Hello World
The first app one writes when learning a new technology is usually a Hello World app, which consists of the minimum possible code necessary to start a simple application and show the text "Hello World!". Let's do exactly that using Flask.
This book is optimized for Python 2.x, so, that's the version I advise you to use from now on. All the examples and code are aimed at that Python version, which is the default in most Linux distributions.