Python Flask Hello World Example

Python Flask - Hello World Example

In this tutorial, we will write a simple Flask application in Python, that returns a "Hello World" response when a request is made for the root URL.

Prerequisites

Python has to be installed in your computer. This is the only prerequisite for this tutorial.

Install flask using PIP

Run the following command to install flask library in your system.

$ pip install flask

Flask Hello World Application

We will build a simple, and lightweight Web application, where the server responds with the message "Hello World".

Create a python file with the following code.

main.py

from flask import Flask

app = Flask(__name__)

@app.route("/")
def index():
    return "Hello World"

if __name__ == "__main__":
    app.run(host="127.0.0.1", port=8080, debug=True)

Open a Terminal and run this python program.

Now the flask application is running on localhost at port 8080.

Open web browser of your choice and enter the URL http://127.0.0.1:8080.

For the route / URL, the server has responded with a message Hello World.

Now let us break down the code in the python file main.py.

From flask library, we imported Flask class.
We created flask application using Flask() class constructor and stored it in the variable app.
We defined a function index() that is called for the root URL "/", which in this case would be http://127.0.0.1:8080, and returns string "Hello World".
We called the run() function on the flask application app and passed host IP address 127.0.0.1 and port number 8080 as arguments.

This is just a basic application to understand how to build a flask application.

Summary

In this Python Flask Tutorial, we learned how to build a basic server application with flask library.