Class or Static Variables in Python?

In this article we are going to learn about class or static variables in Python. The class variables or static variables are used to share data across all the instances of a class. Unlike instance variables which are specific to each object, class variables maintain a common value for all objects of the class.

They are defined within the class but outside any instance methods, making them accessible to all instances. These variables are used when we want to have a single shared state or value among all instances.

Syntax

class ClassName:
    class_variable = value  # Defined at class level
    
    def __init__(self):
        self.instance_variable = value  # Instance-specific

Example 1: Counting Objects Created

Let's look at the following example, where we demonstrate how class variables can be used to count the objects created ?

class Demo:
    count = 0
    
    def __init__(self, name):
        self.name = name
        Demo.count += 1

x1 = Demo("Ravi")
x2 = Demo("Suresh")
print("Total objects created:", Demo.count)

The output of the above program is ?

Total objects created: 2

Example 2: Shared Configuration Value

Consider the following example, where we illustrate how class variables provide a shared configuration value across different objects ?

class Student:
    course = "B.tech"
    
    def __init__(self, name):
        self.name = name

x1 = Student("Sunil")
x2 = Student("Poorna")
print("Student 1 course:", x1.course)
print("Student 2 course:", x2.course)

The output of the above program is ?

Student 1 course: B.tech
Student 2 course: B.tech

Example 3: Modifying Class Variables

In the following example, we modify the class variable from outside the class, affecting all instances ?

class Vehicle:
    engine_type = "2stroke"
    
    def __init__(self, model):
        self.model = model

print("Initial engine type:", Vehicle.engine_type)

# Modify class variable
Vehicle.engine_type = "4stroke"

v1 = Vehicle("Ciaz")
print("After modification:", v1.engine_type)

The output of the above program is ?

Initial engine type: 2stroke
After modification: 4stroke

Key Points

• Class variables are shared among all instances of a class
• They are defined at the class level, not inside methods
• Access using ClassName.variable or instance.variable
• Modifying through the class name affects all instances
• Useful for constants, counters, and shared configuration

Conclusion

Class variables in Python provide a way to share data across all instances of a class. They are perfect for maintaining counters, shared configurations, or any data that should be common to all objects of the class.