Class (Static) and Instance Vairbales in Python

Last Updated : 22 Jan, 2026

In Python, variables defined inside a class can be either class variables (static variables) or instance variables. Class variables are shared by all objects of a class, whereas instance variables are unique to each object.

Unlike languages such as Java or C++, Python does not require a static keyword. Any variable assigned directly inside the class body automatically becomes a class variable.

Class Variables

A class variable is a variable that belongs to the class itself rather than to any specific object. All instances of the class share the same copy of this variable.

  • Defined inside the class but outside all methods
  • Accessed using the class name or an object
  • Memory is allocated only once
Python 
class Student:
    school = "ABC School"   # Class variable

s1 = Student()
s2 = Student()

print(s1.school)
print(s2.school)

Output
ABC School
ABC School

Explanation:

  • school is a class variable
  • Both s1 and s2 share the same value
  • The variable belongs to the class, not to individual objects

Instance Variables

Instance variables are variables that belong to a specific object. Each object maintains its own copy of these variables.

  • Defined inside methods (usually __init__())
  • Unique for each object
  • Changes affect only that particular object
Python 
class Student:
    def __init__(self, name):
        self.name = name   # Instance variable

s1 = Student("Jake")
s2 = Student("Emily")

print(s1.name)
print(s2.name)

Output
Jake
Emily

Explanation:

  • name is an instance variable
  • Each object has its own value
  • Changing one object’s value does not affect the other

Example: Class Variable vs Instance Variable

This example demonstrates how class variables are shared and instance variables remain separate.

Python 
class CSStudent:
    stream = 'cse'          # Class variable

    def __init__(self, name, roll):
        self.name = name    # Instance variable
        self.roll = roll    # Instance variable

# Creating objects
a = CSStudent('Rose', 1)
b = CSStudent('Nat', 2)

print(a.stream)
print(b.stream)
print(a.name)
print(b.name)

Output
cse
cse
Rose
Nat

Explanation:

  • stream is shared by all objects
  • name and roll are unique for each object
  • Both objects access the same class variable

Modifying Class Variables

Class variables can be modified in two ways. However, only one method is recommended because it avoids confusion.

Modifying via an Instance (Not Recommended)

Python 
a.stream = 'ece'
print(a.stream)
print(b.stream)

Output

ece
cse

When we change the class variable using an object (a.stream), Python does not change the class variable. Instead, it creates a new instance variable only for a. So:

  • a gets its own stream = 'ece'
  • b still uses the original class variable value cse

This can be confusing because it looks like the class variable was changed, but it was not.

Modifying via the Class Name (Recommended)

Python 
CSStudent.stream = 'mech'
print(a.stream)
print(b.stream)

Output

ece
mech

When we modify the variable using the class name, the class variable is actually updated. All objects that do not have their own instance variable will reflect this change. Here:

  • b shows the updated value mech
  • a still shows ece because it already has its own instance variable
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