Adventures in Machine Learning

Flask REST API Development: Building Scalable APIs with Connexion and Swagger UI

Introduction to Flask REST API Development

In recent years, modern applications have become increasingly reliant on web-based interfaces and cloud-based storage. As a result, the development of robust, flexible, and reliable REST APIs has become vital for the success of any web application.

Flask, a micro web framework, is one of the most popular frameworks used for developing REST APIs. This article aims to introduce you

to Flask REST API Development and the key concepts behind it.

Building a REST API with Flask

The first step to building a Flask REST API is to understand what a RESTful API is. REST stands for Representational State Transfer.

It refers to a web architecture that uses HTTP requests to GET, POST, PUT, and DELETE data. A RESTful API provides a way for two or more systems to communicate and exchange data.

Flask provides a framework for building RESTful APIs that are both fast and simple to develop. The key to building a Flask REST API is separation of concerns.

Separating the front-end and back-end logic means you can update one part of the application without affecting the other. Separation of concerns not only makes code more scalable, but it also makes the code more manageable and easier to maintain.

Flask also supports server-side caching, which can improve the performance of your REST API significantly.

Testing API with Swagger UI API documentation

As your Flask REST API grows, it can become difficult to keep track of all the endpoints. Swagger UI is a tool that can help you manage and document your Flask REST API.

Swagger UI provides a UI interface for your API, which makes it much easier to test. It also documents your API in real-time, which saves developers time and reduces the complexity of code.

Planning Part One

Creating a REST API for a people collection

In this section, we will create a Flask REST API for a people collection. A people collection stores information about individuals in a database or file system.

We can then retrieve, insert, update, or delete records about an individual using a RESTful API. Flask provides a way to build RESTful APIs by creating routes that map to specific methods.

Keying people to last names and updating with timestamps

We can improve the functionality of our Flask REST API by keying individuals by their last names and updating their records with timestamps. The API can be programmed to retrieve information about an individual by searching for their last name.

Using timestamps will help us track when a record was updated and ensure data consistency.

Decoupling data from application for hiding implementation details

To keep our Flask REST API scalable and maintainable, it’s essential to separate the data from the application. Hiding implementation details provides us with an abstraction layer between the data and the application that adds a layer of security and helps protect the data.

We can use different tools such as Object Relational Mappers (ORM) to achieve decoupling of data.


In conclusion, Flask has become a popular choice for developers looking to create fast and scalable RESTful APIs. Separating the front-end and back-end logic, using Swagger UI for testing and documentation, keying people to last names, updating with timestamps, and decoupling data from the application are all essential steps toward successful Flask REST API development. These concepts should help you get a good understanding of Flask REST API development and get started building your own Flask RESTful API.

3) Getting Started

When starting a Flask project, the first step is always to create a virtual environment for the project. A virtual environment is an isolated Python environment that allows you to install dependencies without affecting your system’s global Python environment.

This ensures that your project dependencies are kept separate from other Python projects and your Python environment. To create a virtual environment, type the following command in your terminal:

“`python -m venv environment-name“`

Replace “`environment-name“` with the desired name for your virtual environment.

Once you have created your virtual environment, activate it by typing the following command in your terminal:

“`source environment-name/bin/activate“`

Next, initiate your Flask project by creating an application instance. In this instance, we define the app directory and Flask instance.

You can create a new Flask project directory and add an “`“` file in the directory. Inside the “`“` file, you can create the Flask instance as shown:


from flask import Flask

app = Flask(__name__)


This initializes the Flask application instance and the “`__name__“` parameter tells Flask to find the template and static files in the directory with this file. With a basic application instance defined, it is possible to create routing rules for the Flask app by defining the URL in a decorator.

Here is how to create the home route:



def home():

return “

Hello World!


This home route allows you to return a basic HTML page.

4) Adding Your First REST API Endpoint

To add a REST API endpoint to your Flask app, there are several configurations you’ll need to make. One of the first configurations is setting up the OpenAPI specification using a Swagger YAML file.

The OpenAPI specification allows your Flask app to be documented and describes the behavior of the REST API. You can add Swagger YAML specification to your Flask API project directory.

Next, you’ll need to add an API configuration file that specifies the path to the Swagger YAML file. The API configuration file is used to configure Connexion, which is a tool that connects your OpenAPI specification to the Flask app.

To add the API configuration file, create a new YAML file in your project directory and provide the details needed for your API. Here’s an example configuration file:


swagger: “2.0”


title: People API

description: REST API for managing People data

basePath: /v1


– name: people

description: Operations related to people.


– http





– people

summary: Get all people

operationId: get_people


– application/json



description: A list of people


type: array


$ref: “#/definitions/Person”



– people

summary: Add a new people to the collection

operationId: add_people


– application/json


– application/json


– in: body

name: body

description: “People object that needs to be added to the collection”

required: true


$ref: “#/definitions/Person”



description: People created



type: object



type: integer

example: 1

description: Unique identifier for a person.


type: string

example: “John Doe”

description: Name of the person.


This specification defines the endpoint path, method, produces and consumes content types, input validation, response types and schema. The specification above shows two operations, one that allows clients to retrieve all the people available and the other allows them to add a people to the collection.

After adding the specifications to the project directory, you can connect the Flask app to Connexion using the following code:


import connexion

app = connexion.App(__name__, specification_dir=’./’)


if __name__ == ‘__main__’:


The above code initializes Connexion, which then loads the OpenAPI specification at the given file path. When the application is run, the app will be running on port 8080.

Now, you can configure a route for the API endpoint by defining it in the “`“` file. Heres how to define the people endpoint and return sample data:



def get_people():

return jsonify({‘people’: [

{‘id’: 1, ‘name’: ‘John Doe’},

{‘id’: 2, ‘name’: ‘Jane Doe’}



In the above code snippet, we returned a JSON response containing a list of 2 people with IDs 1 and 2 and their respective names.

Overall, these steps serve as a foundation for building REST APIs in Flask. Adding routes, connecting to Connexion, OpenAPI configuration, and defining endpoints are all crucial aspects of developing Flask RESTful API.

With these steps, you can get started with building your own APIs in no time!

5) Explore Your API Documentation

After building a Flask RESTful API with Connexion, the next step is to explore the APIs documentation. Connexion automatically generates API documentation from an OpenAPI specification file.

This documentation provides a clear outline of the APIs capabilities and how to use it. Below are some important points to consider when exploring the API documentation.

Automatic creation of API documentation with Connexion

Connexion is a Python web framework that automates a lot of the tedious work associated with developing a RESTful API, including generating API documentation. This documentation is automatically created by Connexion using the OpenAPI specification file.

To access API documentation, go to your browser and type in the URL for your API. Append “/ui” to the end of the URL to bring up the Swagger UI documentation.

The Swagger UI documentation provides a user-friendly interface for exploring the API.

Swagger UI for exploring API

Swagger UI is a tool that allows users to explore and test an API by providing a user interface for the OpenAPI specification file. The Swagger UI tool provides a sandbox environment for testers to experiment with different API calls and see the resulting responses.

Using Swagger UI with Connexion makes it much easier for developers to understand what endpoints are available, what data is required, and what responses can be expected. The Swagger UI provides developers with a user-friendly interface that provides feedback in real-time.

In addition, Swagger UI shows possible formats for input, output and responses.

Testing API functionality with API documentation

API documentation serves as a blueprint for your API, so it’s important to test the functionality of each endpoint. The documentation provides all the details required to make working API calls, including URLs, Request formats, required parameters, and expected responses.

When testing an API, start by making basic requests to determine the overall available resources and their composition. After getting an overall idea of available resources, explore the API in more depth.

Test each endpoint with the request and response bodies that mimic your production application as closely as possible, making sure to include edge cases. API testing with the documentation helps verify whether the API meets the desired requirements.

By following the documentation, developers can uncover issues with an API that may not have been noticed otherwise and can be corrected earlier in the development cycle.


In conclusion, exploring API documentation is essential to understanding the capability of your API. Connexion automates the creation of documentation through the OpenAPI specification file and offers a user-friendly interface to explore your API using Swagger UI.

Testing API functionality with the documentation helps in uncovering issues and verifying the API is compliant with the requirements. By following these best practices, you can develop functional and reliable RESTful APIs with Flask and Connexion.

In this article, we discussed Flask REST API development and its primary concepts. We explored how to build a Flask REST API, separate the front-end and back-end logic, test with Swagger UI documentation, and key individuals by their last names while hiding implementation details.

We also covered how to configure OpenAPI, add an API configuration file, and connect Flask with Connexion. Lastly, we emphasized the importance of exploring API documentation, automated creation of API documentation with Connexion,

Swagger UI for exploring APIs, and testing API functionality with API documentation.

By following these best practices, developers can build functional and reliable RESTful APIs. The takeaway from this article is that Flask REST API development provides a fast and simple way to build modern RESTful APIs and is essential in today’s web-based applications.

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