17. Development Cycle

The responsibilities of a core developer shift based on what kind of branch of Python a developer is working on and what stage the branch is in.

To clarify terminology, Python uses a major.minor.micro nomenclature for production-ready releases. So for Python 3.1.2 final, that is a major version of 3, a minor version of 1, and a micro version of 2.

  • new major versions are exceptional; they only come when strongly incompatible changes are deemed necessary, and are planned very long in advance;
  • new minor versions are feature releases; they get released roughly every 18 months, from the current in-development branch;
  • new micro versions are bugfix releases; they get released roughly every 6 months, although they can come more often if necessary; they are prepared in maintenance branches.

We also publish non-final versions which get an additional qualifier: Alpha, Beta, release candidate. These versions are aimed at testing by advanced users, not production use.

17.1. Branches

There is a branch for each feature version, whether released or not (e.g. 2.7, 3.5). Development is handled separately for Python 2 and Python 3: no merging happens between 2.x and 3.x branches.

In each of the 2.x and 3.x realms, the branch for a feature version is always a descendant of the previous feature version: for example, the 3.3 branch is a descendant of the 3.2 branch.

Therefore, each change should be made first in the oldest branch to which it applies and forward-ported as appropriate: if a bug must be fixed in both Python 3.4 and 3.5, first fix it in 3.4 and then merge 3.4 into default (which holds the future 3.5).

17.1.1. In-development (main) branch

The default branch is the branch for the next feature release; it is under active development for all kinds of changes: new features, semantic changes, performance improvements, bug fixes. As the name indicates, it is the branch checked out by default by Mercurial.

Once a Final release (say, 3.3) is made from the in-development branch, a new maintenance branch is created to host all bug fixing activity for further micro versions (3.3.1, 3.3.2, etc.).

17.1.2. Maintenance branches

A branch for a previous feature release, currently being maintained for bug fixes. There are usually two maintenance branches at any given time: one for Python 3.x and one for Python 2.x. At some point in the future, Python 2.x will be closed for bug fixes and there will be only one maintenance branch left.

The only changes allowed to occur in a maintenance branch without debate are bug fixes. Also, a general rule for maintenance branches is that compatibility must not be broken at any point between sibling minor releases (3.3.1, 3.3.2, etc.). For both rules, only rare exceptions are accepted and must be discussed first.

Sometime after a new maintenance branch is created (after a new minor version is released), the old maintenance branch on that major version will go into security mode, usually after one last maintenance release at the discretion of the release manager. For example, the 3.2 maintenance branch was put into security mode after the 3.2.4 final maintenance release following the release of 3.3.0.

17.1.3. Security branches

A branch less than 5 years old but no longer in maintenance mode.

The only changes made to a security branch are those fixing issues exploitable by attackers such as crashes, privilege escalation and, optionally, other issues such as denial of service attacks. Any other changes are not considered a security risk and thus not backported to a security branch.

Commits to security branches are to be coordinated with the release manager for the corresponding feature version, as listed below in the Summary. Any release made from a security branch is source-only and done only when actual security patches have been applied to the branch.

17.1.4. Summary

There are 5 open branches right now in the Mercurial repository:

  • the default branch holds the future 3.5 version and descends from 3.4 (future RM: Larry Hastings)
  • the 3.4 branch holds bug fixes for future 3.4.x maintenance releases and descends from 3.3 (RM: Larry Hastings)
  • the 3.3 branch holds security fixes for future 3.3.x maintenance releases and descends from 3.2 (RM: Georg Brandl)
  • the 3.2 branch holds security fixes for future 3.2.x security releases (RM: Georg Brandl)
  • the 3.1 branch holds security fixes for future 3.1.x security releases (RM: Benjamin Peterson)
  • the 2.7 branch holds bug fixes for future 2.7.x maintenance releases and descends from 2.6 (RM: Benjamin Peterson)

17.2. Stages

Based on what stage the in-development version of Python is in, the responsibilities of a core developer change in regards to commits to the VCS.

17.2.1. Pre-alpha

The branch is in this stage when no official release has been done since the latest final release. There are no special restrictions placed on commits, although the usual advice applies (getting patches reviewed, avoiding breaking the buildbots).

17.2.2. Alpha

Alpha releases typically serve as a reminder to core developers that they need to start getting in changes that change semantics or add something to Python as such things should not be added during a Beta. Otherwise no new restrictions are in place while in alpha.

17.2.3. Beta

After a first beta release is published, no new features are accepted. Only bug fixes can now be committed. This is when core developers should concentrate on the task of fixing regressions and other new issues filed by users who have downloaded the alpha and beta releases.

Being in beta can be viewed much like being in RC but without the extra overhead of needing commit reviews.

17.2.4. Release Candidate (RC)

A branch preparing for an RC release can only have bugfixes applied that have been reviewed by other core developers. Generally, these issues must be severe enough (e.g. crashes) that they deserve fixing before the final release. All other issues should be deferred to the next development cycle, since stability is the strongest concern at this point.

You cannot skip the peer review during an RC, no matter how small! Even if it is a simple copy-and-paste change, everything requires peer review from a core developer.

17.2.5. Final

When a final release is being cut, only the release manager (RM) can make changes to the branch. After the final release is published, the full development cycle starts again for the next minor version.