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

Mutable objects at class level are shared across all instances:

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

p1 = Problem()
p2 = Problem()
p1.values.append(1)
print(p2.values)  # [1] — p2 sees p1's change!

Fix by initializing in __init__:

class Fixed:
    def __init__(self):
        self.values = []  # each instance gets its own list

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