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 = gradeCreate 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) # BAccess class variables (shared by all objects):
print(alice.school) # Python High School
print(bob.school) # Python High School
print(Student.school) # Python High SchoolNow change the class variable:
Student.school = "Python Academy"Check how the change affects all instances:
print(alice.school) # Python Academy
print(bob.school) # Python AcademyOutput:
Alice
Bob
A
B
Python High School
Python High School
Python High School
Python Academy
Python AcademyYou 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 ageKey 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
EasyComplete 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 theBankAccountclass with attributes and methodsdriver.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.
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
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Object Oriented Programming
1Fundamentals of OOP
External FilesIntroduction to OOPClasses vs ObjectsThe self ParameterMethodsAttributesConstructor Method (__init__)Recap - Simple Calculator4Inheritance
Basic InheritanceThe super() FunctionMethod OverridingMultiple InheritanceMethod Resolution OrderRecap - Employee Hierarchy7Special Methods
Magic Methods IntroductionOperator OverloadingContainer Magic MethodsRecap - Custom List10Design Patterns Part 1
Intro to design patternSingleton PatternFactory PatternObserver PatternStrategy Pattern2Decorators
Introduction to DecoratorsProperty DecoratorStatic Method DecoratorClass Method Decorator5Polymorphism
Method Overriding RevisitedDuck TypingAbstract Classes and MethodsInterface DesignRecap - Shape Calculator8Advanced OOP Concepts
Composition vs InheritanceMixinsStatic and Class MethodsClass DecoratorsContext Managers3Class Properties
Instance vs Class VariablesProperty DecoratorsPrivate AttributesRecap - Bank Account Manager6Encapsulation
Public, Protected, Private MemAccess ModifiersInformation HidingProperty Decorators AdvancedRecap - Student Records System12Project: Library Management
Project OverviewBook and User ClassesBorrowing SystemSearch FunctionalityAdmin InterfaceTesting and Integration