Inheritance

Inheritance

Inheritance allows us to define classes based on other classes.

A child class is a class that inherits the attributes and methods of a parent class.

Child classes can extend and modify the attributes and methods of a parent class.

Generalisation

The concept of collecting shared attributes and methods into a common parent class is called generalisation. This is what allows us to avoid repeating the same code for multiple classes. It also can make our code easier to read and understand by eliminating redundant code.

Creating a child class that extends or modifies a parent class is called specialisation.

Terminology

There is various terminology used to describe parent and child classes. The list below serves to introduce the different terminology used and you may find that we interchange terminology in this section:

• Parent classes are also known as “super” or “base” classes

• Child classes are also known as “sub”, “derived” or “extended” classes

Syntax

To specify the class to inherit from we modify the class name definition to:

class ChildClassName(ParentClassName):

where

• ChildClassName is the name of the class you are defining

• ParentClassName is the parent class you want to inherit from

• The ( ) separate the names

With two classes the syntax looks like:

class ParentClassName:

    def __init__(self, parameter_1, ..., parameter_n):
        # Set values here

class ChildClassName(ParentClassName):

    def __init__(self, parameter_1, ..., parameter_n):
        # Set values here

Example

In the example below the SavingsAccount class inherits from the BaseAccount

class BaseAccount:

    def __init__(self, account_number, balance, holder_name):
        self.account_number = account_number
        self.balance = balance
        self.holder_name = holder_name

    def deposit(self, amount):
        self.balance += amount

    def withdraw(self, amount):
        self.balance -= amount

class SavingsAccount(BaseAccount):

    interest_rate = 0.03  # 3% interest rate

    def get_earned_interest(self):
        return SavingsAccount.interest_rate * self.balance

Rules to Know

1. If a child class does not define its own constructor, __init__, the parent class’s constructor is used.

2. If a child class defines its own constructor, it must initialise any attributes used by the parent class.

3. A child class automatically inherits methods from its parent class.

4. A child class can define a method with the same name as one in the parent class. This is called overriding. When this happens, the child’s version replaces the parent’s version for instances of the child class.