16. Accepting Pull Requests

This page is aimed to core developers, and covers the steps required to accept, merge, and possibly backport a pull request on the main repository.

16.1. Is the PR ready to be accepted?

Before a PR is accepted, you must make sure it is ready to enter the public source tree. Use the following as a checklist of what to check for before merging (details of various steps can be found later in this document):

  1. Has the submitter signed the CLA? (delineated by a label on the pull request)
  2. Did the test suite pass? (delineated by a pull request check)
  3. Did code coverage increase or stay the same? (delineated by a comment on the pull request)
  4. Are the changes acceptable?
  5. Was configure regenerated (if necessary)?
  6. Was pyconfig.h.in regenerated (if necessary)?
  7. Was the submitter added to Misc/ACKS (as appropriate)?
  8. Was an entry added under Misc/NEWS.d/next (as appropriate)?
  9. Was “What’s New” updated (as appropriate)?
  10. Were appropriate labels added to signify necessary backporting of the pull request?

Note

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

16.1.1. Does the test suite still pass?

You must run the whole test suite to ensure that it passes before merging any code changes.

Note

You really need to run the entire test suite. Running a single test is not enough as the changes may have unforeseen effects on other tests or library modules.

Running the entire test suite doesn’t guarantee that the changes will pass the continuous integration tests, as those will exercise more possibilities still (such as different platforms or build options). But it will at least catch non-build specific, non-platform specific errors, therefore minimizing the chance for breakage.

16.1.2. Patch checklist

Along with running the tests, a simple automated patch checklist, patchcheck, guides a developer through the common patch generation checks. To run patchcheck:

On UNIX (including Mac OS X):

make patchcheck

On Windows (after any successful build):

python.bat Tools/scripts/patchcheck.py

The automated patch checklist runs through:

  • Are there any whitespace problems in Python files? (using Tools/scripts/reindent.py)
  • Are there any whitespace problems in C files?
  • Are there any whitespace problems in the documentation? (using Tools/scripts/reindent-rst.py)
  • Has the documentation been updated?
  • Has the test suite been updated?
  • Has an entry under Misc/NEWS.d/next been added?
  • Has Misc/ACKS been updated?
  • Has configure been regenerated, if necessary?
  • Has pyconfig.h.in been regenerated, if necessary?

The automated patch check doesn’t actually answer all of these questions. Aside from the whitespace checks, the tool is a memory aid for the various elements that can go into making a complete patch.

16.2. Commit Style

Once a change patch is ready and tested, it can be committed to the repository. We usually prefer to put a whole feature or bugfix into a single commit, but no more. In particular:

  • Do not fix more than one issue in the same commit (except, of course, if one code change fixes all of them).
  • Do not do cosmetic changes to unrelated code in the same commit as some feature/bugfix.

It is of course okay to pile up several commits to one branch and merge them into another in one commit.

16.3. Handling Others’ Code

As a core developer you will occasionally want to commit a patch created by someone else. When doing so you will want to make sure of some things.

First, make sure the patch is in a good state. Both Lifecycle of a Pull Request and Helping Triage Issues explain what is to be expected of a patch. Typically patches that get cleared by triagers are good to go except maybe lacking Misc/ACKS and Misc/NEWS.d entries (which a core developer should make sure are updated appropriately).

Second, make sure the patch does not break backwards-compatibility without a good reason. This means running the entire test suite to make sure everything still passes. It also means that if semantics do change there must be a good reason for the breakage of code the change will cause (and it will break someone’s code). If you are unsure if the breakage is worth it, ask on python-dev.

Third, ensure the patch is attributed correctly with the contributor’s name in Misc/ACKS if they aren’t already there (and didn’t add themselves in their patch) and by mentioning “Patch by <x>” in the Misc/NEWS.d entry and the check-in message. If the patch has been heavily modified then “Initial patch by <x>” is an appropriate alternate wording.

If you omit correct attribution in the initial check-in, then update ACKS and NEWS.d in a subsequent check-in (don’t worry about trying to fix the original check-in message in that case).

Finally, make sure that the submitter of the patch has a CLA in place (indicated by an asterisk following their username in the issue tracker or by the “CLA Signed” label on the pull request). If the submitter lacks a signed CLA and the patch is non-trivial, direct them to the electronic Contributor Licensing Agreement to ensure the PSF has the appropriate authorizations in place to relicense and redistribute their code.

16.4. Contributor Licensing Agreements

Always get a Contributor Licensing Agreement (CLA) signed unless the change has no possible intellectual property associated with it (e.g. fixing a spelling mistake in documentation). Otherwise it is simply safer from a legal standpoint to require the contributor to sign the CLA.

These days, the CLA can be signed electronically through the form linked above, and this process is strongly preferred to the old mechanism that involved sending a scanned copy of the signed paper form.

As discussed on the PSF Contribution page, it is the CLA itself that gives the PSF the necessary relicensing rights to redistribute contributions under the Python license stack. This is an additional permission granted above and beyond the normal permissions provided by the chosen open source license.

Some developers may object to the relicensing permissions granted to the PSF by the CLA. They’re entirely within their rights to refuse to sign the CLA on that basis, but that refusal does mean we can’t accept their patches for inclusion.

16.5. What’s New and News Entries

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), an entry in the What's New in Python document (in Doc/whatsnew/) should be added as well.

There are two notable exceptions to this general principle, and they both relate to changes that already have a news entry, and have not yet been included in any formal release (including alpha and beta releases). These exceptions are:

  • 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).
  • 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.

Needing a What’s New entry almost always means that a change is not suitable for inclusion in a maintenance release. A small number of exceptions have been made for Python 2.7 due to the long support period - when implemented, these changes must be noted in the “New Additions in Python 2.7 Maintenance Releases” section of the Python 2.7 What’s New document.

News entries go into the Misc/NEWS.d directory as individual files. The easiest way to create a news entry is to use the blurb tool and its blurb add command.

If you are unable to use the tool you can create the news entry file manually. The Misc/NEWS.d directory contains a sub-directory named next which itself 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 of the format <date>.bpo-<issue-number>.<nonce>.rst:

  • <date> is today’s date in YYYY-MM-DD format, e.g. 2017-05-27
  • <issue-number> is the issue number the change is for, e.g. 12345 for bpo-12345
  • <nonce> is some “unique” string to guarantee the file name is unique across branches, e.g. Yl4gI2 (typically six characters, but it can be any length of letters and numbers, and its uniqueness can be satisfied by typing random characters on your keyboard)

So a file name may be Misc/NEWS.d/next/Library/2017-05-27.bpo-12345.Yl4gI2.rst.

The contents of a news file should be valid reStructuredText. The “default role” (single backticks) in reST can be used to refer to objects in the documentation. 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 as it’s part of the file name itself. Example news entry:

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

(In other .rst files the single backticks should not be used. They are allowed here because news entries are meant to be as readable as possible unprocessed.)

16.6. Commit Messages

Every commit has a commit message to document why a change was made and to communicate that reason to other core developers. Python core developers have developed a standard way of formatting commit messages that everyone is expected to follow.

Our usual convention mimics that used in news entries (it is actually common to start by pasting the news entry into the commit message). The only key difference when compared to a news entry is the inclusion of the issue number as the beginning of the commit message. Here is an example:

bpo-42: the spam module is now more spammy.

The spam module sporadically came up short on spam. This change
raises the amount of spam in the module by making it more spammy.

Thanks to Monty Python for the patch.

The first line or sentence is meant to be a dense, to-the-point explanation of what the purpose of the commit is. If this is not enough detail for a commit, a new paragraph(s) can be added to explain in proper depth what has happened (detail should be good enough that a core developer reading the commit message understands the justification for the change). Also, if a non-core developer contributed to the resolution, it is good practice to credit them.

17. Working with Git

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

  • You should not push new branches to the main repository. You can still use them in your fork that you use for development of patches; you can also push these branches to a separate public repository that will be dedicated to maintenance of the work before the work gets integrated in the main repository.
  • You should not commit directly into the master branch, or any of the maintenance branches (2.7, 3.5, or 3.6). You should commit against your own feature branch, and create a pull request.

It is recommended to keep a fork of the main repository around, as it allows simple reversion of all local changes (even “committed” ones) if your local clone gets into a state you aren’t happy with.

17.1. Active branches

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

17.2. Backporting Changes to an Older Version

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

After the pull request has been merged, it can be backported using cherry_picker.py.

The commit hash can be obtained from the original pull request, or by using git log on the master branch. To display the 10 most recent commit hashes and their first line of the commit message:

git log -10 --oneline

Prefix the backport pull request with the branch, for example:

[3.6] bpo-12345: Fix the Spam Module

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 can apply labels to GitHub pull requests).

17.3. Reverting a Merged Pull Request

To revert a merged pull request, press the Revert button at the bottom of the pull request. It will bring up the page to create a new pull request where the commit can be reverted. It also creates 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, for example:

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

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