python-doc/Docs/Basic/05-oop.md

05 – Object-Oriented Programming (OOP) in Python

This document explains the basics of Object-Oriented Programming (OOP) in Python using simple examples. We cover:

• Classes and objects
• Attributes and methods
• Class attributes vs instance attributes
• Inheritance
• Special (magic) methods

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1. Basic Class, Attribute, and Method

Code

class test_class():
    def __init__(self, input):
        self.parm = input
        print("Class Created")

    def result(self):
        print(f"param is : {self.parm}") 

var = test_class('abbas')
var.result()

Explanation

Class

• test_class is a class, which acts as a blueprint for creating objects.

__init__ method (Constructor)

• __init__ is a special method that runs automatically when a new object is created.
• input is a parameter passed when creating the object.
• self.parm = input creates an instance attribute called parm.

Attribute

• parm is an attribute (a variable that belongs to the object).
• It stores data specific to each object.

Method

• result() is a method (a function that belongs to the class).
• It uses self.parm to access the object’s data.

Object Creation

var = test_class('abbas')

• Creates an object named var.
• Calls __init__ automatically.

Method Call

var.result()

• Calls the result method on the object.

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2. Class Attributes vs Instance Attributes

Code

class test_class():
    test_value = 'abbas'

    def __init__(self, input):
        self.parm = input
        print("Class Created")

    def result(self):
        print(f"param is : {self.parm}") 

var = test_class('abbas')
var2 = test_class('mmd')

var.result()
var.test_value

var2.test_value = 'mmd'
var2.test_value
var.test_value

Explanation

Class Attribute

test_value = 'abbas'

• This is a class attribute.
• It belongs to the class itself.
• Shared by all objects unless overridden.

Instance Attribute

self.parm = input

• This is an instance attribute.
• Each object has its own value.

Behavior Analysis

var.test_value

• Accesses the class attribute → 'abbas'

var2.test_value = 'mmd'

• Creates a new instance attribute for var2.
• Does not change the class attribute.

var2.test_value

• Returns 'mmd' (instance attribute)

var.test_value

• Still returns 'abbas' (class attribute)

Key Rule

• Instance attributes override class attributes only for that object.

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3. Inheritance

Code

class class_1():
    def __init__(self):
        print("Class 1 Created") 

    def hi(self):
        print("Hi")

class class_2(class_1):
    def __init__(self):
        print("Class 2 Created") 
        self.hi()

b = class_2()

Explanation

Parent Class

class class_1():

• class_1 is the parent (base) class.

Child Class

class class_2(class_1):

• class_2 inherits from class_1.
• It automatically has access to all public methods of class_1.

Method Usage

self.hi()

• hi() is defined in class_1.
• Because of inheritance, class_2 can call it.

Output Order

Class 2 Created
Hi

Important Note

• class_1.__init__() is not called automatically here.
• To call it, you would need:

super().__init__()

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4. Special (Magic) Methods

Code

class class_1():
    def __init__(self):
        print("Class 1 Created") 

    def __len__(self):
        return 1

    def __str__(self):
        return 'print command on class'

    def __del__(self):
        return 'on del value'

Explanation

Special methods start and end with double underscores (__) and control built-in behavior.

__init__

• Runs when an object is created.

__len__

len(object)

• Defines the behavior of len() on the object.
• Returns 1 in this example.

__str__

print(object)

• Defines the string representation of the object.
• Used by print() and str().

__del__

• Runs when the object is deleted or garbage-collected.
• Used rarely in modern Python.
• Return value is ignored.

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Summary

• Class: Blueprint for objects
• Object: Instance of a class
• Attribute: Data stored in an object
• Method: Function inside a class
• Class Attribute: Shared across all objects
• Instance Attribute: Unique per object
• Inheritance: Child class reuses parent class logic
• Magic Methods: Customize built-in Python behavior