How do I run a Flask app?

Set FLASK_APP=app.py and run flask run , or run the file directly with python app.py when it ends in app.run(debug=True) . The built-in development server is for local use only — serve production traffic with a WSGI server such as Gunicorn.

Why do I get "request is not defined" in Flask?

You used request without importing it. Import it from Flask alongside the other helpers: from flask import Flask, render_template, request . The request object is needed to read form and query data.

How do I render an HTML template in Flask?

Put the file in a templates/ folder and call render_template('index.html', title='Home', message='Hello') . Flask fills the Jinja2 placeholders such as {% raw %}{{ title }}{% endraw %} with the values you pass.

Where do CSS and image files go in a Flask app?

Place them in a folder named static/ . Flask serves them automatically at the /static/<filename> URL, for example /static/style.css .

Joking Flask App

10 Dec 2023 (updated: 24 Aug 2026) / 22 minutes to read

Elena Daehnhardt

Introduction: Build a Flask Web App in Python

Flask is a lightweight Python web framework for building web applications and APIs using @app.route() endpoints and Jinja2 templates. Flask is the closest thing Python has to a magic trick: a few lines of code and you have a working web server. In this post I will show you how to build a small web application that serves random jokes from a text file — perfect for learning the core ideas without drowning in boilerplate.

We will cover:

Installing Flask and setting up a virtual environment
Routing URLs to Python functions
Using Jinja2 templates to render HTML
Serving static files (CSS, images)
Handling form submissions
Wiring it all together into a working Joker App

The full source code is in the GitHub repository flask-random-joke.

What Is Python Flask?

Flask is a lightweight WSGI web framework for Python. It gives you routing, a templating engine, and a development server — nothing more, nothing less. This minimalism is exactly why it is a great first framework: you understand every line you write.

Flask is built on two libraries: Werkzeug (the WSGI toolkit that handles HTTP requests and responses) and Jinja2 (the templating engine). Both are worth knowing a little about even if Flask abstracts them away.

Building the Joker Flask App Step by Step

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ChatGPT prompts I used

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Conclusion: Flask Routing, Templates, and Static Files

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References

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Joking Flask App

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

Introduction: Build a Flask Web App in Python

What Is Python Flask?

Building the Joker Flask App Step by Step

ChatGPT prompts I used

Conclusion: Flask Routing, Templates, and Static Files

References

References

Citation

Joking Flask App

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

Introduction: Build a Flask Web App in Python

What Is Python Flask?

Building the Joker Flask App Step by Step

ChatGPT prompts I used

Conclusion: Flask Routing, Templates, and Static Files

References

You're in

References

Citation