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OOP – Class Variables vs Instance Variables

A class variable is shared across all instances; an instance variable is unique to each object

What it does: Class variables are defined directly in the class body, outside any method. self.name stores data that belongs to one specific object. Instance variables use self — they're created in __init__ and belong to each object. count is defined on the class itself, so all Dog instances share the same count. self.name is unique to each dog; Dog.species is one value shared by all dogs. Modifying Dog.count changes it for every instance — it's the class-level value. Class variables suit things like instance counters or constants shared by all objects. Assigning self.species = 'Feline' creates an instance variable hiding the class one for that dog only.

Common mistakes: count should be a class variable, not instance. Define at class level. Redundant species is already a class variable. No need to set in __init__.

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Learning

Duo

Where is a class variable defined?

Inside the class but outside methodsInside __init__

Matches

❌ species = 'Canine' # in __init__

↓

Error: Class variables belong outside __init__

Applying

Meanings

Term

Class variable

Meaning

Variable defined in the class body and shared by all instances.

Practice

class Dog:

Testing

Gaps

= 'Canine'

Check Answer

Spots

3 self.name = name

4 self.count += 1 # track total dogs

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Quick Questions

1. Where is a class variable defined?

A. Inside the class but outside methods ✓

B. Inside __init__

Class variables sit directly under the class line, not inside any method

2. Where is an instance variable defined?

A. Outside the class

B. Inside __init__ using self.var ✓

Instance variables use self — they're created in __init__ and belong to each object

3. class Dog:\n count = 0\nIs count shared between objects?

A. Yes it's a class variable ✓

B. No each object has its own

count is defined on the class itself, so all Dog instances share the same count

4. self.name vs Dog.species what's the difference?

A. They are the same

B. self.name is per-object, Dog.species is shared ✓

self.name is unique to each dog; Dog.species is one value shared by all dogs

5. Dog.count += 1 what happens?

A. Increments the shared counter ✓

B. Creates a new object

Modifying Dog.count changes it for every instance — it's the class-level value

Match the Pairs

species = 'Canine' # in __init__ → Error: Class variables belong outside __init__

species = Canine → Error: Missing quotes around string value

species = 'Canine' # in class body → Valid: Class variable shared by all instances

self.name = name → Ok: Instance variable unique to each object

bool(0) → Output: False

bool([]) → Output: False

Key Term

Class variable

Variable defined in the class body and shared by all instances.~~Instance variable

Fill in the Blanks

1. class Dog: species = 'Canine'

2. class Dog: count = 0 def __init__(self, name): Dog.count += 1

3. class Dog: count = 0 def __init__(self, name): self.count += 1

4. d = Dog('Rex') print(d.name)

5. print(Dog.species)

Spot the Error

Question 1

A class Dog:

B def __init__(self, name):

C self.name = name

D self.count += 1 # track total dogs

Line D — count should be a class variable, not instance. Define at class level.

Question 2

A class Dog:

B species = 'Canine'

C def __init__(self, name):

D species = 'Canine'

E self.name = name

Line D — Redundant species is already a class variable. No need to set in __init__.

Question 3

A class Dog:

B count = 0

C def __init__(self, name):

D self.name = name

E count += 1

Line E — Use Dog.count to modify the class variable count alone is undefined

Question 4

A class Dog:

B count = 0

C def __init__(self, name):

D self.name = name

E self.count += 1

Line E — self.count creates an instance variable that shadows the class variable

Question 5

A class Dog:

B species = 'Canine'

D d = Dog()

E d.species = 'Feline'

F print(Dog.species) # expects Feline

Line E — d.species = 'Feline' only changes that object. Use Dog.species to change for all.

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