Adventures in Flask: Building a Simple Python Web App

Phew — close call!  

A few hours after publishing this article I noticed a sensitive link had been included in the post that pointed to an internal repository. That link has been removed from this post. If it had been exposed to threat actors, it could have led to attacks against our organization and potentially exposed sensitive internal information. Lesson learned: always double-check links before publishing!

Anyways! If you're just starting with Python web development, Flask is a fantastic place to begin: it's tiny, clear, and gives you full control over how your app is structured. In this post we'll build a small "Notes" app that demonstrates the core Flask concepts: routes, templates, forms, and configuration.

What you'll need

Python 3.8+ installed  

pip (Python package manager)  

A text editor (VS Code, vim, etc.)

Quick setup

1. Create a virtual environment:  

   python -m venv venv  

   source venv/bin/activate   # or venv\Scripts\activate on Windows

2. Install Flask:  

   pip install Flask

3. Project layout (simple):  

   /notes-app  

   /templates  

   index.html  

   app.py  

   requirements.txt

Minimal app.py  

The smallest Flask app looks like:  

from flask import Flask, render_template, request, redirect, url_for

app = Flask(__name__)

# Simple in-memory storage for demo purposes

NOTES = []

@app.route('/', methods=['GET'])  

def index():  

    return render_template('index.html', notes=NOTES)

@app.route('/add', methods=['POST'])  

def add_note():  

    content = request.form.get('content')  

    if content:  

        NOTES.append(content)  

    return redirect(url_for('index'))

if __name__ == '__main__':  

    app.run(debug=True)

Templates  

Create templates/index.html with a form to submit notes and a list to display them. This demonstrates how Flask renders templates and uses request data.

Configuration & Debug Mode  

In development you can set FLASK_ENV=development or run app.run(debug=True) for auto-reload and better error pages. Never leave debug enabled on production servers — it can reveal internal information.

Continuing the tutorial  

Below are some safe next steps to grow your Flask skills that don't require any external repo:

Add form validation with WTForms or simple server-side checks.  

Persist notes to SQLite using SQLAlchemy: define a Note model and perform CRUD.  

Add simple authentication with Flask-Login and protect the note-creation route.  

Containerize the app with Docker for consistent deployment.  

Write unit tests for your routes using Flask's test client.