Python 2 or Python 3?

There are two main branches of Python: 2.7.x and 3.x. At the time of the revision of this third edition of the book, the Python foundation (www.python.org/) is offering downloads for Python Version 2.7.15 (release date January 5, 2018) and 3.6.5 (release date January 3, 2018). Although the Python 3 version is the newest, the older Python 2 has still been in use in both scientific (20% adoption) and commercial (30% adoption) areas in 2017, as depicted in detail by this survey by JetBrains: https://www.jetbrains.com/research/python-developers-survey-2017. If you are still using Python 2, the situation could turn quite problematic soon, because in just one year's time Python 2 will be retired and maintenance will be ceased (pythonclock.org/ will provide you with the countdown, but for an official statement about this, just read https://www.python.org/dev/peps/pep-0373/), and there are really only a handful of libraries still incompatible between the two versions (py3readiness.org/) that do not give enough reasons to stay with the older version.

In addition to all these reasons, there is no immediate backward compatibility between Python 3 and 2. In fact, if you try to run some code developed for Python 2 with a Python 3 interpreter, it may not work. Major changes have been made to the newest version, and that has affected past compatibility. Some data scientists, having built most of their work on Python 2 and its packages, are reluctant to switch to the new version.

In this third edition of the book, we will continue to address the larger audience of data scientists, data analysts, and developers, who do not have such a strong legacy with Python 2. Consequently, we will continue working with Python 3, and we suggest using a version such as the most recently available Python 3.6. After all, Python 3 is the present and the future of Python. It is the only version that will be further developed and improved by the Python foundation, and it will be the default version of the future on many operating systems.

Anyway, if you are currently working with version 2 and you prefer to keep on working with it, you can still use this book and all of its examples. In fact, for the most part, our code will simply work on Python 2 after having the code itself preceded by these imports:

from __future__ import (absolute_import, division, 
                        print_function, unicode_literals) 
from builtins import * 
from future import standard_library 
standard_library.install_aliases() 

As described in the Python-future website (python-future.org), these imports will help convert several Python 3-only constructs to a form that's compatible with both Python 3 and Python 2 (and in any case, most Python 3 code should just simply work on Python 2, even without the aforementioned imports).

In order to run the upward commands successfully, if the future package is not already available on your system, you should install it (version >= 0.15.2) by using the following command, which is to be executed from a shell:

$> pip install -U future

If you're interested in understanding the differences between Python 2 and Python 3 further, we recommend reading the wiki page offered by the Python foundation itself: https://wiki.python.org/moin/Python2orPython3.