YTEP-0033: Dropping Python2 Support


Created: November 28, 2017 Author: Nathan Goldbaum Revision: Matt Turk

We will be dropping support for Python 2.7 in all “major” releases after 3.5, which will include both 3.6 and 4.0.



Detailed Description


Python 2.7, the last major release in the Python 2 series, is currently in maintenance mode, only receiving critical bugfixes. Even this minimal level of support will end on April 12, 2020, when Python 2.7 will go end of life. At that point, Python 2.7 will no longer receive even security fixes. It is thus incumbent on us to not encourage our users to use an insecure, unsupported version of Python that may not even necessarily build on future versions of operating systems by maintaining support for Python 2.7 indefinitely.

Beyond that, for purely selfish reasons, we as a community have taken on costs to support both Python 2 and Python 3 in the same codebase. All contributions must happen in the dialect of Python that simultaneously functions under both major versions. We make heavy use of six, requiring contributors to learn about six to make even trivial contributions that happen to touch on Python 2/3 incompatibilities. We must run tests on both versions to avoid version-specific bugs and regressions. We are unable to use new features that have been added in Python 3 since these features are unavailable in Python 2.

For this reason, many projects in the scientific python ecosystem have either already ended support or proposed ending support for Python 2.7 in the near future. These efforts are summarized in the Python 3 statement, including a timeline that depicts the end of Python 2.7 support for projects that have signed the statement.

Proposed Solution

This YTEP proposes that yt 3.5 be the last major release of yt that will support Python 2.7. Subsequent releases, including 3.6 and 4.0 (but not including 3.5.1, 3.5.2, etc) will not support Python 2.7. In the past, we had suggested that yt 3.6 would not exist, or would not drop python 2.7 support, but the delay in yt 4.0 release has changed this timeline.

The development bandwidth for yt is somewhat restricted. In the past, we have had to ensure installation and utilization on many reasonably slow-moving environments (in particular HPC) but in the last five years or so the change in environments on HPC resources has largely eliminated this as a concern; we do not need to ensure extremely long-term compatibility with python 2.7. Some projects are treating their last release that supports Python 2.7 as a “long term support” (LTS) release. Given our development resources, we generally do not have the bandwidth available to simultaneously support both yt 4.0 and and python2-enabled yt 3.5 at the same level.

We are thus proposing that we make “best effort” compatibility with python2 in the 3.6 series (i.e., “soft” backcompat) and we will continue to accept patches for 3.5, although we anticipate these will taper with time, and we will cease to accept them one calendar year after support for python2.7 has ended. yt 4.0 will not have any backcompat built in.

To summarize:

  • yt 3.5: This will be the last release series that explicitly supports python 2. No further development is anticipated once yt 3.6 has been released.
  • yt 3.6: No hard python2 compatibility requirements, and “best efforts” may not be retained over the development lifetime. Testing of python2 will be dropped prior to release.
  • yt 4.0: No back-compat with python 2.

These timelines are roughly comparable with projects that we directly depend on. In particular:

In principle we don’t need to be constrained by the timelines of projects we depend on, since we need only use the last version that supports Python 2.7, but that would add yet another maintenance burden, since we would not be able to use the latest and greatest version of a downstream project for development.

Since we will be backporting bugfixes to the yt 3.6 release branch while yt 4.0 is under development, we will keep the uses of six and other python 2/3 compatibility code in an effort to make backporting easier. Once yt 4.0 is released we will be able to begin removing compatibility shims and __future__ imports. We will be able to immediately drop the tests for Python 2.7 support on th development branch.

Community Outreach

I am planning to make both yt-dev and yt-users aware of this proposal, with the hope of soliciting feedback from interested users who do not normally participate in development discussions. Additionally, if this proposal is accepted, we will announce the future timeline for releases, along with the planned timeline for dropping Python 2.7 support in all release announcements. This should give more than a year of warning for community members to either say their peace about Python 2.7 support, or to make preparations to migrate user scripts to use Python 3.

Backwards Compatibility

Users who must remain on Python 2.7 for whatever reason will no longer be able to run the latest version of yt. Existing versions will continue to function, however. Users who currently run yt under python 3 (this is the default option in the install script since April) will see no change.


  • Maintain support for Python 2.7 indefinitely.
  • Maintain support for Python 2.7 for the yt 4.x series, dropping support in yt 5.0 or in a subsequent verison. This would likely mean maintaining support beyond 2020.