calibre is completely open source, licensed under the GNU GPL v3. This means that you are free to download and modify the program to your heart’s content. In this section, you will learn how to get a calibre development environment set up on the operating system of your choice. calibre is written primarily in Python with some C/C++ code for speed and system interfacing. Note that calibre is not compatible with Python 3 and requires at least Python 2.7.