Jinja2 is a fast, flexible, and secure templating engine. It allows you to define your website in small blocks that are pieced together to form complete pages. On our blog, for instance, we will have blocks for the header, the sidebar, the footer, as well as templates, for rendering blog posts. This approach is DRY (Don't Repeat Yourself), which means that the markup contained in each block should not be copied or pasted elsewhere. Since the HTML for each part of the site exists in only one place, making changes and fixing bugs is much easier. Jinja2 also allows you to embed display logic in the template. For instance, we may wish to display a log out button to users who are logged in, but display a log in form to users browsing anonymously. As you will see, it is very easy to accomplish these types of things with a bit of template logic.

From the beginning, Flask was built with Jinja2 in mind, so working with templates in your Flask app is extremely easy. Since Jinja2...