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1 Answers

Jk Malhotra
Jk Malhotra 23 Oct 2025
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1. What is a Class?

A class is a blueprint for creating objects. It defines the structure (data) and behavior (functions) that the objects created from it will have.

Example:

class Dog:
    # class attribute (shared by all objects)
    species = "Canine"

    # constructor (called when creating an object)
    def __init__(self, name, age):
        self.name = name  # instance attribute
        self.age = age

    # instance method
    def bark(self):
        print(f"{self.name} says woof!")

2. What is an Object?

An object (also called an instance) is a real-world entity created from a class. Each object has its own unique data.

Example:

dog1 = Dog("Buddy", 3)
dog2 = Dog("Max", 5)

print(dog1.name)     # Buddy
print(dog2.age)      # 5
dog1.bark()          # Buddy says woof!

Each Dog object has its own name and age, but both share the same species attribute.

3. Class vs Instance Attributes

Type Definition Example
Class Attribute Shared by all instances species = "Canine"
Instance Attribute Unique to each object self.name, self.age

Example:

Dog.species = "Dog"
print(dog1.species)  # Dog

Changing species in the class affects all dogs (unless overridden).

4. Methods in Classes

  1. Instance methods → work with specific objects (self)
  2. Class methods → work with the class itself (cls)
  3. Static methods → general utility methods (don’t depend on either)

Example:

class Vehicle:
    count = 0  # class attribute

    def __init__(self, brand):
        self.brand = brand
        Vehicle.count += 1

    def show(self):
        print(f"This is a {self.brand}")

    @classmethod
    def total(cls):
        print(f"Total vehicles: {cls.count}")

    @staticmethod
    def is_motorcycle(wheels):
        return wheels == 2

Usage:

v1 = Vehicle("Toyota")
v2 = Vehicle("Honda")

v1.show()           # This is a Toyota
Vehicle.total()     # Total vehicles: 2
print(Vehicle.is_motorcycle(2))  # True

5. Special Methods (a.k.a. Magic or Dunder Methods)

Python classes can have “special” methods that start and end with double underscores.

Method Purpose
__init__ Constructor (runs when creating object)
__str__ String representation (when using print())
__len__ Defines behavior for len(obj)
__add__ Defines behavior for obj1 + obj2

Example:

class Book:
    def __init__(self, title, pages):
        self.title = title
        self.pages = pages

    def __str__(self):
        return f"Book: {self.title}"

    def __len__(self):
        return self.pages

b = Book("Python 101", 300)
print(b)        # Book: Python 101
print(len(b))   # 300

6. Inheritance (OOP Concept)

A child class can inherit attributes and methods from a parent class.

Example:

class Animal:
    def speak(self):
        print("Some sound")

class Dog(Animal):
    def speak(self):  # overriding parent method
        print("Woof!")

dog = Dog()
dog.speak()  # Woof!
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