Introduction to Flask

Flask is a lightweight Python web framework for building websites and APIs, giving you routing, request handling, and HTML templating in a small, flexible package you control.

Learn Introduction to Flask in our free Flask course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Flask course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

In this first lesson you'll install Flask, understand how a web framework works, and run your very first Flask application locally.

What You'll Learn in This Lesson

1 What Is Flask?

Flask is a web framework — a toolkit that handles the repetitive, low-level work of running a website so you can focus on your app's logic.

When someone visits a URL, their browser sends an HTTP request to your server. Flask receives that request, figures out which Python function should handle it, runs your code, and sends back an HTTP response (usually HTML or JSON). You write the functions; Flask wires everything together.

• Small core: routing, requests, and templating come built in
• Unopinionated: you choose your database, your project layout, your tools
• Extensible: add features like login or databases through extensions when you need them
• Beginner-friendly: a complete web page is under 10 lines of code

2 Installing Flask

Flask is a Python package, so you install it with pip . It's best practice to do this inside a virtual environment so each project keeps its own dependencies:

3 Your First Flask App

Here is the smallest complete Flask app. Save it in a file named app.py :

The code below creates an application object, defines a single route at the home URL / , and returns a greeting. Run it and Flask will print a local address you can open in your browser.

When you run flask run , Flask starts a development server and prints something like Running on http://127.0.0.1:5000 . Open that address and you'll see "Hello, Flask!" in your browser.

🔬 Anatomy of a Flask App

Line

What it does

from flask import Flask

Imports the Flask class from the package

app = Flask(__name__)

Creates your application instance

@app.route("/")

Connects the URL path / to a function

def home():

The view function — runs when the route is visited

return "Hello, Flask!"

The response body sent back to the browser

🎯 Your Turn: Fill in the Blanks

Complete the Flask app below. Replace each ___ so it imports Flask, creates the app, defines a home route, and returns a message.

Common Errors & Quick Reference

❌ ModuleNotFoundError: No module named 'flask'

Flask isn't installed in your active environment. Run pip install flask and make sure your virtual environment is activated.

❌ flask run says "Could not locate a Flask application"

Flask looks for a file named app.py or wsgi.py by default. Either name your file app.py or set FLASK_APP=yourfile.py before running.

Task

Command / Code

Install Flask

pip install flask

Run the dev server

flask run

Create the app

Define a route

🏆 Mini Challenge: Add an About Page

Extend the app so it has two pages: the home page and an about page.

• Keep the home route returning a greeting
• Add a second route at /about
• Make the about function return a short description of you or your site

❓ Frequently Asked Questions

Lesson 1 complete — you've built your first Flask app!

You now know what Flask is, how to install it, and how to define a route and run a development server with flask run . That five-line app is the seed of every Flask project.

🚀 Up next: Your First Route — go deeper into @app.route and learn how to handle multiple pages cleanly.

Practice quiz

Continue this course

• Next: Your First Route

Related lessons

• Python course