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1. What is a Class?

A class is a blueprint for creating objects.

Real-Life Example

Imagine a car factory:

• Class = Car blueprint
• Object = Actual car created from the blueprint

Syntax

class Car:
    pass

This creates an empty class.

My Notes

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2. Creating an Object

Objects are instances of a class.

Example

class Car:
    pass

car1 = Car()
car2 = Car()

print(car1)
print(car2)

Output

<__main__.Car object at ...>
<__main__.Car object at ...>

Key Points

• Each object is independent.
• Objects are created using the class name followed by parentheses.

My Notes

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3. Constructor (__init__)

The constructor runs automatically whenever an object is created.

Example

class Car:
    def __init__(self):
        print("Car Created")

car1 = Car()

Output

Car Created

Key Points

• Special method.
• Automatically called.
• Used to initialize object data.

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4. Understanding self

self refers to the current object.

Example

class Car:
    def __init__(self):
        self.brand = "Toyota"

car1 = Car()

print(car1.brand)

Output

Toyota

Key Points

• Represents the current object.
• Used to access variables and methods inside the class.
• Passed automatically by Python.

My Notes on self

Read more details of self

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5. Attributes (Variables Inside a Class)

Attributes store data related to an object.

Example

class Student:
    def __init__(self):
        self.name = "John"
        self.age = 20

s1 = Student()

print(s1.name)
print(s1.age)

Output

John
20

My Notes

Learn more on Constructor vs attributes and passing values to attributes

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6. Passing Values to Constructor

We can pass values when creating objects.

Example

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

s1 = Student("John", 20)
s2 = Student("Alice", 22)

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

Output

John
Alice

My Notes

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7. Methods

Methods are functions inside a class.

Example

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

    def greet(self):
        print("Hello", self.name)

s1 = Student("John")
s1.greet()

Output

Hello John

My Notes

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8. Multiple Methods

A class can have many methods.

Example

class Calculator:

    def add(self, a, b):
        return a + b

    def multiply(self, a, b):
        return a * b

c = Calculator()

print(c.add(10, 5))
print(c.multiply(10, 5))

Output

15
50

My Notes

Understand how and why the Calculator without constructor, yes, that correct.

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9. Instance Variables

Each object gets its own copy of instance variables.

Example

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

d1 = Dog("Tommy")
d2 = Dog("Rocky")

print(d1.name)
print(d2.name)

Output

Tommy
Rocky

My Notes

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10. Class Variables

Class variables are shared by all objects.

Example

class Student:

    school = "ABC School"

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

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

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

Output

ABC School
ABC School

My Notes

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11. Instance Variables vs Class Variables

Example

class Student:

    school = "ABC School"

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

s1 = Student("John")

print(s1.name)
print(s1.school)

Output

John
ABC School

Comparison

Instance Variable	Class Variable
Unique to object	Shared by all objects
Stored in object	Stored in class
Created with self	Created directly in class

My Notes

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12. Class Methods

Used to work with class variables.

Example

class Student:

    school = "ABC School"

    @classmethod
    def get_school(cls):
        return cls.school

print(Student.get_school())

Output

ABC School

My Notes

Link on, understanding more in class methods vs instance methods

The class method can access and modify class variables.

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13. Static Methods

Static methods do not use object or class data.

Example

class Math:

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

print(Math.add(5, 3))

Output

8

My Notes

Learn More on static method

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14. Encapsulation

Encapsulation means hiding internal data.

Example

class Bank:

    def __init__(self):
        self.__balance = 1000

Key Points

• Double underscore (__) makes a variable private.
• Prevents direct access.

My Notes

Link to understand more on encapsulation

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15. Getters and Setters

Used to safely access and modify private variables.

Example

class Bank:

    def __init__(self):
        self.__balance = 1000

    def get_balance(self):
        return self.__balance

    def set_balance(self, amount):
        self.__balance = amount

My Notes

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16. Inheritance

A child class inherits properties from a parent class.

Example

class Animal:

    def eat(self):
        print("Eating")

class Dog(Animal):
    pass

d = Dog()

d.eat()

Output

Eating

My Notes

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17. Types of Inheritance

Single Inheritance

class Father:
    pass

class Son(Father):
    pass

Multiple Inheritance

class Father:
    pass

class Mother:
    pass

class Child(Father, Mother):
    pass

Multilevel Inheritance

class GrandFather:
    pass

class Father(GrandFather):
    pass

class Son(Father):
    pass

My Notes

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18. Method Overriding

A child class can replace a parent method.

Example

class Animal:

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

class Dog(Animal):

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

d = Dog()

d.sound()

Output

Bark

My Notes

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19. super()

Used to call parent class methods.

Example

class Animal:

    def __init__(self):
        print("Animal")

class Dog(Animal):

    def __init__(self):
        super().__init__()
        print("Dog")

Output

Animal
Dog

My Notes

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20. Polymorphism

Same method name, different behavior.

Example

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

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

animals = [Dog(), Cat()]

for animal in animals:
    animal.sound()

Output

Bark
Meow

My Notes

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21. Magic Methods

Special methods beginning and ending with __.

Common Magic Methods

Method	Purpose
__init__	Constructor
__str__	String Representation
__len__	Length
__add__	Addition
__eq__	Equality

Example

def __str__(self):
    return self.name

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22. Operator Overloading

Allows operators to work with custom objects.

Example

class Number:

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

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

My Notes

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23. Abstract Classes

Used to define required methods.

Example

from abc import ABC, abstractmethod

class Animal(ABC):

    @abstractmethod
    def sound(self):
        pass

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24. Composition

One class contains another class.

Example

class Engine:
    pass

class Car:

    def __init__(self):
        self.engine = Engine()

My Notes

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25. Real-World Project Example

Bank Account

class BankAccount:

    def __init__(self, owner, balance):
        self.owner = owner
        self.balance = balance

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

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

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

My Notes

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Revision Checklist

• Class
• Object
• Constructor
• self
• Attributes
• Methods
• Instance Variables
• Class Variables
• Class Methods
• Static Methods
• Encapsulation
• Getters & Setters
• Inheritance
• Method Overriding
• super()
• Polymorphism
• Magic Methods
• Operator Overloading
• Abstract Classes
• Composition