Instance vs Class Variables

Part of the Object Oriented Programming section of Coddy's Python journey — lesson 13 of 64.

Instance variables are unique to each object, while class variables are shared by all objects of the same class.

Here is an example of a class with both types of variables

class Student:
    # Class variable - shared by all instances
    school = "Python High School"
    
    def __init__(self, name, grade):
        # Instance variables - unique to each student
        self.name = name
        self.grade = grade

Create student objects:

alice = Student("Alice", "A")
bob = Student("Bob", "B")

Access instance variables (unique to each object):

print(alice.name)  # Alice
print(bob.name)    # Bob
print(alice.grade) # A
print(bob.grade)   # B

Access class variables (shared by all objects):

print(alice.school)     # Python High School
print(bob.school)       # Python High School
print(Student.school)   # Python High School

Now change the class variable:

Student.school = "Python Academy"

Check how the change affects all instances:

print(alice.school)   # Python Academy
print(bob.school)     # Python Academy

Output:

Alice
Bob
A
B
Python High School
Python High School
Python High School
Python Academy
Python Academy

You can also create instance variables after object creation:

alice.age = 16  # This creates an instance variable
print(alice.age)  # 16
# print(bob.age)  # This would cause an error - bob doesn't have age

Key Difference: Instance variables (created with self.variable_name) are unique to each object, while class variables (defined directly in the class) are shared by all objects. Changing a class variable affects all instances.

Challenge

Easy

Complete a banking system.. You'll define a BankAccount class in one file and use it in another.

You'll work with:

bank_account.py: Define the BankAccount class with attributes and methods
driver.py: Import the class, create accounts, and demonstrate class variables

Follow the TODO comments in both files for step-by-step instructions. The comments will guide you through creating the class with proper variables, methods, and showing how class variables affect all instances.

Cheat sheet

Instance variables are unique to each object, while class variables are shared by all objects of the same class.

Class with both variable types:

class Student:
    # Class variable - shared by all instances
    school = "Python High School"
    
    def __init__(self, name, grade):
        # Instance variables - unique to each student
        self.name = name
        self.grade = grade

Access instance variables (unique to each object):

alice = Student("Alice", "A")
bob = Student("Bob", "B")
print(alice.name)  # Alice
print(bob.name)    # Bob

Access class variables (shared by all objects):

print(alice.school)     # Python High School
print(bob.school)       # Python High School
print(Student.school)   # Python High School

Changing class variables affects all instances:

Student.school = "Python Academy"
print(alice.school)   # Python Academy
print(bob.school)     # Python Academy

Create instance variables after object creation:

alice.age = 16  # This creates an instance variable
print(alice.age)  # 16

Try it yourself

# TODO: Import the BankAccount class from bank_account.py
from bank_account import BankAccount

# TODO: Create two accounts with these exact details:
# - account1: holder "Alice Smith" with $1000 balance
# - account2: holder "Bob Jones" with $2000 balance

# TODO: Call display_info() for both accounts

# TODO: Change the class variable interest_rate to 0.03 (3%)
# Use this format: BankAccount.interest_rate = 0.03

# TODO: Print the text "After interest rate change:"

# TODO: Call display_info() for both accounts again to show they both have the new interest rate