent:
    def __init__(self, a):
        self.a = a

    def method(self):
        return "parent version"

class Child(Parent):                  # Child inherits from Parent
    def __init__(self, a, b):
        super().__init__(a)           # run Parent's __init__ first
        self.b = b                    # then add something new

    def method(self):                 # override the parent's method
        return "child + " + super().method()

isinstance(Child(1, 2), Parent)       # True

5Python Inheritance: run the example

Press Run (or Ctrl + Enter). The output (step 6) appears directly below the code. Change anything and run it again.

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

    def yearly_pay(self):
        return self.salary * 12

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

    def yearly_pay(self):
        return super().yearly_pay() + self.bonus

staff = [Employee("Asha", 3000), Manager("Ben", 4000, 5000)]
for person in staff:
    print(person.name, person.yearly_pay(), isinstance(person, Manager))
print("Is Ben an Employee?", isinstance(staff[1], Employee))

Expected outputAsha 36000 False
Ben 53000 True
Is Ben an Employee? True

7Python Inheritance: how the example works

Hover or tap a step to highlight its lines in the example above.

1. Line 1-4The parent class Employee stores a name and a monthly salary on every new object.
2. Line 6-7yearly_pay() returns twelve months of salary. Every employee — and every child class — gets this method.
3. Line 9class Manager(Employee): creates a child class. A Manager automatically has everything an Employee has.
4. Line 10-12Manager needs an extra bonus, so it defines its own __init__. super().__init__(name, salary) runs the parent's setup, then the child adds self.bonus.
5. Line 14-15The child overrides yearly_pay(). Instead of repeating the maths, it calls the parent's version with super().yearly_pay() and adds the bonus.
6. Line 17One list holds both kinds of objects.
7. Line 18-19Polymorphism: the same call person.yearly_pay() runs Employee's version for Asha (36000) and Manager's version for Ben (48000 + 5000). isinstance shows which objects are managers.
8. Line 20A Manager is also an Employee, because isinstance() counts parent classes too.

8Python Inheritance: common mistakes

class Vehicle:
    def __init__(self, wheels):
        self.wheels = wheels

class Bike(Vehicle):
    def __init__(self, brand):
        self.brand = brand

trek = Bike("Trek")
print(trek.wheels)

class Vehicle:
    def __init__(self, wheels):
        self.wheels = wheels

class Bike(Vehicle):
    def __init__(self, brand):
        super().__init__(2)
        self.brand = brand

trek = Bike("Trek")
print(trek.wheels)

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

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

rex = Dog("Rex")

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

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

rex = Dog("Rex")
print(rex.name)

class Shape:
    def area(self):
        return 0

class Square(Shape):
    def __init__(self, side):
        self.side = side

    def Area(self):
        return self.side ** 2

print(Square(4).area())

class Shape:
    def area(self):
        return 0

class Square(Shape):
    def __init__(self, side):
        self.side = side

    def area(self):
        return self.side ** 2

print(Square(4).area())

9Python Inheritance: exercises

Write your answer, then press Check. The checks run in your browser.

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

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

# create class Cat here


kitty = Cat("Tom")
print(kitty.speak())

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

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

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

kitty = Cat("Tom")
print(kitty.speak())

class Account:
    def __init__(self, owner):
        self.owner = owner
        self.balance = 0

class SavingsAccount(Account):
    def __init__(self, owner, rate):
        self.rate = rate

    def add_interest(self):
        self.balance += self.balance * self.rate

savings = SavingsAccount("Asha", 0.05)
savings.balance = 200
savings.add_interest()
print(savings.owner, savings.balance)

class Account:
    def __init__(self, owner):
        self.owner = owner
        self.balance = 0

class SavingsAccount(Account):
    def __init__(self, owner, rate):
        super().__init__(owner)
        self.rate = rate

    def add_interest(self):
        self.balance += self.balance * self.rate

savings = SavingsAccount("Asha", 0.05)
savings.balance = 200
savings.add_interest()
print(savings.owner, savings.balance)

2 more exercises → 4 more examples

Further reading

• Classes
• dataclasses
• Data model

10Next lesson

Related:
Python Polymorphism and Duck TypingPython Multiple Inheritance and MROPython Composition: Has-A vs Is-A

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