Different versions of Python exist. At the moment of creating this book, the most popular versions are Python 2 and Python 3. Python 3 is, as can be expected, an update of Python 2. Python 2 programs are, unfortunately, not completely compatible with Python 3. Since a lot of Python 2 code is still in use, Python 2 is still an active language, and still being maintained.

The reason why Python 3 was created is to resolve a number of inconsistencies and idiosyncrasies in the Python 2 language. For people new to programming, this is a big plus, because there are less “weird” language elements they need to learn and understand if they choose Python 3 instead of Python 2.

To give an example, when you calculate $7/4$ in Python 2, the answer is $1$, and not $1.75$ as you might expect. The reason is that both $7$ and $4$ are whole numbers (“integers”), and therefore the result of their division is a whole number. If you want to make sure that the result is $1.75$, you must make at least one of the numbers involved a floating-point number. Therefore, $7.0/4$ gives the result $1.75$. This is how almost all computer languages do calculations. Naturally, for people who are not familiar with programming computers, this is counter-intuitive. Python 3 has resolved this issue, and automatically does the floating-point conversion when a floating-point result would be expected, i.e., in Python 3, $7/4$ gives the result $1.75$. Many Python 2 programs, however, are written with the assumption that integer-division rounds down, which means that, when you run them as Python 3 programs, they no longer give the desired results. Thus, Python 2 and Python 3 are not compatible.

Since Python 3 is more intuitive than Python 2, and since nowadays most Python programs and modules have been converted to Python 3, this book is written for Python 3. If you ever have to revert back to Python 2, it is not hard to make the change. An overview of the differences between Python 2 and Python 3 is given in Appendix 30¹ (which is not a complete overview, but contains all the differences that I am aware of). If you are only using Python 3, you can ignore this appendix. However, considering how often I see the question “What exactly are the differences between Python 2 and Python 3?,” and how hard it seems to be to find an answer to that question, I thought it prudent to add it.