What Is a Web Framework?¶

A web framework is a software architecture that provides tools and libraries needed to build a web application. It simplifies tasks such as handling requests, routing URLs, rendering templates, and managing data.

What Is Flask?¶

Flask is a micro-framework for Python. It is called a micro-framework because, unlike full-fledged frameworks such as Django, it does not include built-in features like authentication, admin panels, or ORM.
Instead, Flask keeps things minimal, flexible, and modular, allowing developers to add only the components they need.

Flask is built on:

Werkzeug – A WSGI toolkit that handles request and response objects.
Jinja2 – A templating engine used for rendering HTML with Python variables.

Setting Up the Flask Environment¶

Before starting, ensure Python 3 is installed.

1. Create a virtual environment¶

python3 -m venv env

2. Activate the virtual environment¶

source env/bin/activate

3. Install Flask¶

pip install flask

That’s all you need to create your first Flask application.

Your First Flask App¶

Create a new Python file and add:

from flask import Flask

app = Flask(__name__)


@app.route("/")
def index():
    return "hello, world"

Run the application¶

flask run

Understanding the Code¶

`from flask import Flask`¶

You import the Flask class from the flask package. This gives your script access to the essential components needed to create a web application.

`app = Flask(name)`¶

__name__ is a special built-in Python variable.
It is also called a dunder (double-underscore) variable.
Flask uses __name__ to determine:
- the root path of your application
- where to find templates and static files
- the execution context for routing and configuration

This line creates the Flask application instance, stored in the variable app.

`@app.route("/")` – The Route Decorator¶

app.route() is a decorator that maps a URL path to a function.
Here, the root URL / is associated with the index() function.

Whenever someone visits your home page (http://127.0.0.1:5000/), Flask runs the function below the decorator.

`def index():`¶

Defines the function to run when the / route is accessed.

`return "hello, world"`¶

Whatever this function returns becomes the response shown in the browser.

In this case, the text:

hello, world

What Happens When You Run the App?¶

flask run starts the built-in Flask development server.
The default URL is:
http://127.0.0.1:5000/
When you visit /, Flask:
- matches the route
- calls the index() function
- displays hello, world in the browser