Python OOP: Mastering Classes and Objects

What is a Class?

A template that defines the structure and behavior of objects.

code Attributes

Variables that store object data

self.name = "Buddy"

Stored in each object's memory

functions Methods

Functions that define behavior

def bark(self): print("Woof!")

Operate on the object's data

terminal Class Declaration Syntax

  class ClassName:
      def __init__(self, params):  # Constructor
          self.attr = params       # Attribute init
      
      def method_name(self):     # Method
          # Function body

class def self __init__

integration_instructions Practical Implementation

      class User:
          def __init__(self, username, email):
              self.username = username  # String attribute
              self.email = email        # String attribute
              self.login_count = 0      # Integer attribute
          
          def login(self):             # Method
              self.login_count += 1
              print(f"{self.username} logged in")

→ Usage: user1 = User("alice", "alice@example.com")

→ Key Point: Attributes define state, methods define behavior.

compare Class vs Module: When to Use Each

Classes	Modules
class DataProcessor: def __init__(self, data): self.data = data	# processor.py def clean_data(data): return data.strip()
Need multiple instances Require state management	Single stateless functionality Utility functions collection

Pro Tip: Use modules for utilities, classes when you need to create objects with identity.

Objects (Instances)

Concrete individual units created from a class blueprint.

create Instantiation

  my_dog = Dog("Buddy", 3)  # Creates object
  print(my_dog.name)        # Access attribute

fingerprint Object Identity

  print(id(my_dog))         # Unique memory address
  print(type(my_dog))       # Returns: <class '__main__.Dog'>

memory Memory Representation

my_dog

Reference

→

Dog Object

name: "Buddy"
age: 3
__class__: Dog

edit Instance Mutability

  dog1 = Dog("Buddy", 3)
  dog2 = Dog("Max", 5)
  
  # Modify instance attribute
  dog1.age = 4

Key Behavior:

Changes to dog1 don't affect dog2
Each object maintains independent state

architecture Practical Usage Patterns

Collections of Objects

  dogs = [Dog("Buddy", 3), 
          Dog("Max", 5)]

Object as Arguments

  def print_dog_age(dog_obj):
      print(dog_obj.age)

Dynamic Attributes

  dog1.breed = "Golden"  # Added at runtime

The `init` Method

The constructor that initializes new object instances.

schedule

Automatic Execution Timeline

my_obj = MyClass() → Python allocates memory
__new__ method creates raw object (implicit)
__init__ initializes object state
Reference assigned to my_obj

input Required Parameters

  class User:
      def __init__(self, username):  # Mandatory
          self.username = username
  
  # user = User()  # ERROR: missing username

Forgetting arguments raises TypeError

settings Default Values

  class Account:
      def __init__(self, balance=0):  # Optional
          self.balance = balance
  
  acc = Account()  # Uses balance=0

Default args enable flexible instantiation

construction Advanced Initialization Techniques

Type Validation

  def __init__(self, name: str):
      if not isinstance(name, str):
          raise TypeError("Name must be string")
      self.name = name

Computed Attributes

  def __init__(self, first, last):
      self.fullname = f"{first} {last}"  # Derived value

warning Critical Mistakes

❌ Mutable Default Arguments

  def __init__(self, items=[]):  # Dangerous!
      self.items = items

All instances share the same list. Use None instead.

❌ Forgetting self

  def __init__(name):  # Missing self
      self.name = name

Raises TypeError: __init__() takes 1 positional argument...

Instance vs. Class Attributes

Understand where data lives and how it's shared.

Instance Attributes Class Attributes

  class Dog:
      def __init__(self, name):
          self.name = name  # Per-instance
  
  dog1 = Dog("Buddy")
  dog2 = Dog("Max")

Stored in individual objects
Unique to each instance
Defined in __init__ or methods

  class Dog:
      species = "Canis familiaris"  # Shared
  
  dog1 = Dog()
  dog2 = Dog()

Stored in the class itself
Shared across all instances
Defined at class level

memory Memory Allocation

Dog Class

→

species: "Canis familiaris"

dog1

→ name: "Buddy"

→ __class__: Dog

dog2

→ name: "Max"

→ __class__: Dog

Class attributes exist once in memory, instance attributes are per-object

warning Shadowing Class Attributes

  class Dog:
      species = "Canis familiaris"
  
  dog1 = Dog()
  dog1.species = "Felis catus"  # Creates instance attribute
  
  print(Dog.species)  # Original unchanged: "Canis familiaris"

Instance assignment doesn't modify class attribute

update Dynamic Class Changes

  Dog.species = "Canis lupus"  # Affects all instances
  
  print(dog1.species)  # "Canis lupus" (if no shadowing)
  print(dog2.species)  # "Canis lupus"

Class-level changes propagate to instances

rule When to Use Each

✅ Use Instance Attributes For:

Object-specific state (name, age, balance)
Mutable data unique to each instance

✅ Use Class Attributes For:

Constants (DEFAULT_SIZE, MAX_LIMIT)
Shared configuration across instances

Common Mistakes

error

1. Missing `self` Parameter

  class Dog:
      def bark():  # Missing self
          print("Woof!")

❌ Error Triggered:

TypeError: bark() takes 0 positional arguments but 1 was given

Python automatically passes the instance as first argument.

✅ Fix:

def bark(self):

All instance methods must take self as first parameter.

warning

2. Mutable Default Arguments

  class ShoppingCart:
      def __init__(self, items=[]):  # Dangerous!
          self.items = items

❌ Hidden Behavior:

cart1 = ShoppingCart()
  cart1.items.append("Apple")
  cart2 = ShoppingCart()
  print(cart2.items)  # ['Apple'] 😱

Default list is created once at class definition.

✅ Fix:

def __init__(self, items=None):
      self.items = items or []

Use None and initialize per-instance.

compare_arrows

3. Class vs Instance Attribute Confusion

  class Counter:
      count = 0  # Class attribute
      
      def increment(self):
          self.count += 1  # Accidentally creates instance attr

❌ Unexpected Output:

c1 = Counter()
  c1.increment()
  c2 = Counter()
  print(c2.count)  # 0 (Expected 1)

+= creates new instance attribute instead of modifying class attr.

✅ Fix:

def increment(self):
      Counter.count += 1  # Explicit class reference

Or use @classmethod for class-level operations.

auto_awesome

You're Now a Class Designer!

In this lesson, you've transformed from writing procedural scripts to architecting object-oriented systems. You can now:

Design classes that logically bundle data and behavior
Instantiate objects with independent state
Leverage __init__ for clean object initialization
Choose between class vs instance attributes strategically

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Python Classes: Blueprints for Cleaner Code

What is a Class?

code Attributes

functions Methods

terminal Class Declaration Syntax

integration_instructions Practical Implementation

compare Class vs Module: When to Use Each

Objects (Instances)

create Instantiation

fingerprint Object Identity

edit Instance Mutability

architecture Practical Usage Patterns

Collections of Objects

Object as Arguments

Dynamic Attributes

The `init` Method

Automatic Execution Timeline

input Required Parameters

settings Default Values

Type Validation

Computed Attributes

warning Critical Mistakes

Instance vs. Class Attributes

memory Memory Allocation

warning Shadowing Class Attributes

update Dynamic Class Changes

rule When to Use Each

Common Mistakes

1. Missing `self` Parameter

2. Mutable Default Arguments

3. Class vs Instance Attribute Confusion

You're Now a Class Designer!

Expand Your Knowledge

quiz Test Your Knowledge

We'd Like to Hear Your Feedback

Comments

Python Classes: Blueprints for Cleaner Code

What is a Class?

code Attributes

functions Methods

terminal Class Declaration Syntax

integration_instructions Practical Implementation

compare Class vs Module: When to Use Each

Objects (Instances)

create Instantiation

fingerprint Object Identity

edit Instance Mutability

architecture Practical Usage Patterns

Collections of Objects

Object as Arguments

Dynamic Attributes

The __init__ Method

Automatic Execution Timeline

input Required Parameters

settings Default Values

Type Validation

Computed Attributes

warning Critical Mistakes

Instance vs. Class Attributes

memory Memory Allocation

warning Shadowing Class Attributes

update Dynamic Class Changes

rule When to Use Each

Common Mistakes

1. Missing self Parameter

2. Mutable Default Arguments

3. Class vs Instance Attribute Confusion

You're Now a Class Designer!

Expand Your Knowledge

quiz Test Your Knowledge

We'd Like to Hear Your Feedback

Comments

The `init` Method

1. Missing `self` Parameter