Instance vs Class Variables

OOP Core

Instance vs Class Variables

Python classes hold both shared and per-instance state. Know where your data lives to avoid cross-instance side effects.

Compare attributesFix accidental sharing

Comparison

Type	Defined	Accessed	Use for
Class variable	Inside class body	Class.attr or instance.attr	Defaults, configuration, counters
Instance variable	Inside methods (self.attr)	instance.attr	Per-object state

class Cart:
    tax_rate = 0.07  # class variable

    def __init__(self):
        self.items = []  # instance variable

Mutable class attributes

Be careful assigning mutable objects at class level—they are shared:

class Problem:
    values = []  # shared across all instances

Prefer instantiating inside __init__.

Next up in your learning path

Encapsulation

Wrap attributes behind safe methods.

Design

Inheritance

See how class variables behave in subclasses.

Architecture

Dataclasses

Use type hints to document attribute intent.

Modern