Introduction to OOP

Object-Oriented Programming (OOP) is a programming approach that organizes programs using classes and objects. It models real-world entities, making code reusable, modular, secure, and easier to maintain.

1. Class

A class is a blueprint or template used to create objects. It defines the data (attributes) and behavior (methods) that objects will have.

Example

class Student:
    def display(self):
        print("This is a Student class.")

s = Student()
s.display()

Output

This is a Student class.

2. Object

An object is an instance of a class. It represents a real entity and can access the class’s variables and methods.

Example

class Car:
    def start(self):
        print("Car Started")

c = Car()
c.start()

Output

Car Started

3. Constructor (`init`)

A constructor is a special method that is automatically called whenever an object is created. It is mainly used to initialize object variables.

Example

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

s = Student("Alice")
print(s.name)

Output

Alice

4. `self` Keyword

self refers to the current object of the class. It is used to access instance variables and instance methods.

Example

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

    def show(self):
        print(self.name)

s = Student("David")
s.show()

Output

David

5. Instance Variables

Instance variables belong to an object. Every object has its own separate copy.

Example

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

s1 = Student("John")
s2 = Student("Emma")

print(s1.name)
print(s2.name)

Output

John
Emma

6. Instance Methods

Instance methods work with object data and always receive self as the first parameter.

Example

class Student:
    def show(self):
        print("Instance Method")

s = Student()
s.show()

Output

Instance Method

7. Class Variables

Class variables belong to the class itself and are shared among all objects.

Example

class Student:
    school = "ABC School"

s1 = Student()
s2 = Student()

print(s1.school)
print(s2.school)

Output

ABC School
ABC School

8. Class Methods (`@classmethod`)

Class methods work with class variables. They receive cls instead of self.

Example

class Student:
    school = "ABC School"

    @classmethod
    def show_school(cls):
        print(cls.school)

Student.show_school()

Output

ABC School

9. Static Methods (`@staticmethod`)

Static methods do not use instance variables or class variables. They behave like normal functions inside a class.

Example

class Math:
    @staticmethod
    def add(a, b):
        return a + b

print(Math.add(5, 3))

Output

10. Encapsulation

Encapsulation combines data and methods into a single class while controlling access to the data.

Example

class Bank:
    def __init__(self):
        self.__balance = 5000

    def show_balance(self):
        print(self.__balance)

b = Bank()
b.show_balance()

Output

11. Access Modifiers

Public

Public members can be accessed from anywhere.

class Student:
    def __init__(self):
        self.name = "Alice"

s = Student()
print(s.name)

Protected

Protected members are indicated using a single underscore (_). They are intended for internal use and subclasses.

class Student:
    def __init__(self):
        self._marks = 90

s = Student()
print(s._marks)

Private

Private members use double underscores (__) and cannot be accessed directly outside the class.

class Student:
    def __init__(self):
        self.__age = 20

    def show(self):
        print(self.__age)

s = Student()
s.show()

12. Inheritance

Inheritance allows a child class to reuse the properties and methods of a parent class.

Example

class Animal:
    def sound(self):
        print("Animal Sound")

class Dog(Animal):
    pass

d = Dog()
d.sound()

Output

Animal Sound

13. Types of Inheritance

Single Inheritance

class A:
    pass

class B(A):
    pass

Multiple Inheritance

class A:
    pass

class B:
    pass

class C(A, B):
    pass

Multilevel Inheritance

class A:
    pass

class B(A):
    pass

class C(B):
    pass

Hierarchical Inheritance

class A:
    pass

class B(A):
    pass

class C(A):
    pass

Hybrid Inheritance

Combination of two or more inheritance types.

14. Polymorphism

Polymorphism allows the same method name to perform different actions depending on the object.

Example

class Dog:
    def sound(self):
        print("Bark")

class Cat:
    def sound(self):
        print("Meow")

for animal in (Dog(), Cat()):
    animal.sound()

Output

Bark
Meow

15. Method Overriding

Method overriding occurs when a child class provides its own version of a parent class method.

Example

class Animal:
    def sound(self):
        print("Animal Sound")

class Dog(Animal):
    def sound(self):
        print("Bark")

d = Dog()
d.sound()

Output

Bark

16. Operator Overloading

Operator overloading allows operators such as +, -, and * to work with user-defined objects.

Example

class Number:
    def __init__(self, value):
        self.value = value

    def __add__(self, other):
        return self.value + other.value

n1 = Number(10)
n2 = Number(20)

print(n1 + n2)

Output

17. Abstraction

Abstraction hides implementation details and exposes only the essential features.

Example

from abc import ABC, abstractmethod

class Shape(ABC):

    @abstractmethod
    def area(self):
        pass

class Square(Shape):
    def area(self):
        print("Area = side × side")

s = Square()
s.area()

Output

Area = side × side

18. `super()` Function

super() is used to call the parent class constructor or methods.

Example

class Parent:
    def __init__(self):
        print("Parent Constructor")

class Child(Parent):
    def __init__(self):
        super().__init__()
        print("Child Constructor")

c = Child()

Output

Parent Constructor
Child Constructor

19. Magic (Dunder) Methods

Magic methods begin and end with double underscores. They define the behavior of objects.

Common Magic Methods

__init__() – Constructor
__str__() – String representation
__len__() – Returns object length
__add__() – Operator overloading
__repr__() – Official object representation

Example

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

    def __str__(self):
        return self.name

s = Student("Alice")
print(s)

Output

Alice

20. Object Introspection (Additional Important Topic)

Object introspection helps inspect an object’s type, attributes, and inheritance.

Common Functions

type()
isinstance()
issubclass()
hasattr()
getattr()
setattr()

Example

class Student:
    pass

s = Student()

print(type(s))
print(isinstance(s, Student))

21. Four Pillars of OOP

Encapsulation

Wrapping data and methods into a single class while restricting direct access.

Abstraction

Hiding implementation details and exposing only essential functionality.

Inheritance

Creating a new class from an existing class to reuse code.

Polymorphism

Using the same interface or method name with different implementations.

Advantages of OOP

Code reusability
Better maintainability
Easy debugging
Modular programming
Data security through encapsulation
Real-world problem modeling
Improved scalability

​Introduction to OOP

​1. Class

​Example

​2. Object

​Example

​3. Constructor (__init__)

​Example

​4. self Keyword

​Example

​5. Instance Variables

​Example

​6. Instance Methods

​Example

​7. Class Variables

​Example

​8. Class Methods (@classmethod)

​Example

​9. Static Methods (@staticmethod)

​Example

​10. Encapsulation

​Example

​11. Access Modifiers

​Public

​Protected

​Private

​12. Inheritance

​Example

​13. Types of Inheritance

​Single Inheritance

​Multiple Inheritance

​Multilevel Inheritance

​Hierarchical Inheritance

​Hybrid Inheritance

​14. Polymorphism

​Example

​15. Method Overriding

​Example

​16. Operator Overloading

​Example

​17. Abstraction

​Example

​18. super() Function

​Example

​19. Magic (Dunder) Methods

​Common Magic Methods

​Example

​20. Object Introspection (Additional Important Topic)

​Common Functions

​Example

​21. Four Pillars of OOP

​Encapsulation

​Abstraction

​Inheritance

​Polymorphism

​Advantages of OOP

Introduction to OOP

1. Class

Example

2. Object

Example

3. Constructor (`init`)

Example

4. `self` Keyword

Example

5. Instance Variables

Example

6. Instance Methods

Example

7. Class Variables

Example

8. Class Methods (`@classmethod`)

Example

9. Static Methods (`@staticmethod`)

Example

10. Encapsulation

Example

11. Access Modifiers

Public

Protected

Private

12. Inheritance

Example

13. Types of Inheritance

Single Inheritance

Multiple Inheritance

Multilevel Inheritance

Hierarchical Inheritance

Hybrid Inheritance

14. Polymorphism

Example

15. Method Overriding

Example

16. Operator Overloading

Example

17. Abstraction

Example

18. `super()` Function

Example

19. Magic (Dunder) Methods

Common Magic Methods

Example

20. Object Introspection (Additional Important Topic)

Common Functions

Example

21. Four Pillars of OOP

Encapsulation

Abstraction

Inheritance

Polymorphism

Advantages of OOP