Build a REST API using Flask - Python

Last Updated : 22 Nov, 2025

REST (Representational State Transfer) is an approach used to build web services that interact using standard HTTP methods such as GET, POST, PUT and DELETE. APIs built using REST follow predictable rules, making them suitable for data exchange between applications.

In this article, we will build a REST API using Python’s Flask framework through two methods:

  • Using Flask
  • Using Flask-RESTful

Before starting this, let us install the necessary libraries.

Installation

Install all required packages in one command:

pip install flask flask-restful

This installs both Flask and Flask-RESTful, preparing the environment for building APIs using both methods.

Using Flask

This method use Flask framework only. Here, API is created using regular functions called “routes.” Flask checks the URL the user visits and executes the matching function. One route sends a simple message, and another route receives a number from the URL and returns its square.

Python 
from flask import Flask, jsonify
app = Flask(__name__)

@app.route('/', methods=['GET'])
def home():
    return jsonify({'data': 'hello world'})

@app.route('/home/<int:num>', methods=['GET'])
def disp(num):
    return jsonify({'data': num ** 2})

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

Explanation:

  • app = Flask(name) creates the main application object where routes are defined.
  • Home Route ('/') returns a simple JSON message when opened in a browser.
  • Square Route ('/home/<int:num>') receives a number through the URL and returns its square.
  • app.run(debug=True) starts the server and reloads automatically while developing.

Output

Open your browser and visit - http://127.0.0.1:5000/. You will see a JSON message returned as below: