Web API is a kind of web application. Simply put, web APIs are built for other applications to get info from the web server. For Python, Flask can be used to build web applications. In this post, I want to share how to build web APIs with Flask.

A simple web API

Supposing that we want to build a simple web API that returns the square root a number, the following code shows how to do that:

import math
from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/sqrt', methods=["POST"])
def sqrt():
    num = request.form.get('num', type=int)

    return jsonify({'result': math.sqrt(num), 'msg': 'success'})

In the above code, we first create a flask application using Flask(__name__), which lets Flask know that we are building an application. We then use the @app.route() decorator to define a URL path that we can later incorparate with IP and port number to access our web API. The methods parameter is used to specify what kinds of HTTP requests the server can process from the client.

Below the app decorator, we define a view function to process the client request, the actual name of this view function does not matter. The view function name does not have be the same as app.route path (ref here).

Processing the client request

To process the client requests in the server side, Flask provides the request module. When a client is using a POST request to post a JSON object, the posted data will be available in request.form, which is like a Python dictionary. Note that by default, the value for a key is of string type. You have to specify the type parameter to convert the value to desired type:

num = request.form.get('num', type=int)

If you do not use type=int, num will be a string instead of an integer..

Return JSON object to client

Flask provides the jsonify method to serialize any compatible data type to JSON format. Finally, we return the result in JSON format.

Running the development server

After we build the simple web API, we need to test if it works. First, we need to run the web server. Flask ships with a simple web server for testing purposes. There are two ways that we can run the web server: via app.run() or via the Flask command line interface.

Run the HTTP server via app.run()

Add the following code to the Flask application module:

if __name__ == "__main__":
    # the server will listen on port 1234. Default is port 5000.

If we do not specify the host address, the default will be, which means that we can only access the service in our machine. To use the IP address of our machine, we can specify the host parameter in app.run:

app.run(host="", port=1234)

Then other machines in the same network can access the web service.

Finally, we can start the HTTP server with python application.py (suppose that our module is named application.py). We will see the following message on command line:

 * Serving Flask app "application" (lazy loading)
 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: off
 * Running on (Press CTRL+C to quit)

Run the server via Flask command line interface

The second and officially-recommended way to run Flask application is via the Flask command line interface (flask run).

When we run the application using flask run, the following settings does not work:

if __name__ == "__main__":
    # when debug is True, flask will reload app once code changes.
    app.run(host="", port=8081)

The correct way to run the application is to run the following command:

export FLASK_APP=application
FLASK_ENV=development flask run --host= --port 1234

FLASK_APP variable is used to specify which file our flask application resides. FLASK_ENV is used to specify the flask running environment. The default is production. If you use development, Flask will activate debug mode, i.e., it will reload the application once the code changes.

--host is used to specify the host address. --port is used to specify the listening port. We can also change the port number via FLASK_RUN_PORT env variable:

export FLASK_APP=application
export FLASK_RUN_PORT=1234
FLASK_ENV=development flask run --host=

Auto reloading on code changes

Flask provides a debug mode where the app will be reloaded once the code changes, so we do not need to manually stop the server and run it again.

If we run the server via app.run(), we can activate debug mode via debug=True:

if __name__ == "__main__":
    # when debug is True, flask will reload app once code changes.
    app.run(host="", port=1234, debug=True)

We may encounter following errors when we run python application.py:

OSError: [Errno 8] Exec format error

The reason for this error can be found here. To put it simply, if application.py has execution right, then Flaks assumes that the script can be run via ./application.py. So we must add shebang at the beginning of the module:

!#/usr/bin/env python

Or we must disable the execution right of this module. When the application is reloaded, it will be run via python application.py.

Another way is to run the app via Flask command line interface, as we have mentioned earlier. Set the FLASK_ENV variable to development or set FLASK_DEBUG to 1:

export FLASK_APP=application
FLASK_ENV=development flask run --host= --port 1234

# or the following
# FLASK_DEBUG=1 flask run --host= --port 1234

Test the web API

The full URL for the web API is:

We can test the web API via curl:

curl -XPOST -d num=4 ""

It is more convenient to test the API via requests package using the following code:

import requests

def post_data():
    url = ""

    payload = {'num': 4}
    response = requests.post(url, data=payload)

    return response.json()

def main():
    result = post_data()

if __name__ == "__main__":

If everything is OK, you will see the following output:

{'msg': 'success', 'result': 2.0}