Lesson 1of 3

Class and Instance Attributes in Python | OOP Basics

Learn the difference between class attributes and instance attributes in Python with simple examples and clear explanations for beginners.

Introduction

In Python, attributes are variables that belong to a class or an object. There are two main types of attributes:

Class Attributes
Instance Attributes

Understanding the difference between them is very important in Object-Oriented Programming (OOP).

1. Class Attributes

Definition

A class attribute is shared by all objects (instances) of a class.

It is defined inside the class but outside methods.

Syntax

class ClassName:
    class_attribute = value

Example

class Student:
    school = "Ghazi University"   # Class Attribute
 
# Creating objects
s1 = Student()
s2 = Student()
 
print(s1.school)
print(s2.school)

Output

Ghazi University
Ghazi University

Explanation

school is a class attribute.
All objects share the same value.
Memory is allocated only once for class attributes.

Accessing Class Attributes

You can access class attributes using:

1. Object Name

print(s1.school)

2. Class Name

print(Student.school)

Modifying Class Attributes

class Student:
    school = "Ghazi University"
 
Student.school = "Punjab University"
 
s1 = Student()
print(s1.school)

Output

Punjab University

2. Instance Attributes

Definition

An instance attribute belongs to a specific object.

Each object has its own copy of instance attributes.

They are usually created inside the __init__() method using self.

Example

class Student:
    def __init__(self, name, roll_no):
        self.name = name
        self.roll_no = roll_no
 
# Creating objects
s1 = Student("Ali", 101)
s2 = Student("Sara", 102)
 
print(s1.name)
print(s2.name)

Output

Ali
Sara

Explanation

name and roll_no are instance attributes.
Every object stores its own data.
Changing one object's attributes does not affect others.

Modifying Instance Attributes

class Student:
    def __init__(self, name):
        self.name = name
 
s1 = Student("Ali")
s2 = Student("Sara")
 
s1.name = "Ahmed"
 
print(s1.name)
print(s2.name)

Output

Ahmed
Sara

Difference Between Class and Instance Attributes

Feature	Class Attribute	Instance Attribute
Shared or Separate	Shared by all objects	Separate for each object
Defined	Inside class	Inside methods using `self`
Memory Usage	One copy	Separate copy for each object
Accessed By	Class name or object	Object only
Purpose	Common data	Object-specific data

Example Combining Both

class Student:
    school = "Ghazi University"   # Class Attribute
 
    def __init__(self, name):
        self.name = name          # Instance Attribute
 
s1 = Student("Ali")
s2 = Student("Sara")
 
print(s1.school)
print(s2.school)
 
print(s1.name)
print(s2.name)

Output

Ghazi University
Ghazi University
Ali
Sara

Important Concept: Attribute Lookup

Python first checks:

Instance attributes
Then class attributes

Example

class Student:
    school = "Ghazi University"
 
s1 = Student()
 
print(s1.school)
 
s1.school = "UET"
 
print(s1.school)
print(Student.school)

Output

Ghazi University
UET
Ghazi University

Explanation

Initially, s1.school uses the class attribute.
After assignment, Python creates an instance attribute for s1.
The class attribute remains unchanged.

Real-Life Example

class Car:
    wheels = 4   # Class Attribute
 
    def __init__(self, color):
        self.color = color   # Instance Attribute
 
c1 = Car("Red")
c2 = Car("Black")
 
print(c1.wheels)
print(c2.wheels)
 
print(c1.color)
print(c2.color)

Advantages of Class Attributes

Saves memory
Stores common data
Easy to update shared information

Advantages of Instance Attributes

Stores unique object data
Allows flexibility
Each object behaves independently

Common Mistake

class Test:
    value = []
 
t1 = Test()
t2 = Test()
 
t1.value.append(10)
 
print(t2.value)

Output

[10]

Why?

Because value is a class attribute shared by all objects.

Best Practice

Use:

Class attributes for shared/common values
Instance attributes for object-specific data

Summary

Concept	Description
Class Attribute	Shared by all objects
Instance Attribute	Unique for each object
`self.attribute`	Creates instance attributes
`ClassName.attribute`	Accesses class attributes

Tasks

Task 1

Create a Teacher class with:

Class attribute: school
Instance attributes: name, subject

Create two objects and display values.

Task 2

Create a Mobile class with:

Class attribute: brand
Instance attribute: model

Change the class attribute and observe the result.

Conclusion

Class and instance attributes are fundamental concepts in Python OOP.

Use class attributes for shared data.
Use instance attributes for unique object information.

←Back To Python Development Path Continue to Lab exercise #1 →