OOP – Class Variables vs Instance Variables

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

1. Where is a class variable defined?

A Inside the class but outside methods

B Inside __init__

2. Where is an instance variable defined?

A Outside the class

B Inside __init__ using self.var

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

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

A self.name is per-object, Dog.species is shared

B They are the same

5. Dog.count += 1 what happens?

A Creates a new object

B Increments the shared counter

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1species = 'Canine' # in __init__		AValid: Class variable shared by all instances
2species = Canine		BError: Class variables belong outside __init__
3species = 'Canine' # in class body		COutput: False
4self.name = name		DOutput: False
5bool(0)		EOk: Instance variable unique to each object
6bool([])		FError: Missing quotes around string value

Definitions

Class variable

Fill in the Blanks

Complete each line of code by filling in the missing parts.

class Dog: _______ = 'Canine'

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

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

d = Dog('Rex') print(d.____)

print(___.species)

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A class Dog:

B def __init__(self, name):

C self.name = name

D self.count += 1 # track total dogs

Question 2

A class Dog:

B species = 'Canine'

C def __init__(self, name):

D species = 'Canine'

E self.name = name

Question 3

A class Dog:

B count = 0

C def __init__(self, name):

D self.name = name

E count += 1

Question 4

A class Dog:

B count = 0

C def __init__(self, name):

D self.name = name

E self.count += 1

Question 5

A class Dog:

B species = 'Canine'

D d = Dog()

E d.species = 'Feline'

F print(Dog.species) # expects Feline

Answer Key

OOP – Class Variables vs Instance Variables

Section A: Quick Questions

A — Inside the class but outside methods

B — Inside __init__ using self.var

A — Yes it's a class variable

A — self.name is per-object, Dog.species is shared

B — Increments the shared counter

Section B: 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

Section C: Definitions

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

Section D: Fill in the Blanks

class Dog:
    species = 'Canine'

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

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

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

print(Dog.species)

Section E: Spot the Error

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

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

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

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

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