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Classes & Objects in Python

In Python:

👉 Class = Blueprint
👉 Object = Instance created from blueprint

A class defines:

✔ Data (attributes)
✔ Behavior (methods)

An object is the actual entity in memory.

Mental Model (CRITICAL)

Think of:

  • Class → Design of a Car 🚗
  • Object → Actual Car Built

You can build many objects from one class.

1. Creating a Class

A class is defined using the class keyword.

class Person:
    pass

pass means “empty placeholder”

2. Creating an Object (Instance)

Objects are created by calling the class like a function.

p1 = Person()
p2 = Person()

Each object:

✔ Lives in memory
✔ Has a unique identity

print(id(p1))
print(id(p2))

3. Attributes — Object Data

Attributes are variables stored inside objects.

Instance Attributes

Defined inside the constructor.

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age
p1 = Person("Alice", 25)

print(p1.name)
print(p1.age)

Output:

Alice
25

Each object gets its own copy.

Class Attributes (Shared)

Defined directly inside the class.

class Person:
    species = "Human"
p1 = Person()
p2 = Person()

print(p1.species)
print(p2.species)

Shared across all objects.

Instance vs Class Attribute

Feature Instance Attribute Class Attribute
Belongs To Object Class
Shared? ❌ No ✅ Yes
Memory Per object Single copy

4. Methods — Object Behavior

Methods are functions defined inside classes.

class Person:
    def greet(self):
        print("Hello!")
p1 = Person()
p1.greet()

What Really Happens?

p1.greet() → Person.greet(p1)

Python automatically passes the object.

5. The self Keyword (ABSOLUTELY CRITICAL)

self refers to:

👉 The current object calling the method

class Person:
    def greet(self):
        print(self)

6. Constructor — __init__

Constructor runs automatically during object creation.

class Person:
    def __init__(self):
        print("Object created!")
p1 = Person()

Output:

Object created!

7. Object State (Why Objects Exist)

Objects store state (data).

class Counter:
    def __init__(self):
        self.count = 0

    def increment(self):
        self.count += 1
c1 = Counter()
c1.increment()
c1.increment()

print(c1.count)

Output:

2

8. Dynamic Attribute Creation

Python allows attributes to be added dynamically.

class Person:
    pass

p1 = Person()
p1.name = "Alice"

print(p1.name)

Flexible but dangerous in large systems.

9. Inspecting Object Internals

View Object Attributes

print(p1.__dict__)

Shows stored data.

View Class Attributes

print(Person.__dict__)

10. Method Types

Instance Method (Default)

def method(self)

Works with objects.

Class Method

Works with class itself.

@classmethod
def method(cls)

Static Method

No object / class access.

@staticmethod
def method()

Why Different Method Types Exist?

Method Type Access
Instance Method Object data
Class Method Class data
Static Method Utility logic

11. Object Identity & Equality

Identity (is)

Checks memory location.

a = [1, 2]
b = a

print(a is b)

Output:

True

Equality (==)

Checks values.

a = [1, 2]
b = [1, 2]

print(a == b)
print(a is b)

Output:

True
False

12. Object Lifecycle

Objects:

✔ Created → __init__()
✔ Used → Methods
✔ Destroyed → Garbage Collector

13. Memory Behavior

Each object:

✔ Separate identity
✔ Separate instance attributes

Class attributes → Shared memory

Common Errors

Error Cause
TypeError: missing self Forgot self parameter
AttributeError Accessing non-existent attribute
Overwriting class attribute Accidentally shadowing
Mutable class attributes bug Shared mutation

Dangerous Bug: Mutable Class Attribute

class Person:
    hobbies = []
p1 = Person()
p2 = Person()

p1.hobbies.append("Coding")

print(p2.hobbies)

Output:

['Coding']

🚨 Shared across objects → subtle bug

Best Practices

✔ Use instance attributes for object data
✔ Avoid mutable class attributes
✔ Always include self
✔ Use constructor for initialization
✔ Keep responsibilities clear

Summary

Concept Meaning
Class Blueprint
Object Instance
Instance Attribute Per object data
Class Attribute Shared data
Method Behavior
self Current object
init Constructor
dict Internal storage

Practice Tasks

  1. Create a Car class with attributes & methods
  2. Create multiple objects & compare identities
  3. Implement instance vs class attributes
  4. Demonstrate mutable class attribute bug
  5. Inspect __dict__
  6. Write class method & static method
  7. Build stateful object (Counter, BankAccount)