Python basics - Day 22

Day 22 – OOP Basics: Class & Object

Project: Build a “Student Management App” using Python classes and objects.

01. Learning Goal

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

• Understand the core idea of Object-Oriented Programming (OOP)
  


• Define a class and create objects (instances)
  


• Use attributes and methods effectively
  


• Understand how the __init__() constructor works
  

02. Problem Scenario

Imagine you’re building a system to manage students in a school.


Each student has a name, an age, and the ability to introduce themselves.


Instead of storing data in plain variables, you’ll group related data and actions together inside a class.

03. Step 1 – What is OOP?

OOP (Object-Oriented Programming) is a paradigm where you model real-world entities as objects.


Each object has:

• 
  Attributes → data (e.g., name, age)
  


• 
  Methods → behaviors or actions (e.g., introduce, study)

In Korean:


객체지향 프로그래밍(OOP)은 현실 세계의 개체를 객체(object) 로 모델링하는 방식으로,


객체는 속성(데이터) 과 메서드(동작) 를 가진다.

04. Step 2 – Defining a Class

Let’s define a simple Student class with attributes and methods.

class Student:

    def __init__(self, name, age):  # Constructor

        self.name = name            # Attribute

        self.age = age



    def introduce(self):            # Method

        print(f"My name is {self.name}, I am {self.age} years old.")

05. Step 3 – Creating Objects

A class is just a blueprint — we create objects (instances) from it.

s1 = Student("Sabin", 30)

s2 = Student("Anna", 22)



s1.introduce()  # My name is Sabin, I am 30 years old.

s2.introduce()  # My name is Anna, I am 22 years old.

06. Step 4 – The __init__() Constructor

The __init__() method runs automatically whenever an object is created.

It initializes the object’s attributes.

class Car:

    def __init__(self, brand, model):

        self.brand = brand

        self.model = model



car1 = Car("Tesla", "Model 3")

print(car1.brand, car1.model)  # Tesla Model 3

07. Step 5 – Attributes & Methods

Attributes store data; methods define actions.

class Dog:

    def __init__(self, name):

        self.name = name



    def bark(self):

        print(f"{self.name} is barking! / {self.name}가 짖고 있어요!")



d = Dog("Buddy")

print(d.name)  # Buddy

d.bark()       # Buddy is barking! / Buddy가 짖고 있어요!

08. Step 6 – Practice Examples

Example 1: Book Class

class Book:

    def __init__(self, title, author):

        self.title = title

        self.author = author



    def info(self):

        print(f"'{self.title}' written by {self.author}")



b = Book("Python Basics", "Sabin")

b.info()

Output:

'Python Basics' written by Sabin

Example 2: Calculator Class

class Calculator:

    def add(self, a, b):

        return a + b



    def sub(self, a, b):

        return a - b



calc = Calculator()

print(calc.add(10, 5))  # 15

print(calc.sub(10, 5))  # 5

09. Step 7 – Mini Project: Student Management App

Let’s expand our Student class to include a grade and a study method.

class Student:

    def __init__(self, name, age, grade):

        self.name = name

        self.age = age

        self.grade = grade



    def introduce(self):

        print(f"My name is {self.name}, I’m {self.age} years old and in grade {self.grade}.")



    def study(self, subject):

        print(f"{self.name} is studying {subject}!")



s = Student("Sabin", 30, "A")

s.introduce()

s.study("Python")

Output:

My name is Sabin, I’m 30 years old and in grade A.

Sabin is studying Python!

10. Reflection

You have learned how to:

• Define and instantiate classes


• Use attributes and methods to represent data and behavior


• Understand how the __init__() constructor works


• Build your first object-oriented program
  

Next → Day 23 – OOP: Class Variables & Methods

Learn how to share data across all instances using class-level attributes and methods.