How do I fix jinja2.exceptions.TemplateNotFound: index.html in Flask?

Flask looks for templates in a folder named templates next to app.py . Create that folder and place index.html and edit.html inside it, matching the filename passed to render_template exactly (including case).

Why does Flask say the address is already in use on port 5000?

Another process is holding port 5000 — on macOS this is often the AirPlay Receiver. Free it by disabling AirPlay Receiver in System Settings, or run the app on a different port with app.run(debug=True, port=5001) .

How do I prevent SQL injection in a Flask SQLite app?

Never build SQL strings by concatenating form input. Use parameterised queries with ? placeholders and a value tuple, e.g. db.execute('INSERT INTO tasks (task, priority, complete) VALUES (?, ?, ?)', (task, priority, False)) , which safely escapes the values.

Do I need a virtual environment to run a Flask app?

It is optional but recommended. A virtual environment ( python3 -m venv .venv ) keeps Flask and its dependencies isolated from your system Python, avoiding version conflicts across projects.

Introduction: Building a Python Flask TODO App

Flask is a lightweight Python web framework that maps URLs to Python functions and renders HTML templates. Python is famous for being able to build almost anything, and web applications are no exception. When it comes to web development in Python, my absolute favorite starting point is the Flask framework. It is incredibly lightweight and stays out of your way, especially compared to larger, more opinionated frameworks like Django.

In this post, we are going to build a classic TODO web application from scratch. We will use Python, Flask for the web routing, and SQLite for persistent data storage.

If you think Flask is just a “toy” framework for beginners, think again. Its minimal core does not stop it running real production workloads: plenty of companies build on top of it precisely because it gets out of the way and lets you add only the pieces you need. Let’s get started.

Prerequisites: Python 3, an Editor, and pip

🔒 Subscribe to keep reading.

Setting Up a Python Virtual Environment for Flask

🔒 Subscribe to keep reading.

Installing Flask with pip

🔒 Subscribe to keep reading.

Creating the Flask TODO App

🔒 Subscribe to keep reading.

Conclusion

🔒 Subscribe to keep reading.

References

🔒 Subscribe to keep reading.

You've hit a Deep Dive tutorial.

I spend dozens of hours researching, coding, and breaking things to write these guides. This content is free, but reserved for my subscriber community. Drop your email below to unlock this guide (and all past/future deep dives):

Already a subscriber? Use the magic link from your last newsletter, or reset your password.

New subscribers get an inbox mail: Set a password to unlock articles. The form does not log you in — use the same email afterwards.

Python Flask TODO App

📚 This post is part of the "Python Projects & Utilities" series

Introduction: Building a Python Flask TODO App

Prerequisites: Python 3, an Editor, and pip

Setting Up a Python Virtual Environment for Flask

Installing Flask with pip

Creating the Flask TODO App

Conclusion

References

References

Citation

Python Flask TODO App

📚 This post is part of the "Python Projects & Utilities" series

Introduction: Building a Python Flask TODO App

Prerequisites: Python 3, an Editor, and pip

Setting Up a Python Virtual Environment for Flask

Installing Flask with pip

Creating the Flask TODO App

Conclusion

References

You're in

References

Citation