How to Create Class Variable in Python: Simple Guide

In Python, create a class variable by defining it directly inside the class but outside any methods using variable_name = value. This variable is shared by all instances of the class and accessed via ClassName.variable_name or self.variable_name inside methods.

Syntax

Define a class variable inside the class but outside any method. It belongs to the class itself, not to any single object.

• ClassName: The name of your class.
• variable_name: The name of the class variable.
• value: The value assigned to the class variable.

python

class ClassName:
    variable_name = value

Example

This example shows a class variable count that keeps track of how many objects have been created from the class.

python

class Dog:
    count = 0  # class variable shared by all dogs

    def __init__(self, name):
        self.name = name  # instance variable unique to each dog
        Dog.count += 1

# Create dogs
dog1 = Dog('Buddy')
dog2 = Dog('Lucy')

print(Dog.count)  # Access class variable via class
print(dog1.count) # Access class variable via instance
print(dog2.count) # Same value shared by all instances

Output

2
2
2

Common Pitfalls

One common mistake is trying to create a class variable inside the __init__ method using self.variable_name. This creates an instance variable instead, which is unique to each object and does not share the value across instances.

Another pitfall is modifying a class variable via an instance, which creates a new instance variable and leaves the class variable unchanged.

python

class Cat:
    count = 0  # class variable

    def __init__(self, name):
        self.name = name
        # Wrong: this creates instance variable, not changes class variable
        self.count = Cat.count + 1

cat1 = Cat('Milo')
cat2 = Cat('Luna')

print(Cat.count)  # Still 0, class variable unchanged
print(cat1.count) # 1, instance variable
print(cat2.count) # 1, instance variable

# Correct way:
class CatCorrect:
    count = 0

    def __init__(self, name):
        self.name = name
        CatCorrect.count += 1

cat3 = CatCorrect('Simba')
cat4 = CatCorrect('Nala')

print(CatCorrect.count)  # 2, class variable updated correctly

Output

0
1
1
2

Quick Reference

Class Variable Cheat Sheet:

• Define inside class, outside methods: variable = value
• Access via ClassName.variable or self.variable
• Shared by all instances
• Modify using ClassName.variable = new_value to affect all instances
• Modifying via instance creates a new instance variable (avoid this)

Key Takeaways

Class variables are defined inside the class but outside any method using simple assignment.

They are shared by all instances of the class and accessed via the class name or instances.

Avoid creating class variables inside __init__ with self; that makes instance variables instead.

Modify class variables using the class name to update the shared value for all objects.

Accessing a class variable via an instance is allowed but modifying it via instance creates a new instance variable.