In this chapter, we will explore modern methods of the so-called social login, where we allow a user to authenticate with our application using derived credentials from another web application. Currently, the most widespread third-party applications that support this mechanism are, somewhat unsurprisingly, Twitter and Facebook.

While there exist several other widespread web applications that support this type of integration (for example, LinkedIn, Dropbox, Foursquare, Google, and GitHub to name a few), the majority of your potential users will be in possession of at least one account on either Twitter or Facebook, the two major social networks of this time.

To do this, we will be adding, configuring, and deploying the Flask-OAuthlib extension. This extension abstracts out some of the usual difficulties and roadblocks that are often experienced when dealing with an OAuth-based authorization flow (which we will explain shortly) and includes functionalities...