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Python | Build a REST API using Flask

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  • Difficulty Level : Basic
  • Last Updated : 25 Feb, 2022

Prerequisite: Introduction to Rest API

REST stands for REpresentational State Transfer and is an architectural style used in modern web development. It defines a set or rules/constraints for a web application to send and receive data.

In this article, we will build a REST API in Python using the Flask framework. Flask is a popular micro framework for building web applications. Since it is a micro-framework, it is very easy to use and lacks most of the advanced functionality which is found in a full-fledged framework. Therefore, building a REST API in Flask is very simple.

There are two ways of creating a REST API in Flask:

  1. Using Flask without any external libraries
  2. Using flask_restful library

Libraries required:

flask_restful can be installed via the pip command:

 sudo pip3 install flask-restful 

Method 1: using only Flask

Here, there are two functions: One function to just return or print the data sent through GET or POST and another function to calculate the square of a number sent through GET request and print it.

# Using flask to make an api
# import necessary libraries and functions
from flask import Flask, jsonify, request
# creating a Flask app
app = Flask(__name__)
# on the terminal type: curl
# returns hello world when we use GET.
# returns the data that we send when we use POST.
@app.route('/', methods = ['GET', 'POST'])
def home():
    if(request.method == 'GET'):
        data = "hello world"
        return jsonify({'data': data})
# A simple function to calculate the square of a number
# the number to be squared is sent in the URL when we use GET
# on the terminal type: curl / home / 10
# this returns 100 (square of 10)
@app.route('/home/<int:num>', methods = ['GET'])
def disp(num):
    return jsonify({'data': num**2})
# driver function
if __name__ == '__main__': = True)


Executing the square function:

Method 2: Using flask-restful

Flask Restful is an extension for Flask that adds support for building REST APIs in Python using Flask as the back-end. It encourages best practices and is very easy to set up. Flask restful is very easy to pick up if you’re already familiar with flask.

In flask_restful, the main building block is a resource. Each resource can have several methods associated with it such as GET, POST, PUT, DELETE, etc. for example, there could be a resource that calculates the square of a number whenever a get request is sent to it. Each resource is a class that inherits from the Resource class of flask_restful. Once the resource is created and defined, we can add our custom resource to the api and specify a URL path for that corresponding resource.

# using flask_restful
from flask import Flask, jsonify, request
from flask_restful import Resource, Api
# creating the flask app
app = Flask(__name__)
# creating an API object
api = Api(app)
# making a class for a particular resource
# the get, post methods correspond to get and post requests
# they are automatically mapped by flask_restful.
# other methods include put, delete, etc.
class Hello(Resource):
    # corresponds to the GET request.
    # this function is called whenever there
    # is a GET request for this resource
    def get(self):
        return jsonify({'message': 'hello world'})
    # Corresponds to POST request
    def post(self):
        data = request.get_json()     # status code
        return jsonify({'data': data}), 201
# another resource to calculate the square of a number
class Square(Resource):
    def get(self, num):
        return jsonify({'square': num**2})
# adding the defined resources along with their corresponding urls
api.add_resource(Hello, '/')
api.add_resource(Square, '/square/<int:num>')
# driver function
if __name__ == '__main__': = True)


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