Class vs Instance Attributes

Object-Oriented Programming (OOP) in Python revolves around classes and objects. One of the most important concepts in class design is understanding the difference between Class Attributes and Instance Attributes.

What are Class Variables?

• Class Variables are variables that are shared by all objects (instances) of a class.
• They belong to the class itself, not to individual objects.
• All objects access the same memory location for class variables.

Simple Definition

A class variable is defined inside the class but outside all methods, and it is shared by every instance of the class.

Syntax

class ClassName:
  class_variable = value

Example

class Compnay:
  company_name = "ABC"

# here,
  # company_name is a class variable

Example of Class Variables

class Employee:
  compnay = "ABC PVT. LID."
  
  def __init__(self,name, salary):
    self.name = name
    self.salary = salary

emp_1 = Employee("Rahul",25000)
emp_1 = Employee("Priya",15000)


print(emp1.company)
print(emp2.company)

# output: 
  # ABC PVT. LID.
  # ABC PVT. LID.

# here,
  # Both emp1 and emp2 share the same company variable.

Memory Structure of Class Variables

Class (Employee)
│
├── company = "ABC PVT. LID."
│
├── Object emp1
│     ├── name = Rahul
│     └── salary = 25000
│
└── Object emp2
      ├── name = Priya
      └── salary = 15000

company variable is stored once in class memory

All objects refer to it.

Accessing Class Variables

There are two ways.

1. Using Class Name (Recommended)

print(Employee.company)

2. Using Object

print(emp1.company)

Modifying Class Variables

Class variables can be changed in two ways.

1. Modify using Class Name

class Employee:
  company = "ABC PVT. LID."

Employee.company = "CDE PVT. LID."
print(Employee.company)

2. Modify using Object

emp1.company = "Amazon"

Important:

• This does not modify the class variable.
• Instead it creates a new instance variable for that object.

Example:

class Employee:
  company = "ABC"

emp1 = Employee()
emp2 = Employee()

emp1.company = "DEF"
print(emp1.company)
print(emp2.company)

# output:
  # ABC
  # DEF

What are Instance Variables?

Instance variables are variables that belong to individual objects (instances) of a class.

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Each object gets its own copy of these variables.

Simple Definition

Instance variables are variables defined inside a method (usually __init__) using self and they store data specific to each object.

Syntax

class ClassName:
    
    def __init__(self):
        self.variable_name = value

Example:

class Student:

    def __init__(self, name, age):
        self.name = name
        self.age = age

# here,
  # self.name
  # self.age
# are instance variables.

Example

class Employee:

    def __init__(self, name, salary):
        self.name = name
        self.salary = salary


emp1 = Employee("Rahul", 50000)
emp2 = Employee("Priya", 70000)

print(emp1.name, emp1.salary)
print(emp2.name, emp2.salary)

Memory Structure of Instance Variables

Class: Employee
│
├── Object emp1
│     ├── name = Rahul
│     └── salary = 50000
│
└── Object emp2
      ├── name = Priya
      └── salary = 70000

Instance variables are stored separately for each object

Where Can Instance Variables Be Created?

Instance variables can be created in three places.

1. Inside Constructor (__init__)

class Student:

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

2. Inside Instance Methods

class Student:

    def set_age(self, age):
        self.age = age

3. Outside the Class (Using Object)

class Student:
    pass

s1 = Student()
s1.name = "Rahul"

Accessing Instance Variables

Instance variables are accessed using object reference.

class Car:

    def __init__(self, brand):
        self.brand = brand


c1 = Car("BMW")

print(c1.brand)

Modifying Instance Variables

Instance variables can be modified easily.

class Student:

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


s1 = Student("Rahul")

s1.name = "Amit"

print(s1.name)

Deleting Instance Variables

You can delete them using del.

class Student:

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


s1 = Student("Rahul")

del s1.name

Questions

1. What is a Class Variable in Python?
2. What is an Instance Variable?
3. How do you access a class variable?
4. How do you create a class in Python?
5. How do you access an instance variable?
6. How many copies of class variables exist?

Table of Contents

• Python: A Beginner’s Guide to Programming
• Object-Oriented Programming (OOP) in Python