There are two major versions of the Python language, Python 2 which is the widely deployed legacy language and Python 3 which is the present and future of the language. Much Python code will work without modification between the last version of Python 2 (which is Python 2.7 (https://www.python.org/download/releases/2.7/)) and recent versions of Python 3, such as Python 3.5 (https://www.python.org/download/releases/3.5.1/). However, there are some key differences between the major versions, and in a strict sense the languages are incompatible. We'll be using Python 3.5 for this book, but we'll point out key differences with Python 2 as we go. It's also very likely that, this being a book on Python fundamentals, everything we present will apply to future versions of Python 3, so don't be afraid to try those as they become available.

Before we can start programming in Python we need to get hold of a Python environment. Python is a highly portable language and is available on all major operating systems. You will be able to work through this book on Windows, Mac or Linux, and the only major section where we diverge into platform specifics is coming right up — as we install Python 3. As we cover the three platforms, feel free to skip over the sections which aren’t relevant for you.