🧩 Design Principles of Software: Building Better Code with Python

#softwareengineering #design #programming #solidprinciples

🚀 TL;DR

This article explains key software design principles (SOLID + DRY +
KISS) with a real-world Python example.\
You'll also find step-by-step code, a GitHub repo with automation, and a
short guide to present this as a 5-minute video. 🎥

📌 Outline

Why software design principles matter 🤔\
Key principles (SOLID, DRY, KISS, YAGNI) 📚\
Python example: Order Processing System 🐍\
Refactoring with design principles applied ✨\
GitHub repo structure & CI/CD automation ⚙️

1️⃣ Why software design principles matter 🤔

🔧 Maintainability → Easier to update and fix bugs.\
♻️ Reusability → Write once, use multiple times.\
📈 Scalability → Design that grows with your app.\
👩‍💻 Team collaboration → Principles help everyone understand and extend code.

2️⃣ Core Design Principles 📚

SOLID → Five core OOP principles:
- S: Single Responsibility\
- O: Open/Closed\
- L: Liskov Substitution\
- I: Interface Segregation\
- D: Dependency Inversion
DRY → Don't Repeat Yourself\
KISS → Keep It Simple, Stupid\
YAGNI → You Ain't Gonna Need It

3️⃣ Python Example: Order Processing System 🐍

❌ Bad code (violates SRP, DRY):

class Order:
    def __init__(self, items):
        self.items = items

    def calculate_total(self):
        return sum(item['price'] for item in self.items)

    def save_to_db(self):
        print("Saving order to database...")

    def send_email(self):
        print("Sending email to customer...")

👉 Problem: One class is doing everything (violates SRP).

✅ Refactored code with design principles:

class Order:
    def __init__(self, items):
        self.items = items

    def calculate_total(self):
        return sum(item['price'] for item in self.items)


class OrderRepository:
    def save(self, order: Order):
        print(f"Saving order with total {order.calculate_total()} to database...")


class EmailService:
    def send_order_confirmation(self, order: Order):
        print(f"Email sent for order total: {order.calculate_total()}")


# Usage
items = [{"name": "Pizza", "price": 12}, {"name": "Cola", "price": 3}]
order = Order(items)

repo = OrderRepository()
repo.save(order)

email = EmailService()
email.send_order_confirmation(order)

✅ SRP: Each class has a single responsibility.\
✅ DRY: No duplicate calculation logic.\
✅ KISS: Simple methods and classes.\
✅ Extensible: Swap EmailService with SMS later (Open/Closed
Principle).

4️⃣ Repo Structure & Automation ⚙️

software-design-principles/
├─ order.py
├─ tests/
│   └─ test_order.py
├─ requirements.txt
├─ README.md
└─ .github/
   └─ workflows/
      └─ ci.yml

requirements.txt

pytest==8.3.3

ci.yml (GitHub Actions)

name: Python CI

on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - uses: actions/setup-python@v4
        with:
          python-version: '3.11'
      - run: pip install -r requirements.txt
      - run: pytest

🚀Repository link
🔑Repository to Practice

DEV Community