Python Class Attributes – Shared State Across Instances

Introduction – Why Use Class Attributes?

In Python OOP, class attributes are attributes that are shared across all instances of a class. Unlike instance attributes (defined with self), class attributes:

• Belong to the class itself, not to individual objects
• Are stored only once in memory
• Can be accessed or modified using either the class or any of its objects

They are useful when you want shared data or constants across all objects.

In this guide, you’ll learn:

• What class attributes are
• How they differ from instance attributes
• How to access and modify them
• Use cases and best practices
• Real-world examples and gotchas

What Are Class Attributes?

A class attribute is defined inside the class but outside all methods.

class Car:
    wheels = 4  # class attribute

    def __init__(self, brand):
        self.brand = brand  # instance attribute

Accessing Class Attributes

Using the class name:

print(Car.wheels)  # 4

Using an instance:

c1 = Car("Tesla")
print(c1.wheels)  # 4

Even though you’re using an object, it refers to the class attribute if no instance attribute of the same name exists.

Modifying Class Attributes

Change it for all instances:

Car.wheels = 6
print(c1.wheels)  # 6

Overriding with an instance attribute:

c1.wheels = 8
print(c1.wheels)      # 8
print(Car.wheels)     # 6 (unchanged)

When you assign c1.wheels = 8, it creates an instance attribute that shadows the class attribute.

Class vs Instance Attributes

Real-World Example – Counting Instances

class User:
    count = 0  # class attribute

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

u1 = User("Alice")
u2 = User("Bob")
print(User.count)  # 2

The count attribute is shared and updated across all objects.

Example – Shared Configuration

class Config:
    debug = True
    version = "1.0.0"

print(Config.debug)  # True
Config.debug = False
print(Config.debug)  # False

Common Mistakes

Best Practices

Summary – Recap & Next Steps

Python class attributes provide a powerful way to share common data across all instances of a class. Use them to define default values, constants, or track instance counts.

Key Takeaways:

• Class attributes are shared by all instances
• Defined at the class level, outside methods
• Modifying via instance creates a new instance variable
• Useful for defaults, counters, and configuration flags

Real-World Relevance:
Used in web configurations, singleton designs, ORMs, data models, and analytics counters.

FAQ – Python Class Attributes

Are class attributes shared across all objects?

Yes. They are stored once and shared unless overridden in an instance.

Can class attributes be changed?

Yes, using ClassName.attribute = value. This change will reflect in all objects (unless overridden).

Can class attributes be accessed using self?

Yes, but it’s better to use ClassName.attr for clarity when modifying them.

What happens if I modify a class attribute using an instance?

It creates an instance attribute, not modifying the class one. Always update using the class name.

Should I store mutable defaults in class attributes?

Be careful. Shared mutable objects like lists/dicts can cause unexpected behavior across instances. Prefer creating a fresh copy in __init__().