Python Classes - Complete Notes

Table of Contents

1. Introduction to OOP
2. Classes and Objects
3. Constructor (__init__)
4. Understanding self
5. Attributes
6. Methods
7. Instance Variables
8. Class Variables
9. Class Methods
10. Static Methods
11. Encapsulation
12. Getters and Setters
13. Inheritance
14. Method Overriding
15. super()
16. Polymorphism
17. Magic Methods
18. Operator Overloading
19. Abstract Classes
20. Composition
21. Dataclasses
22. Design Patterns
23. Metaclasses
24. Interview Questions
25. Practice Exercises

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1. Introduction to OOP

What is OOP?

Object-Oriented Programming (OOP) is a programming paradigm that organizes code using objects and classes.

Benefits

• Code Reusability
• Modularity
• Scalability
• Maintainability
• Security through Encapsulation

Four Pillars of OOP

1. Encapsulation
2. Inheritance
3. Polymorphism
4. Abstraction

Notes

Add your understanding here.

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2. Classes and Objects

Definition

A class is a blueprint for creating objects.

Example

class Student:
    pass

Creating Objects

s1 = Student()
s2 = Student()

Notes

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3. Constructor (__init__)

Definition

Constructor is automatically executed when an object is created.

Example

class Student:
    def __init__(self):
        print("Object Created")

Notes

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4. Understanding self

Definition

self refers to the current object.

Example

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

Key Points

• Refers to current instance
• First parameter of instance methods
• Passed automatically by Python

Notes

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5. Attributes

Instance Attributes

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

Class Attributes

class Student:
    school = "ABC School"

Notes

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6. Methods

Instance Method

class Student:
    def greet(self):
        print("Hello")

Notes

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7. Instance Variables

Definition

Variables unique to each object.

Example

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

Notes

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8. Class Variables

Definition

Shared among all objects.

Example

class Student:
    school = "ABC School"

Notes

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9. Class Methods

Syntax

@classmethod
def method(cls):
    pass

Example

class Student:
    school = "ABC"

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

Notes

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10. Static Methods

Syntax

@staticmethod
def method():
    pass

Example

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

Notes

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11. Encapsulation

Definition

Restrict direct access to data.

Example

class Bank:
    def __init__(self):
        self.__balance = 1000

Notes

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12. Getters and Setters

Getter

def get_balance(self):
    return self.__balance

Setter

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

Notes

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13. Inheritance

Definition

Acquire properties from another class.

Example

class Animal:
    pass

class Dog(Animal):
    pass

Types

• Single
• Multiple
• Multilevel
• Hierarchical
• Hybrid

Notes

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14. Method Overriding

Example

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

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

Notes

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15. super()

Purpose

Call parent class methods.

Example

class Dog(Animal):
    def __init__(self):
        super().__init__()

Notes

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16. Polymorphism

Definition

One interface, multiple implementations.

Example

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

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

Notes

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17. Magic Methods

Common Magic Methods

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

Example

def __str__(self):
    return self.name

Notes

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18. Operator Overloading

Example

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

Notes

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19. Abstract Classes

Example

from abc import ABC, abstractmethod

class Animal(ABC):

    @abstractmethod
    def sound(self):
        pass

Notes

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20. Composition

Definition

One class contains another class.

Example

class Engine:
    pass

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

Notes

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21. Dataclasses

Example

from dataclasses import dataclass

@dataclass
class Student:
    name: str
    age: int

Benefits

• Less boilerplate code
• Auto-generated methods

Notes

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22. Design Patterns

Patterns to Learn

• Singleton
• Factory
• Observer
• Strategy
• Adapter
• Decorator

Notes

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23. Metaclasses

Definition

A class that creates classes.

Example

class MyMeta(type):
    pass

Notes

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24. Interview Questions

Basic

• What is OOP?
• What is a class?
• What is an object?
• What is self?

Intermediate

• Difference between class and instance variable?
• Difference between classmethod and staticmethod?
• What is encapsulation?

Advanced

• What are magic methods?
• What is MRO?
• What are metaclasses?

My Answers

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25. Practice Exercises

Beginner

• Create Student class
• Create Car class
• Create BankAccount class

Intermediate

• Build Employee Management System
• Build Library Management System

Advanced

• Build E-Commerce System
• Build Inventory Management System
• Build School ERP

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Personal Revision Notes

Important Concepts

• self
• init
• Class Variables
• Class Methods
• Static Methods
• Inheritance
• Polymorphism
• Encapsulation
• Magic Methods
• Abstract Classes

Revision Date

Date	Topic	Status

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Questions To Ask Later

1. 2. 3. 4. 5.

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Useful Resources

• Python Official Documentation
• OOP Concepts
• Design Patterns
• Django Models (uses OOP heavily)

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Summary

Classes are blueprints for objects.

Core topics to master:

• Class
• Object
• Constructor
• self
• Methods
• Variables
• Inheritance
• Encapsulation
• Polymorphism
• Abstraction
• Magic Methods

Master these and you'll be comfortable building real-world Python applications.