16. Accepting Pull Requests

This page is a step-by-step guide for core developers who need to assess, merge, and possibly backport a pull request on the main repository.

16.1. Assessing a pull request

Before you can accept a pull request, you need to make sure that it is ready to enter the public source tree. Ask yourself the following questions:

  • Are there ongoing discussions at bugs.python.org (b.p.o.)?

    Read the linked b.p.o. issue. If there are ongoing discussions, then we need to have a resolution there before we can merge the pull request.

  • Was the pull request first made against the appropriate branch?

    The only branch that receives new features is main, the in-development branch. Pull requests should only target bug-fix branches if an issue appears in only that version and possibly older versions.

  • Are the changes acceptable?

    If you want to share your work-in-progress code on a feature or bugfix, then you can open a WIP-prefixed pull request, publish patches on the issue tracker, or create a public fork of the repository.

  • Do the checks on the pull request show that the test suite passes?

    Make sure that all of the status checks are passing.

  • Is the patch in a good state?

    Check Lifecycle of a Pull Request and Helping Triage Issues to review what is expected of a patch.

  • Does the patch break backwards-compatibility without a strong reason?

    Run the entire test suite to make sure that everything still passes. If there is a change to the semantics, then there needs to be a strong reason, because it will cause some peoples’ code to break. If you are unsure if the breakage is worth it, then ask on python-dev.

  • Does documentation need to be updated?

    If the pull request introduces backwards-incompatible changes (e.g. deprecating or removing a feature), then make sure that those changes are reflected in the documentation before you merge the pull request.

  • Were appropriate labels added to signify necessary backporting of the pull request?

    If it is determined that a pull request needs to be backported into one or more of the maintenance branches, then a core developer can apply the label needs backport to X.Y to the pull request. Once the backport pull request has been created, remove the needs backport to X.Y label from the original pull request. (Only core developers and members of the Python Triage Team can apply labels to GitHub pull requests).

  • Does the pull request have a label indicating that the submitter has signed the CLA?

    Make sure that the contributor has signed a Contributor Licensing Agreement (CLA), unless their change has no possible intellectual property associated with it (e.g. fixing a spelling mistake in documentation). To check if a contributor’s CLA has been received, go to Check Python CLA and put in their GitHub username. For further questions about the CLA process, write to contributors@python.org.

  • Were What's New in Python and Misc/NEWS.d/next updated?

    If the change is particularly interesting for end users (e.g. new features, significant improvements, or backwards-incompatible changes), then an entry in the What's New in Python document (in Doc/whatsnew/) should be added as well. Changes that affect only documentation generally do not require a NEWS entry. (See the following section for more information.)

16.2. Updating NEWS and What’s New in Python

Almost all changes made to the code base deserve an entry in Misc/NEWS.d. If the change is particularly interesting for end users (e.g. new features, significant improvements, or backwards-incompatible changes), then an entry in the What's New in Python document (in Doc/whatsnew/) should be added as well. Changes that affect documentation only generally do not require a NEWS entry.

There are two notable exceptions to this general principle, and they both relate to changes that:

  • Already have a NEWS entry

  • Have not yet been included in any formal release (including alpha and beta releases)

These are the two exceptions:

  1. If a change is reverted prior to release, then the corresponding entry is simply removed. Otherwise, a new entry must be added noting that the change has been reverted (e.g. when a feature is released in an alpha and then cut prior to the first beta).

  2. If a change is a fix (or other adjustment) to an earlier unreleased change and the original NEWS entry remains valid, then no additional entry is needed.

If a change needs an entry in What's New in Python, then it very likely not suitable for including in a maintenance release.

NEWS entries go into the Misc/NEWS.d directory as individual files. The NEWS entry can be created by using blurb-it, or the blurb tool and its blurb add command.

If you are unable to use the tool, then you can create the NEWS entry file manually. The Misc/NEWS.d directory contains a sub-directory named next, which contains various sub-directories representing classifications for what was affected (e.g. Misc/NEWS.d/next/Library for changes relating to the standard library). The file name itself should be in the format <datetime>.bpo-<issue-number>.<nonce>.rst:

  • <datetime> is today’s date joined with a hyphen (-) to the current time, in the YYYY-MM-DD-hh-mm-ss format (e.g. 2017-05-27-16-46-23).

  • <issue-number> is the issue number the change is for (e.g. 12345 for bpo-12345).

  • <nonce> is a unique string to guarantee that the file name is unique across branches (e.g. Yl4gI2). It is typically six characters long, but it can be any length of letters and numbers. Its uniqueness can be satisfied by typing random characters on your keyboard.

As a result, a file name can look something like Misc/NEWS.d/next/Library/2017-05-27-16-46-23.bpo-12345.Yl4gI2.rst.

The contents of a NEWS file should be valid reStructuredText. An 80 character column width should be used. There is no indentation or leading marker in the file (e.g. -). There is also no need to start the entry with the issue number since it is part of the file name. You can use inline markups too. Here is an example of a NEWS entry:

Fix warning message when :func:`os.chdir` fails inside
:func:`test.support.temp_cwd`. Patch by Chris Jerdonek.

The inline Sphinx roles like :func: can be used help readers find more information. You can build HTML and verify that the link target is appropriate by using make html.

While Sphinx roles can be beneficial to readers, they are not required. Inline ``code blocks`` can be used instead.

16.3. Working with Git

As a core developer, you have the ability to push changes to the official Python repositories, so you need to be careful with your workflow:

  • You should not push new branches to the main repository. You can still use them in the fork that you use for the development of patches. You can also push these branches to a separate public repository for maintenance work before it is integrated into the main repository.

  • You should not commit directly into the main branch, or any of the maintenance branches. You should commit against your own feature branch, and then create a pull request.

  • For a small change, you can make a quick edit through the GitHub web UI. If you choose to use the web UI, be aware that GitHub will create a new branch in the main CPython repository rather than in your fork. Delete this newly created branch after it has been merged into the main branch or any of the maintenance branches. To keep the CPython repository tidy, remove the new branch within a few days.

Keep a fork of the main repository, since it will allow you to revert all local changes (even committed ones) if you’re not happy with your local clone.

16.3.1. Seeing active branches

If you use git branch, then you will see a list of branches. The only branch that receives new features is main, the in-development branch. The other branches receive only bug fixes or security fixes.

16.3.2. Backporting changes to an older version

If it is determined that a pull request needs to be backported into one or more of the maintenance branches, then a core developer can apply the label needs backport to X.Y to the pull request.

After the pull request has been merged, miss-islington (bot) will first try to do the backport automatically. If miss-islington is unable to do it, then the pull request author or the core developer who merged it should look into backporting it themselves, using the backport generated by cherry_picker.py as a starting point.

You can get the commit hash from the original pull request, or you can use git log on the main branch. To display the 10 most recent commit hashes and their first line of the commit, use the following command:

git log -10 --oneline

You can prefix the backport pull request with the branch, and reference the pull request number from main. Here is an example:

[3.9] bpo-12345: Fix the Spam Module (GH-NNNN)

Note that cherry_picker.py adds the branch prefix automatically.

Once the backport pull request has been created, remove the needs backport to X.Y label from the original pull request. (Only core developers and members of the Python Triage Team can apply labels to GitHub pull requests).

16.3.3. Reverting a merged pull request

To revert a merged pull request, press the Revert button at the bottom of the pull request. That will bring up the page to create a new pull request where the commit can be reverted. It will also create a new branch on the main CPython repository. Delete the branch once the pull request has been merged.

Always include the reason for reverting the commit to help others understand why it was done. The reason should be included as part of the commit message. Here is an example:

Revert bpo-NNNN: Fix Spam Module (GH-111)

Reverts python/cpython#111.
Reason: This commit broke the buildbot.