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All of the major projects making up the Scientific Python stack now support both Python 3.x and Python 2.7, and many projects have been supporting these two versions of the language for several years. While we have developed tools and techniques to maintain compatibility efficiently, it is a small but constant friction in the development of a lot of code.
We are keen to use Python 3, and we currently accept the cost of writing cross-compatible code to allow a smooth transition, but we don’t intend to maintain this indefinitely. Although the transition has not been as quick as we hoped, we do see it taking place, with more and more people using, teaching and recommending Python 3.
The developers of the Python language extended support of Python 2.7 from 2015 to 2020, recognising that many people were still using Python 2. We believe that the extra 5 years is sufficient, and our projects plan to stop supporting Python 2 when upstream support ends in 2020, if not before. We will then be able to simplify our code and take advantage of the many new features in the current version of the Python language and standard library.
In addition, significantly before 2020, some of our projects will step down Python 2.7 support to only fixing bugs, and make new feature releases which require Python 3. This too parallels support for the language itself, as Python 2.7 releases only include bugfixes and security improvements.
Third parties may offer paid support for our projects on old Python versions for longer than we support them ourselves. We won’t obstruct this, and it is a core principle of free and open source software that this is possible. However, if you enjoy the free, first party support for many projects in the Scientific Python stack, please start planning to move to Python 3.
For all of these reasons, the following projects have pledged to drop support for Python 2.7 no later than 2020, coinciding with the Python development team's timeline for dropping support for Python 2.7.