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How to Build Your First Flask Application (Step-by-Step Guide for Beginners) Uploaded: Nov. 29 2025 | Modified: Nov. 29 2025

How to Build Your First Flask Application (Step-by-Step Guide for Beginners)

If you're learning Python web development, building your first Flask app is the perfect starting point. This beginner-friendly guide will walk you through how to create, run, and understand your first Flask application from scratch.

By the end of this tutorial, you will have a working Flask web app running locally on your machine.

Note: If you haven’t installed Flask yet, check out our beginner installation guides for Windows, macOS, and Linux before continuing.

1. Flask Project Structure (Beginner-Friendly Overview)

Before writing code, your project folder should look like this:

myapp
│
├── .venv
│
└── app.py

2. Create Your Flask App File (app.py)

Inside the myapp folder, create a Python file called:

app.py

All the code for your first Flask application will live here.

3. Import the Flask Class

Start by importing the Flask module:

from flask import Flask

This gives your code access to the Flask framework.

4. Initialize the Flask Application Object

Next, create a Flask application instance:

app = Flask(__name__)

This tells Flask where your application is located and initializes its core features.

5. Create Your First Route Using @app.route()

A route controls what your users see when they visit a specific URL.

Here’s how to define your home page (/):

@app.route('/')

6. Return a Response With a View Function

Now, write a function that returns a message to the browser:

def index():
    return 'Hello, World!'

When someone visits your home page, this function runs.

7. Run the Flask Development Server

Add this code at the bottom of app.py:

if __name__ == '__main__':
    app.run(debug=True)

8. How to Run Your First Flask Application

Follow these steps:

Step 1 — Open your project directory

Windows

> cd \path\to\myapp

macOS/Linux

$ cd /path/to/myapp

Step 2 — Activate the virtual environment

Windows

myapp > .venv\Scripts\activate

macOS/Linux

myapp $ source .venv/bin/activate

Step 3 — Run the Flask development server

Windows

(venv) myapp > python app.py

macOS/Linux

(venv) myapp $ python app.py

Your terminal will display:

Running on http://127.0.0.1:5000

Step 4 — Open your Flask app in the browser

Go to:

http://127.0.0.1:5000

You should see:

Hello, World!

Step 5 — Stop the server

Press:

CTRL + C

Step 6 — Deactivate your virtual environment

deactivate

Complete Working Example (Copy & Paste)

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello():
    return 'Hello, World!'

if __name__ == '__main__':
    app.run(debug=True)

FAQ (Frequently Asked Questions)

Why do I need app = Flask(__name__) in my Flask project?

This line initializes your Flask application and tells Flask where to find your resources, such as templates and static files. Without it, your app cannot run.

What does @app.route('/') mean in Flask?

@app.route('/') creates the home page URL for your application. When a user visits http://127.0.0.1:5000/, the route calls the function below it.

Why is my Flask app only showing a blank page?

Common causes include:

What does debug=True do in Flask?

Enabling debug mode automatically reloads your app when you make changes and displays detailed error messages — extremely helpful for beginners.

How do I stop the Flask development server?

Press CTRL + C in the terminal where the server is running. This immediately stops the development server.

Do I need a virtual environment for my first Flask app?

Yes — using a virtual environment keeps your project’s dependencies isolated and prevents version conflicts with other Python projects on your system.

Why does my terminal say “ModuleNotFoundError: No module named 'flask'”?

This usually means:

Where will my Flask app run after I start it?

By default, Flask apps run on:

http://127.0.0.1:5000

This is your local development server.

What should I learn after building my first Flask app?

Flask is simpler and more flexible, making it ideal for beginners. Django is more structured and feature-rich for big projects.