Build a REST API using Flask Python (original) (raw)

Last Updated : 7 Apr, 2026

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.

To build a REST API using Flask, we can use 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

Run the program and open browser to visit - http://127.0.0.1:5000/. You will see a JSON message returned as below:

Output1

Browser output showing the home route response

Now, In the browser, visit: http://127.0.0.1:5000/home/22\. The API reads the number 22 and returns its square.

Output2

Browser output showing square of the number received from the URL.

This shows how passing values through the URL works in a REST API.

Using Flask-RESTful

This method uses the Flask-RESTful extension, which organizes APIs using classes called resources. Each resource represents an endpoint and contains methods such as get(), which run when a GET request is made. One resource returns a message, and another resource calculates the square of a number.

Python `

from flask import Flask, jsonify from flask_restful import Resource, Api

app = Flask(name) api = Api(app)

class Hello(Resource): def get(self): return {'message': 'hello world'}

class Square(Resource): def get(self, num): return {'square': num ** 2}

api.add_resource(Hello, '/') api.add_resource(Square, '/square/int:num')

if name == 'main': app.run(debug=True)

**Explanation:

**Resource represents an API endpoint using a class.
**Hello Resource (/) returns a message when accessed.
**Square Resource (/square/int:num) accepts a number in the URL and returns its square.
**api.add_resource() connects each class to a specific route.

Output

Open the browser and visit: http://127.0.0.1:5000/

Output3

Browser output showing message returned from Hello Resource

Now, In the browser, visit: http://127.0.0.1:5000/square/3

Output4

Browser output showing the square returned by Square Resource

This shows how Flask-RESTful handles different routes cleanly using class-based resources.