Python in a Nutshell, 3rd Edition

Chapter 26. v2/v3 Migration and Coexistence

The earliest release of Python 3 (the precursor of what we call v3 in this book) first appeared in 2008, specifically as “Python 3.0”: that release was not a production-quality one, nor was it meant to be. Its main purpose was to let people start adapting to changes in syntax and semantics; some parts of the implementation were unsatisfactory (particularly the I/O facilities, now much improved). As we’re writing this chapter, the current releases are 2.7.12 and 3.5.2, with a 3.6.0 release just out in December 2016, very close to the deadline for us to finish this book (as a result, while we mention 3.6’s highlights as being “new in 3.6,” that’s in good part based on a beta version of 3.6: we can’t claim thorough coverage of 3.6).

Python’s core developers have done a lot of work to backport v3 features into v2, but this activity has now ended, and v2 is feature-frozen (some v3 standard library backports to v2 are available for pip install, as repeatedly mentioned earlier in this book). v2 is getting only security bug-fix releases, and even those are due to stop once v2 maintenance formally ends in 2020. Guido van Rossum’s PyCon NA keynote in 2014 explicitly ruled out any 2.8 release; should you choose to stick with v2, after 2020 you will be on your own.

For those who have mastered Python’s build system, or who are able to hire third-party developers to support their v2 use, “sticking with v2” may remain a viable strategy even after 2020. ...