Introduction to Flask

Reviewed & published by Brayan K

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.

Part of the free Flask course at LearnCodingFast — hands-on lessons with worked examples and the output they print, 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

• What Flask is and why it's called a 'micro' framework
• How to install Flask with pip
• The structure of a minimal Flask application
• What the @app.route decorator does
• How to run an app locally with flask run
• Where to view your app in the browser

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.

🧩 Flask is a "micro" framework:

• 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:

# Create and activate a virtual environment
python -m venv venv
source venv/bin/activate      # On Windows: venv\Scripts\activate

# Install Flask
pip install flask

# Confirm it installed
flask --version

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.

# app.py — the classic minimal Flask application
from flask import Flask

# Create the application object.
# __name__ tells Flask where to find templates and static files.
app = Flask(__name__)

# @app.route maps a URL path to the function below it.
# Visiting "/" (the home page) runs home() and shows its return value.
@app.route("/")
def home():
    return "Hello, Flask!"

print("Routes registered:", [str(r) for r in app.url_map.iter_rules()])

# Flask gives you a test client, so you can call your own route and see what
# a browser would get — without starting a server at all:
with app.test_client() as client:
    response = client.get("/")
    print("GET / ->", response.status_code, response.data.decode())

# To serve it on your own machine, end the file with the lines below, then
# run "python app.py" (or "flask run") and visit http://127.0.0.1:5000
#     if __name__ == "__main__":
#         app.run(debug=True)

# ✅ Expected output:
# Routes registered: ['/static/<path:filename>', '/']
# GET / -> 200 Hello, Flask!

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

🎯 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.

# 🎯 YOUR TURN: replace each ___ to finish the app
from flask import ___          # import the Flask class

app = ___(__name__)            # create the application object

@app.route("___")              # the home page path
def home():
    return "___"               # your greeting message

print("Rules:", [str(r) for r in app.url_map.iter_rules()])

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.

🏆 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

# 🏆 Mini Challenge: a two-page Flask app
from flask import Flask

app = Flask(__name__)

@app.route("/")
def home():
    return "Welcome to my site!"

# TODO: add a route for /about that returns a description
@app.route("/about")
def about():
    return "This site was built with Flask."

# Print the registered routes to confirm both exist
for rule in app.url_map.iter_rules():
    print(rule)

❓ Frequently Asked Questions

Lesson 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.

Practice quiz

Continue this course

• Next: Your First Route — Map URLs to Python functions with the @app.route decorator
• Quick reference: Flask cheat sheet › Setup & Routes

Frequently asked questions

What is Flask?

Flask is a lightweight Python web framework used to build websites and APIs. It gives you routing, request handling, and templating while staying small and unopinionated.

Do I need to know Python before learning Flask?

Yes — Flask is written in Python and your route functions are plain Python. You should be comfortable with functions, dictionaries, and decorators before starting.

How do I run a Flask app?

Install Flask with 'pip install flask', save your app in a file like app.py, then run 'flask run' (or 'python app.py') in your terminal. Flask serves it locally at http://127.0.0.1:5000.

Is Flask good for beginners?

Yes. Flask's minimal core means you can build a working web page in under 10 lines of code, then add features like databases and authentication as you grow.

Related lessons

• Python course