Understanding Class and Instance Attributes in Python.

Understanding Class and Instance Attributes in Python

Introduction

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

1. Class Attributes
2. 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

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:

1. Instance attributes
2. 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

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.