README.md

Module 044: Class Variables vs Instance Variables

Phase: 5. OOP | Estimated time: 1.5 hours | Milestone Project: No

Prerequisites

• Module 043 (Constructors and self)

Learning Objectives

By the end of this module, you will be able to:

• Define class variables (shared across all instances)
• Define instance variables (unique to each instance)
• Modify class vs instance variables and understand the difference
• Decide when to use each
• Use __dict__ to inspect attributes

Why This Matters

Not all data in a class belongs to individual objects. Class variables let you share state across all instances — like a counter, a default setting, or a constant.

Concept Explanation

Class Variables

Defined outside any method, directly in the class body. They are shared by all instances.

class Employee:
    """Represent an employee."""

    company = "TechCorp"          # class variable
    employee_count = 0            # class variable

    def __init__(self, name):
        """Initialize employee."""
        self.name = name          # instance variable
        Employee.employee_count += 1

e1 = Employee("Alice")
e2 = Employee("Bob")

print(e1.company)      # TechCorp
print(e2.company)      # TechCorp
print(Employee.company)  # TechCorp

Instance Variables

Defined inside methods (usually __init__) with self.attr. Each instance has its own copy.

class Employee:
    """Represent an employee."""

    company = "TechCorp"

    def __init__(self, name, salary):
        """Initialize employee with instance vars."""
        self.name = name
        self.salary = salary

e1 = Employee("Alice", 70000)
e2 = Employee("Bob", 80000)

print(e1.name)    # Alice
print(e2.name)    # Bob  (different!)

Modifying Class vs Instance Variables

class Employee:
    company = "TechCorp"

e1 = Employee()
e2 = Employee()

# Modify class variable via class
Employee.company = "NewCorp"
print(e1.company)  # NewCorp
print(e2.company)  # NewCorp

# Shadow: assign to instance creates new instance var
e1.company = "PersonalCo"
print(e1.company)  # PersonalCo  (instance var shadows class var)
print(e2.company)  # NewCorp     (still class var)
print(Employee.company)  # NewCorp

When to Use Each

Use case	Class Variable	Instance Variable
Constants / defaults	✅ MAX_SPEED = 120	❌
Counters	✅ total_instances += 1	❌
Shared configuration	✅ allowed_roles = ["admin"]	❌
Unique object data	❌	✅ self.name = name
Mutable shared state	⚠️ Use with caution	✅

__dict__ for Attribute Inspection

Every object has a __dict__ that stores its attributes.

class Point:
    """A 2D point."""

    origin_name = "O"

    def __init__(self, x, y):
        self.x = x
        self.y = y

p = Point(3, 4)
print(p.__dict__)        # {'x': 3, 'y': 4}
print(Point.__dict__)    # includes origin_name, __init__, etc.

Common Pitfalls

1. Modifying mutable class variables via one instance: If a class variable is a list and you do instance.list.append(), it affects all instances.
2. Shadowing: Assigning instance.class_var = value creates a new instance variable instead of modifying the class variable.
3. Forgetting self: Instance methods must use self.attr to access instance variables.

Hands-On Walkthrough

1. Create a class Counter with a class variable count = 0 and an __init__ that increments it.
2. Create three instances and print Counter.count.
3. Add an instance method that resets only that instance's tracking without affecting the class counter.
4. Inspect __dict__ on both the class and an instance.

Key Takeaways

• Class variables are shared across all instances; instance variables are per-object.
• Access class variables via ClassName.var or self.var (if not shadowed).
• Assigning to self.var creates an instance variable that may shadow a class variable.
• Use class variables for shared state, constants, or defaults.
• __dict__ lets you inspect all attributes of an object.

Further Reading

• Python docs: Class and instance variables
• Real Python: Class vs Instance Variables

Next Module

Continue to Module 045: Encapsulation.