We've spoken about Werkzeug a few times during the course of this book but we haven't really explained what it is, why we use it, or why it's useful. To understand Werkzeug, we first need to know why it exists. For this, we will need to understand the origins of the Python Web Server Gateway Interface specification, most commonly abbreviated as WSGI.

Today, choosing a Python web application framework is a relatively simple matter of preference: most developers choose a framework based on a previous experience, necessity (for example, one that is designed for an asynchronous request processing such as Tornado), or some other quantifiable or unquantifiable criteria.

Several years ago, however, the choice of an application framework affected the web server that you could use. As all Python web application frameworks at the time implemented their own HTTP request processing in a slightly different manner, they were often only compatible with a subset of web servers. Developers...