Class inheritance

Introduction to Object-Oriented Programming in Python

George Boorman

Curriculum Manager, DataCamp

Code reuse

1. Someone has already done it

Packages are great for fixed functionality
OOP is great for customizing functionality

logos of several python packages

Code reuse

1. Someone has already done it

2. DRY: Don't Repeat Yourself

various gui elements

Code reuse

1. Someone has already done it

2. DRY: Don't Repeat Yourself

don't use various gui elements

Inheritance

$$\Large{\text{New class functionality = Old class functionality + extra}}$$

Example hierarchy

BankAccount class

Example hierarchy

SavingsAccount inheriting from BankAccount

Example hierarchy

CheckingAccount also inheriting from BankAccount

Example hierarchy

CheckingAccount has a modified withdraw method

Implementing class inheritance

class BankAccount:
    def __init__(self, balance):
       self.balance = balance

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


# Class inheriting from BankAccount
class SavingsAccount(BankAccount):
    pass

BankAccount: Parent class whose functionality is being extended/inherited
SavingsAccount: Child/sub-class that will inherit the functionality and add more

Child class has all of the parent data

# Constructor inherited from BankAccount
savings_acct = SavingsAccount(1000)
type(savings_acct)

__main__.SavingsAccount

# Attribute inherited from BankAccount
savings_acct.balance

# Method inherited from BankAccount
savings_acct.withdraw(300)

Inheritance: "is-a" relationship

A SavingsAccount is a BankAccount

(possibly with special features)

savings_acct = SavingsAccount(1000)
isinstance(savings_acct, SavingsAccount)

True

isinstance(savings_acct, BankAccount)

True

acct = BankAccount(500)
isinstance(acct, SavingsAccount)

False

isinstance(acct, BankAccount)

True

Let's practice!

Introduction to Object-Oriented Programming in Python