There are two important versions of the Python language in common use right now:
Python 2 and Python 3. These two versions represent changes in some key elements
of the language, and code written for one will not generally work for the other unless you take special precautions. Python 2 is older and more well-established than Python 3, but Python 3 addresses some known shortcomings in the older version. Python 3 is the definite future of Python, and you should use it if at all possible.

While there are some critical differences between Python 2 and 3, most of the fundamentals of the two version are the same. If you learn one, most of what you know transfers cleanly to the other. In this book we'll be teaching Python 3, but we'll point out important differences between the versions when necessary.