Deepika Pusala

Posted on Aug 24

Week-04,Task- 1: OOP & Dunder Methods

#beginners #coding #python #tutorial

🐍 Python OOP — Simple Tutorial for Beginners

We'll cover:

🧱 Classes, Inheritance & MRO
✨ Dunder / Magic Methods
🏷️ @dataclass + Abstract Base Classes (abc)
🧩 Composition vs Inheritance
📋 Dunder Methods Cheat Sheet

1️⃣ Classes, Inheritance & MRO

Let's start from the absolute basics.

🧱 What is a Class?

A class is like a blueprint for creating objects.

Imagine you want to create students.

Every student might have:

name
age
college

Instead of writing separate code for every student, we create a class.

class Student:

    def __init__(self, name, age):
        self.name = name
        self.age = age

    def introduce(self):
        print("Hi, I am", self.name)

Now we can create objects:

student1 = Student("Deepika", 21)
student2 = Student("Sneha", 22)

student1.introduce()
student2.introduce()

Output:

Hi, I am Deepika
Hi, I am Sneha

🧠 Think like this:

Class
  ↓
Blueprint

Object
  ↓
Actual thing created from blueprint

Example:

Student class
     ↓
 ┌───────────────┐
 │ Deepika       │ ← object
 │ Sneha         │ ← object
 │ Koushik       │ ← object
 └───────────────┘

2️⃣ 👨‍👩‍👧 What is Inheritance?

Inheritance means:

One class can reuse the properties and methods of another class.

Suppose we have:

class Animal:

    def eat(self):
        print("Animal is eating")

    def sleep(self):
        print("Animal is sleeping")

Now we create:

class Dog(Animal):
    pass

Because Dog inherits from Animal, Dog automatically gets:

eat()
sleep()

So:

dog = Dog()

dog.eat()
dog.sleep()

Output:

Animal is eating
Animal is sleeping

🎉 We didn't write eat() or sleep() inside Dog.

That's inheritance.

🌳 Parent and Child

We usually call:

Animal
   ↓
Parent class / Base class / Superclass

and:

Dog
   ↓
Child class / Derived class / Subclass

So:

class Dog(Animal):

means:

"Dog is a type of Animal."

3️⃣ 🔄 Method Overriding

What if Dog wants its own version of eat()?

Easy!

class Animal:

    def eat(self):
        print("Animal is eating")


class Dog(Animal):

    def eat(self):
        print("Dog is eating")

Now:

dog = Dog()
dog.eat()

Output:

Dog is eating

Python chooses the method from Dog.

This is called method overriding.

4️⃣ 🧭 What is MRO?

Now comes the scary-looking word:

MRO = Method Resolution Order

But don't worry 😭.

It's simply:

The order in which Python searches for a method.

Suppose:

class Animal:

    def speak(self):
        print("Animal")


class Dog(Animal):
    pass

When we do:

dog = Dog()
dog.speak()

Python asks:

Does Dog have speak()?
        ↓
       NO
        ↓
Does Animal have speak()?
        ↓
       YES
        ↓
Run Animal.speak()

That's basically method resolution.

🔍 How do I see MRO?

Use:

print(Dog.mro())

or:

print(Dog.__mro__)

You'll get something similar to:

[<class 'Dog'>, <class 'Animal'>, <class 'object'>]

Meaning:

Dog
Animal
object

Python searches in that order.

5️⃣ 😵 Multiple Inheritance + MRO

Here's where MRO becomes REALLY important.

class A:

    def show(self):
        print("A")


class B(A):

    def show(self):
        print("B")


class C(A):

    def show(self):
        print("C")


class D(B, C):
    pass

Now:

obj = D()
obj.show()

What happens?

Python needs to decide:

D
↓
B or C?
↓
A?

Python uses MRO to decide the order.

print(D.mro())

Conceptually:

D → B → C → A → object

Therefore:

obj.show()

finds show() in B first.

Output:

⭐ Important: You don't manually decide which parent's method runs. Python follows its MRO rules.

6️⃣ ✨ Dunder / Magic Methods

Now let's enter the fun part.

You've probably seen things like:

__init__
__str__
__repr__
__eq__
__len__
__add__

These are called Dunder methods.

"Dunder" = Double UNDERscore

__method__
↑       ↑
double  double
underscore

They are also called magic methods.

7️⃣ 🤔 Why do we need Dunder Methods?

Let's start with something familiar.

You can do:

a = 10
b = 20

print(a + b)

Output:

But what if we create our own class?

class Number:

    def __init__(self, value):
        self.value = value

Then:

a = Number(10)
b = Number(20)

print(a + b)

❌ Python doesn't automatically know how to add two Number objects.

That's where __add__() comes in.

8️⃣ `init()` 🏗️

You've already seen this.

class Student:

    def __init__(self, name):
        self.name = name

When you write:

student = Student("Deepika")

Python automatically calls __init__().

Think:

Student("Deepika")
       ↓
__init__("Deepika")
       ↓
self.name = "Deepika"

🧠 Purpose: __init__() is used to initialize an object's data.

9️⃣ `repr()` 🔎

Suppose:

class Student:

    def __init__(self, name, age):
        self.name = name
        self.age = age

Then:

student = Student("Deepika", 21)

print(student)

Without defining a representation, Python gives something ugly like:

<__main__.Student object at 0x000001...>

Not very useful 😭.

So:

class Student:

    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __repr__(self):
        return f"Student(name='{self.name}', age={self.age})"

Now:

print(student)

can show:

Student(name='Deepika', age=21)

🧠 Purpose: __repr__() gives a useful representation of an object, especially for debugging.

🔟 `eq()` ⚖️

eq means equal.

Normally:

a == b

asks: "Are these two things equal?"

For our own class, we can define what equality means.

class Student:

    def __init__(self, name):
        self.name = name

    def __eq__(self, other):
        return self.name == other.name

Now:

s1 = Student("Deepika")
s2 = Student("Deepika")

print(s1 == s2)

Output:

True

Because:

s1.name == s2.name
Deepika == Deepika
True

1️⃣1️⃣ `len()` 📏

Python's len() can work with our class if we define __len__().

Example:

class Team:

    def __init__(self, members):
        self.members = members

    def __len__(self):
        return len(self.members)

Now:

team = Team(["Deepika", "Sneha", "Koushik"])

print(len(team))

Output:

Python internally does something conceptually like:

team.__len__()

1️⃣2️⃣ `add()` ➕

This controls the + operator.

class Number:

    def __init__(self, value):
        self.value = value

    def __add__(self, other):
        return Number(self.value + other.value)

Now:

a = Number(10)
b = Number(20)

c = a + b

print(c.value)

Output:

Behind the scenes:

a + b

roughly means:

a.__add__(b)

🔥 That's the magic.

1️⃣3️⃣ Other Useful Dunder Methods

There are MANY, but you don't need to memorize all of them.

Here's the important beginner set:

Dunder	Used for
`__init__`	Initialize object
`__repr__`	Developer-friendly representation
`__str__`	User-friendly string
`__eq__`	`==`
`__ne__`	`!=`
`__lt__`	`<`
`__gt__`	`>`
`__le__`	`<=`
`__ge__`	`>=`
`__len__`	`len()`
`__add__`	`+`
`__sub__`	`-`
`__mul__`	`*`
`__truediv__`	`/`
`__getitem__`	`obj[index]`
`__setitem__`	`obj[index] = value`
`__contains__`	`x in obj`
`__iter__`	iteration
`__next__`	next item
`__enter__`	`with` block starts
`__exit__`	`with` block ends

🏷️ 1️⃣4️⃣ What is `@dataclass`?

Now imagine you're writing this:

class Student:

    def __init__(self, name, age, college):
        self.name = name
        self.age = age
        self.college = college

And then you also want __repr__ and __eq__.

That's a lot of repetitive code 😭.

Python gives us @dataclass, which can automatically generate common methods for us.

Without `@dataclass`

class Student:

    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __repr__(self):
        return f"Student(name='{self.name}', age={self.age})"

    def __eq__(self, other):
        return self.name == other.name and self.age == other.age

That's a lot.

With `@dataclass`

from dataclasses import dataclass

@dataclass
class Student:
    name: str
    age: int

That's it! 😭❤️

Now:

s1 = Student("Deepika", 21)
s2 = Student("Deepika", 21)

print(s1)
print(s1 == s2)

Output:

Student(name='Deepika', age=21)
True

🧠 Simple definition: @dataclass is a decorator that automatically creates common methods for classes that mainly store data.

It can generate things like:

__init__
__repr__
__eq__

and more depending on the options you use.

🚨 When should you use `@dataclass`?

Use it when your class is mainly a data container.

For example:

@dataclass
class Employee:
    name: str
    salary: float
    department: str

Great use case ✅.

But if your class contains complicated business logic and isn't primarily storing data, a normal class may be better.

🧠 1️⃣5️⃣ Abstract Base Classes — `abc`

This sounds terrifying. It's actually pretty simple.

Imagine we say:

Every animal MUST have a sound() method.

We don't want to provide the actual implementation in the parent class. We just want to create a rule.

That's where an abstract class comes in.

Example

from abc import ABC, abstractmethod

class Animal(ABC):

    @abstractmethod
    def sound(self):
        pass

Now:

class Dog(Animal):

    def sound(self):
        print("Woof")

And:

class Cat(Animal):

    def sound(self):
        print("Meow")

Why is this useful?

We are basically saying:

Animal
  ↓
Every child MUST implement sound()

So:

dog = Dog()
dog.sound()

works.

But:

animal = Animal()

❌ doesn't work because Animal is abstract.

🧠 Think of ABC like a contract

Imagine your manager says:

"Every employee class must have a calculate_salary() method."

You can create:

class Employee(ABC):

    @abstractmethod
    def calculate_salary(self):
        pass

Then:

class Developer(Employee):

    def calculate_salary(self):
        return 50000


class Designer(Employee):

    def calculate_salary(self):
        return 45000

The parent establishes the rule. The child provides the actual implementation.

🧩 1️⃣6️⃣ Composition vs Inheritance

This is SUPER important for interviews.

Let's first understand inheritance.

Inheritance = "IS-A"

Example:

Dog IS-A Animal
Car IS-A Vehicle
Manager IS-A Employee

So:

class Dog(Animal):

makes sense, because:

Dog → Animal
IS-A relationship

🧩 Composition = "HAS-A"

Composition means one object contains/uses another object.

Example:

Car HAS-A Engine
House HAS-A Room
Computer HAS-A CPU

For example:

class Engine:

    def start(self):
        print("Engine started")


class Car:

    def __init__(self):
        self.engine = Engine()

    def start_car(self):
        self.engine.start()
        print("Car started")

Now:

car = Car()
car.start_car()

Output:

Engine started
Car started

Here:

Car
 ↓
HAS-A
 ↓
Engine

That's composition.

⚔️ Composition vs Inheritance

Let's compare.

Inheritance:

class Dog(Animal):
    pass

Means: Dog IS-A Animal

Composition:

class Car:

    def __init__(self):
        self.engine = Engine()

Means: Car HAS-A Engine

🧠 When should I use which?

A simple rule:

Use inheritance when the relationship genuinely is IS-A:

Dog → Animal
Cat → Animal
Manager → Employee

Use composition when the relationship is HAS-A:

Car → Engine
Computer → CPU
House → Room

💡 Why is composition often preferred?

Suppose:

class Car(Engine):

This says: "Car IS-A Engine."

🤨 That doesn't make sense. A car isn't an engine. A car has an engine.

So:

class Car:

    def __init__(self):
        self.engine = Engine()

is much more logical.

🧠 The Entire Topic in One Picture

                    PYTHON OOP
                        │
        ┌───────────────┼────────────────┐
        │               │                │
     Classes       Inheritance       Composition
        │               │                │
    Blueprint         IS-A             HAS-A
        │               │                │
     Objects        Dog-Animal       Car-Engine
                        │
                       MRO
                        │
              Search order for methods


                    DUNDER METHODS
                          │
          ┌───────────────┼───────────────┐
          │               │               │
       __init__        __repr__         __eq__
          │               │               │
     initialize       representation    equality
          │
          ├── __len__ → len()
          ├── __add__ → +
          ├── __str__ → str()
          ├── __getitem__ → []
          └── __iter__ → iteration


                    SPECIAL TOOLS
                          │
              ┌───────────┴───────────┐
              │                       │
          @dataclass                 abc
              │                       │
       Data-focused class        Rules/contracts
       auto-generated methods    abstract methods

📋 Dunder Cheat Sheet — SAVE THIS! ⭐

Method	What it controls	Easy meaning
`__init__`	Object creation	Set up object
`__str__`	`str(obj)` / `print(obj)`	User-friendly text
`__repr__`	`repr(obj)`	Developer representation
`__eq__`	`obj1 == obj2`	Are they equal?
`__ne__`	`obj1 != obj2`	Are they different?
`__lt__`	`obj1 < obj2`	Less than
`__gt__`	`obj1 > obj2`	Greater than
`__le__`	`obj1 <= obj2`	Less/equal
`__ge__`	`obj1 >= obj2`	Greater/equal
`__len__`	`len(obj)`	Object length
`__add__`	`obj1 + obj2`	Addition
`__sub__`	`obj1 - obj2`	Subtraction
`__mul__`	`obj1 * obj2`	Multiplication
`__truediv__`	`obj1 / obj2`	Division
`__getitem__`	`obj[key]`	Get item
`__setitem__`	`obj[key] = value`	Set item
`__contains__`	`x in obj`	Membership
`__iter__`	`for x in obj`	Make iterable
`__next__`	`next(obj)`	Get next item
`__enter__`	Enter `with`	Start context manager
`__exit__`	Exit `with`	Finish context manager
`__call__`	`obj()`	Make object callable

🎯 The 10 Things I Want You to Remember

If you're preparing for your internship/interview, don't try to memorize everything at once.

Remember these:

Class — Blueprint for creating objects.
Object — Actual instance created from a class.
Inheritance — Child class gets/reuses functionality from parent. class Dog(Animal):
Method overriding — Child provides its own version of parent's method.
MRO — Order Python follows when searching for a method. Dog.mro()
Dunder methods — Special methods that let our objects work naturally with Python operations. __init__, __repr__, __eq__, __len__, __add__
@dataclass — Makes data-focused classes much shorter by automatically generating common methods.
Abstract Base Class — Defines rules that child classes must follow. @abstractmethod
Inheritance = IS-A — Dog IS-A Animal
Composition = HAS-A — Car HAS-A Engine

DEV Community

Week-04,Task- 1: OOP & Dunder Methods

🐍 Python OOP — Simple Tutorial for Beginners

1️⃣ Classes, Inheritance & MRO

🧱 What is a Class?

🧠 Think like this:

2️⃣ 👨‍👩‍👧 What is Inheritance?

🌳 Parent and Child

3️⃣ 🔄 Method Overriding

4️⃣ 🧭 What is MRO?

🔍 How do I see MRO?

5️⃣ 😵 Multiple Inheritance + MRO

6️⃣ ✨ Dunder / Magic Methods

7️⃣ 🤔 Why do we need Dunder Methods?

8️⃣ `init()` 🏗️

9️⃣ `repr()` 🔎

🔟 `eq()` ⚖️

1️⃣1️⃣ `len()` 📏

1️⃣2️⃣ `add()` ➕

1️⃣3️⃣ Other Useful Dunder Methods

🏷️ 1️⃣4️⃣ What is `@dataclass`?

Without `@dataclass`

With `@dataclass`

🚨 When should you use `@dataclass`?

🧠 1️⃣5️⃣ Abstract Base Classes — `abc`

Example

Why is this useful?

🧠 Think of ABC like a contract

🧩 1️⃣6️⃣ Composition vs Inheritance

Inheritance = "IS-A"

🧩 Composition = "HAS-A"

⚔️ Composition vs Inheritance

🧠 When should I use which?

💡 Why is composition often preferred?

🧠 The Entire Topic in One Picture

📋 Dunder Cheat Sheet — SAVE THIS! ⭐

🎯 The 10 Things I Want You to Remember

Top comments (0)

🐍 Python OOP — Simple Tutorial for Beginners

1️⃣ Classes, Inheritance & MRO

🧱 What is a Class?

🧠 Think like this:

2️⃣ 👨‍👩‍👧 What is Inheritance?

🌳 Parent and Child

3️⃣ 🔄 Method Overriding

4️⃣ 🧭 What is MRO?

🔍 How do I see MRO?

5️⃣ 😵 Multiple Inheritance + MRO

6️⃣ ✨ Dunder / Magic Methods

7️⃣ 🤔 Why do we need Dunder Methods?

8️⃣ __init__() 🏗️

9️⃣ __repr__() 🔎

🔟 __eq__() ⚖️

1️⃣1️⃣ __len__() 📏

1️⃣2️⃣ __add__() ➕

1️⃣3️⃣ Other Useful Dunder Methods

🏷️ 1️⃣4️⃣ What is @dataclass?

Without @dataclass

With @dataclass

🚨 When should you use @dataclass?

🧠 1️⃣5️⃣ Abstract Base Classes — abc

Example

Why is this useful?

🧠 Think of ABC like a contract

🧩 1️⃣6️⃣ Composition vs Inheritance

Inheritance = "IS-A"

🧩 Composition = "HAS-A"

⚔️ Composition vs Inheritance

🧠 When should I use which?

💡 Why is composition often preferred?

🧠 The Entire Topic in One Picture

📋 Dunder Cheat Sheet — SAVE THIS! ⭐

🎯 The 10 Things I Want You to Remember

8️⃣ `init()` 🏗️

9️⃣ `repr()` 🔎

🔟 `eq()` ⚖️

1️⃣1️⃣ `len()` 📏

1️⃣2️⃣ `add()` ➕

🏷️ 1️⃣4️⃣ What is `@dataclass`?

Without `@dataclass`

With `@dataclass`

🚨 When should you use `@dataclass`?

🧠 1️⃣5️⃣ Abstract Base Classes — `abc`