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Python Enhancement Proposals

PEP 6 – Bug Fix Releases

Aahz <aahz at>, Anthony Baxter <anthony at>
15-Mar-2001, 18-Apr-2001, 19-Aug-2004

Table of Contents


This PEP is obsolete. The current release policy is documented in the devguide. See also PEP 101 for mechanics of the release process.


Python has historically had only a single fork of development, with releases having the combined purpose of adding new features and delivering bug fixes (these kinds of releases will be referred to as “major releases”). This PEP describes how to fork off maintenance, or bug fix, releases of old versions for the primary purpose of fixing bugs.

This PEP is not, repeat NOT, a guarantee of the existence of bug fix releases; it only specifies a procedure to be followed if bug fix releases are desired by enough of the Python community willing to do the work.


With the move to SourceForge, Python development has accelerated. There is a sentiment among part of the community that there was too much acceleration, and many people are uncomfortable with upgrading to new versions to get bug fixes when so many features have been added, sometimes late in the development cycle.

One solution for this issue is to maintain the previous major release, providing bug fixes until the next major release. This should make Python more attractive for enterprise development, where Python may need to be installed on hundreds or thousands of machines.


Bug fix releases are required to adhere to the following restrictions:

  1. There must be zero syntax changes. All .pyc and .pyo files must work (no regeneration needed) with all bugfix releases forked off from a major release.
  2. There must be zero pickle changes.
  3. There must be no incompatible C API changes. All extensions must continue to work without recompiling in all bugfix releases in the same fork as a major release.

Breaking any of these prohibitions requires a BDFL proclamation (and a prominent warning in the release notes).


Where possible, bug fix releases should also:

  1. Have no new features. The purpose of a bug fix release is to fix bugs, not add the latest and greatest whizzo feature from the HEAD of the CVS root.
  2. Be a painless upgrade. Users should feel confident that an upgrade from 2.x.y to 2.x.(y+1) will not break their running systems. This means that, unless it is necessary to fix a bug, the standard library should not change behavior, or worse yet, APIs.

Applicability of Prohibitions

The above prohibitions and not-quite-prohibitions apply both for a final release to a bugfix release (for instance, 2.4 to 2.4.1) and for one bugfix release to the next in a series (for instance 2.4.1 to 2.4.2).

Following the prohibitions listed in this PEP should help keep the community happy that a bug fix release is a painless and safe upgrade.

Helping the Bug Fix Releases Happen

Here’s a few pointers on helping the bug fix release process along.

  1. Backport bug fixes. If you fix a bug, and it seems appropriate, port it to the CVS branch for the current bug fix release. If you’re unwilling or unable to backport it yourself, make a note in the commit message, with words like ‘Bugfix candidate’ or ‘Backport candidate’.
  2. If you’re not sure, ask. Ask the person managing the current bug fix releases if they think a particular fix is appropriate.
  3. If there’s a particular bug you’d particularly like fixed in a bug fix release, jump up and down and try to get it done. Do not wait until 48 hours before a bug fix release is due, and then start asking for bug fixes to be included.

Version Numbers

Starting with Python 2.0, all major releases are required to have a version number of the form X.Y; bugfix releases will always be of the form X.Y.Z.

The current major release under development is referred to as release N; the just-released major version is referred to as N-1.

In CVS, the bug fix releases happen on a branch. For release 2.x, the branch is named ‘release2x-maint’. For example, the branch for the 2.3 maintenance releases is release23-maint


The process for managing bugfix releases is modeled in part on the Tcl system [1].

The Patch Czar is the counterpart to the BDFL for bugfix releases. However, the BDFL and designated appointees retain veto power over individual patches. A Patch Czar might only be looking after a single branch of development - it’s quite possible that a different person might be maintaining the 2.3.x and the 2.4.x releases.

As individual patches get contributed to the current trunk of CVS, each patch committer is requested to consider whether the patch is a bug fix suitable for inclusion in a bugfix release. If the patch is considered suitable, the committer can either commit the release to the maintenance branch, or else mark the patch in the commit message.

In addition, anyone from the Python community is free to suggest patches for inclusion. Patches may be submitted specifically for bugfix releases; they should follow the guidelines in PEP 3. In general, though, it’s probably better that a bug in a specific release also be fixed on the HEAD as well as the branch.

The Patch Czar decides when there are a sufficient number of patches to warrant a release. The release gets packaged up, including a Windows installer, and made public. If any new bugs are found, they must be fixed immediately and a new bugfix release publicized (with an incremented version number). For the 2.3.x cycle, the Patch Czar (Anthony) has been trying for a release approximately every six months, but this should not be considered binding in any way on any future releases.

Bug fix releases are expected to occur at an interval of roughly six months. This is only a guideline, however - obviously, if a major bug is found, a bugfix release may be appropriate sooner. In general, only the N-1 release will be under active maintenance at any time. That is, during Python 2.4’s development, Python 2.3 gets bugfix releases. If, however, someone qualified wishes to continue the work to maintain an older release, they should be encouraged.

Patch Czar History

Anthony Baxter is the Patch Czar for 2.3.1 through 2.3.4.

Barry Warsaw is the Patch Czar for 2.2.3.

Guido van Rossum is the Patch Czar for 2.2.2.

Michael Hudson is the Patch Czar for 2.2.1.

Anthony Baxter is the Patch Czar for 2.1.2 and 2.1.3.

Thomas Wouters is the Patch Czar for 2.1.1.

Moshe Zadka is the Patch Czar for 2.0.1.


This PEP started life as a proposal on comp.lang.python. The original version suggested a single patch for the N-1 release to be released concurrently with the N release. The original version also argued for sticking with a strict bug fix policy.

Following feedback from the BDFL and others, the draft PEP was written containing an expanded bugfix release cycle that permitted any previous major release to obtain patches and also relaxed the strict bug fix requirement (mainly due to the example of PEP 235, which could be argued as either a bug fix or a feature).

Discussion then mostly moved to python-dev, where BDFL finally issued a proclamation basing the Python bugfix release process on Tcl’s, which essentially returned to the original proposal in terms of being only the N-1 release and only bug fixes, but allowing multiple bugfix releases until release N is published.

Anthony Baxter then took this PEP and revised it, based on lessons from the 2.3 release cycle.



Last modified: 2023-11-28 14:46:07 GMT