Python Object-Oriented Programming

Classes and Objects

Object-Oriented Programming (OOP) in Python allows you to define custom data types using classes. A class is a blueprint for creating objects, which are instances of the class.

Syntax

class ClassName:
    def __init__(self, attribute1, attribute2):
        self.attribute1 = attribute1
        self.attribute2 = attribute2

    def method(self):
        # Code for the method
        pass

Python
​x
 
class ClassName:
    def __init__(self, attribute1, attribute2):
        self.attribute1 = attribute1
        self.attribute2 = attribute2
​
    def method(self):
        # Code for the method
        pass

Example

class Car:
    def __init__(self, brand, model):
        self.brand = brand
        self.model = model

    def drive(self):
        print(f"The {self.brand} {self.model} is driving.")

car = Car("Toyota", "Corolla")
car.drive()  # Output: The Toyota Corolla is driving.

Python
 
class Car:
    def __init__(self, brand, model):
        self.brand = brand
        self.model = model
​
    def drive(self):
        print(f"The {self.brand} {self.model} is driving.")
​
car = Car("Toyota", "Corolla")
car.drive()  # Output: The Toyota Corolla is driving.

The __init__ method is a constructor that initializes the attributes of the class. Methods are functions defined inside the class that operate on its attributes.

Instance and Class Variables

Instance variables are unique to each object instance, while class variables are shared among all instances of a class.

Example

class Car:
    wheels = 4  # Class variable

    def __init__(self, brand, model):
        self.brand = brand  # Instance variable
        self.model = model  # Instance variable

car1 = Car("Toyota", "Corolla")
car2 = Car("Honda", "Civic")

print(car1.wheels)  # Output: 4
print(car2.wheels)  # Output: 4

Python
 
class Car:
    wheels = 4  # Class variable
​
    def __init__(self, brand, model):
        self.brand = brand  # Instance variable
        self.model = model  # Instance variable
​
car1 = Car("Toyota", "Corolla")
car2 = Car("Honda", "Civic")
​
print(car1.wheels)  # Output: 4
print(car2.wheels)  # Output: 4

wheels is a class variable, so its value is shared by all instances of the Car class. brand and model are instance variables, unique to each Car object.

Pillars of OOP

Python supports the four main pillars of Object-Oriented Programming:

Encapsulation

Bundling the data (attributes) and methods (functions) that operate on the data into a single unit, or class.

class Account:
    def __init__(self, balance):
        # Private attribute (encapsulated)
        self.__balance = balance

    def withdraw(self, amount):
        try:
            # Check if there's enough balance
            if amount > self.__balance:
                raise ValueError("Insufficient funds")
            # Deduct amount if valid
            self.__balance -= amount
        except ValueError as e:
            # Handle insufficient funds
            print("Error:", e)
        else:
            # Runs only if no exception occurs
            print(f"Withdrew: ${amount}")
        finally:
            # Always runs — show current balance
            print(f"Balance: ${self.__balance}")

# Create an account with $100
a = Account(100)

# First withdrawal: should succeed
a.withdraw(30)

# Second withdrawal: should fail due to insufficient funds
a.withdraw(100)

Python
 
class Account:
    def __init__(self, balance):
        # Private attribute (encapsulated)
        self.__balance = balance
​
    def withdraw(self, amount):
        try:
            # Check if there's enough balance
            if amount > self.__balance:
                raise ValueError("Insufficient funds")
            # Deduct amount if valid
            self.__balance -= amount
        except ValueError as e:
            # Handle insufficient funds
            print("Error:", e)
        else:
            # Runs only if no exception occurs
            print(f"Withdrew: ${amount}")
        finally:
            # Always runs — show current balance
            print(f"Balance: ${self.__balance}")
​
# Create an account with $100
a = Account(100)
​
# First withdrawal: should succeed
a.withdraw(30)
​
# Second withdrawal: should fail due to insufficient funds
a.withdraw(100)
​

Example: demonstrates encapsulation by using a private attribute __balance inside an Account class. It also uses try, except, else, and finally blocks to safely handle withdrawals and exceptions like insufficient funds. The finally block ensures the balance is always printed, regardless of success or failure.

Abstraction

Hiding the complex implementation details and exposing only the essential features of an object.

from abc import ABC, abstractmethod

# Abstract class
class Vehicle(ABC):
    @abstractmethod
    def drive(self):
        pass

# Concrete class
class Car(Vehicle):
    def drive(self):
        print("Car is driving...")

# Using abstraction
my_car = Car()
my_car.drive()  # You don't need to know how it works internally

Python
 
from abc import ABC, abstractmethod
​
# Abstract class
class Vehicle(ABC):
    @abstractmethod
    def drive(self):
        pass
​
# Concrete class
class Car(Vehicle):
    def drive(self):
        print("Car is driving...")
​
# Using abstraction
my_car = Car()
my_car.drive()  # You don't need to know how it works internally
​

Example: When you call the drive() method on a Car object, you don’t need to know how the method works internally.

Inheritance

Creating a new class that is a modified version of an existing class. The new class inherits attributes and methods from the parent class.

Example:

class ElectricCar(Car):
    def __init__(self, brand, model, battery_size):
        super().__init__(brand, model)
        self.battery_size = battery_size

    def charge(self):
        print(f"The {self.brand} {self.model} is charging.")

Python
 
class ElectricCar(Car):
    def __init__(self, brand, model, battery_size):
        super().__init__(brand, model)
        self.battery_size = battery_size
​
    def charge(self):
        print(f"The {self.brand} {self.model} is charging.")

Polymorphism

The ability to present the same interface for different underlying data types or classes.

Example:

class Dog:
    def sound(self):
        return "Woof!”

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

def make_sound(animal):
    print(animal.sound())

dog = Dog()
cat = Cat()
make_sound(dog)  # Output: Woof!
make_sound(cat)  # Output: Meow!

Python
 
class Dog:
    def sound(self):
        return "Woof!”
​
class Cat:
    def sound(self):
        return "Meow!"
​
def make_sound(animal):
    print(animal.sound())
​
dog = Dog()
cat = Cat()
make_sound(dog)  # Output: Woof!
make_sound(cat)  # Output: Meow!