A class variable is shared. An instance variable belongs to one object.

That is the whole lesson in one sentence.

If you create two dogs, both dogs can share the fact that they are canine. But each dog can have its own name.

Start with the smallest example

class Dog:
    species = "canine"

    def __init__(self, name):
        self.name = name

There are two kinds of data here:

• species is a class variable. It is defined on the class and shared as class-level data.
• self.name is an instance variable. Each Dog object gets its own name.

Create two dogs

dog1 = Dog("Buddy")
dog2 = Dog("Max")

print(dog1.name)
print(dog2.name)

print(dog1.species)
print(dog2.species)

Output:

Buddy
Max
canine
canine

Buddy belongs to dog1. Max belongs to dog2. Both objects can read the shared class value canine.

Video 1: Class variables vs. instance variables

Think: shared vs. unique

A simple way to decide which kind of variable makes sense is to ask:

• Should every object normally share this value? A class variable may fit.
• Can each object have a different value? An instance variable usually fits.

For a game, every basic enemy might start with the same category:

class Enemy:
    category = "basic"

    def __init__(self, name, health):
        self.name = name
        self.health = health

category is shared class-level data. But one enemy can be named Goblin with 50 health while another is named Guard with 100 health.

Video 2: Class variables in a simple example

You can read a class variable from the class

print(Dog.species)

Output:

canine

This makes the relationship clear: species lives on Dog as class data.

An instance can have its own value

dog1.species = "robot dog"

print(dog1.species)
print(dog2.species)
print(Dog.species)

Output:

robot dog
canine
canine

Here, assigning dog1.species creates an instance attribute that hides the class value when you read that name through dog1. It does not change Dog.species or dog2.species.

The shared-list mistake

This is one of the most important beginner mistakes to recognize.

class Dog:
    tricks = []

    def __init__(self, name):
        self.name = name

Because tricks is a class variable, the same list is shared.

dog1 = Dog("Buddy")
dog2 = Dog("Max")

dog1.tricks.append("sit")

print(dog2.tricks)

You may be surprised to see:

['sit']

Why? Both objects are using the same class-level list.

The simple fix

If every dog should have its own tricks, create the list on each instance:

class Dog:
    species = "canine"

    def __init__(self, name):
        self.name = name
        self.tricks = []

Now every new Dog gets a separate tricks list.

Video 3: Another direct comparison

How this connects to earlier lessons

OSPython.012: Classes and Objects introduced classes and objects. This lesson zooms in on where object data can live.

The official Python tutorial describes instance variables as data unique to each instance and class variables as data shared by all instances of the class.

Common beginner mistakes

• Putting unique object data in a class variable.
• Using a mutable class variable such as a list when each object needs its own list.
• Assuming changing an instance attribute always changes the class variable.
• Forgetting that self.name belongs to the current object.

Quick practice

1. Create a Player class.
2. Add game = "Hash Race" as a class variable.
3. Give each player its own name in __init__().
4. Create two players with different names.
5. Print both names and the shared game value.
6. Add an empty inventory list to each instance, not to the class.

Key takeaway

Class variable = shared class-level data. Instance variable = data belonging to one object. If every object needs its own mutable value, such as its own list, create that value on the instance.