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Pull request guidelines

Pull requests (PRs) on GitHub are the mechanism for contributing to Matplotlib's code and documentation.

We value contributions from people with all levels of experience. In particular, if this is your first PR not everything has to be perfect. We'll guide you through the PR process. Nevertheless, please try to follow our guidelines as well as you can to help make the PR process quick and smooth. If your pull request is incomplete or a work-in-progress, please mark it as a :ref:`draft pull request <draft-pr>` on GitHub and specify what feedback from the developers would be helpful.

Please be patient with reviewers. We try our best to respond quickly, but we have limited bandwidth. If there is no feedback within a couple of days, please ping us by posting a comment to your PR or reaching out on a :ref:`communication channel <communication-channels>`

Summary for pull request authors

We recommend that you check that your contribution complies with the following guidelines before submitting a pull request:

.. rst-class:: checklist

When opening a pull request on Github, please ensure that:

.. rst-class:: checklist

For guidance on creating and managing a pull request, please see our :ref:`contributing <contributing>` and :ref:`pull request workflow <edit-flow>` guides.

Summary for pull request reviewers

.. redirect-from:: /devel/maintainer_workflow

Please help review and merge PRs!

If you have commit rights, then you are trusted to use them. Please be patient and kind with contributors.

When reviewing, please ensure that the pull request satisfies the following requirements before merging it:

Content

.. rst-class:: checklist
  • Is the feature / bugfix reasonable?
  • Does the PR conform with the :ref:`coding_guidelines`?
  • Is the :ref:`documentation <pr-documentation>` (docstrings, examples, what's new, API changes) updated?
  • Is the change purely stylistic? Generally, such changes are discouraged when not part of other non-stylistic work because it obscures the git history of functional changes to the code. Reflowing a method or docstring as part of a larger refactor/rewrite is acceptable.

Workflow

.. rst-class:: checklist

Detailed guidelines

Draft PRs

Authors may create a draft PR (or change to draft status later) if the code is not yet ready for a regular full review. Typical use cases are posting code as a basis for discussion or signalling that you intend to rework the code as a result of feedback. Authors should clearly communicate why the PR has draft status and what needs to be done to make it ready for review. In particular, they should explicitly ask for targeted feedback if needed. By default, reviewers will not look at the code of a draft PR and only respond to specific questions by the author.

Documentation

  • Every new feature should be documented. If it's a new module, don't forget to add a new rst file to the API docs.
  • Each high-level plotting function should have a small example in the Examples section of the docstring. This should be as simple as possible to demonstrate the method. More complex examples should go into a dedicated example file in the :file:`examples` directory, which will be rendered to the examples gallery in the documentation.
  • Build the docs and make sure all formatting warnings are addressed.
  • See :ref:`documenting-matplotlib` for our documentation style guide.

Labels

  • If you have the rights to set labels, tag the PR with descriptive labels. See the list of labels.
  • If the PR makes changes to the wheel building Action, add the "Run cibuildwheel" label to enable testing wheels.
  • If the PR does not yet have the quality and clarity needed for an effective review, you can use the "status: autoclose candidate" label. This will trigger a two-weeks countdown after which the PR will be automatically closed if no further improvements have been made. See the autoclose workflow for more details.

Milestones

Set the milestone according to these guidelines:

  • New features and API changes are milestoned for the next meso release v3.N.0.
  • Bugfixes, tests for released code, and docstring changes may be milestoned for the next micro release v3.N.M.
  • Documentation changes (only .rst files and examples) may be milestoned v3.N-doc.

If multiple rules apply, choose the first matching from the above list. See :ref:`backport-strategy` for detailed guidance on what should or should not be backported.

The milestone marks the release a PR should go into. It states intent, but can be changed because of release planning or re-evaluation of the PR scope and maturity.

All Pull Requests should target the main branch. The milestone tag triggers an :ref:`automatic backport <automated-backports>` for milestones which have a corresponding branch.

Review

  • Do not let perfect be the enemy of the good, particularly for documentation or example PRs. If you find yourself making many small suggestions, either open a PR against the original branch, push changes to the contributor branch, or merge the PR and then open a new PR against upstream.
  • If you push to a contributor branch leave a comment explaining what you did, ex "I took the liberty of pushing a small clean-up PR to your branch, thanks for your work.". If you are going to make substantial changes to the code or intent of the PR please check with the contributor first.

Approval

As a guiding principle, we require two approvals from core developers (those with commit rights) before merging a pull request. This two-pairs-of-eyes strategy shall ensure a consistent project direction and prevent accidental mistakes. It is permissible to merge with one approval if the change is not fundamental and can easily be reverted at any time in the future.

Some explicit rules following from this:

  • Documentation and examples may be merged with a single approval. Use the threshold "is this better than it was?" as the review criteria.

  • Minor infrastructure updates, e.g. temporary pinning of broken dependencies or small changes to the CI configuration, may be merged with a single approval.

  • Code changes (anything in src or lib) must have two approvals.

    Ensure that all API changes are documented in a file in one of the subdirectories of :file:`doc/api/next_api_changes`, and significant new features have an entry in :file:`doc/user/whats_new`.

    • If a PR already has a positive review, a core developer (e.g. the first reviewer, but not necessarily) may champion that PR for merging. In order to do so, they should ping all core devs both on GitHub and on the dev mailing list, and label the PR with the "Merge with single review?" label. Other core devs can then either review the PR and merge or reject it, or simply request that it gets a second review before being merged. If no one asks for such a second review within a week, the PR can then be merged on the basis of that single review.

      A core dev should only champion one PR at a time and we should try to keep the flow of championed PRs reasonable.

Automated tests

Before being merged, a PR should pass the :ref:`automated-tests`. If you are unsure why a test is failing, ask on the PR or in our :ref:`communication-channels`

Merging

After giving the last required :ref:`approval <pr-approval>`, the author of the approval should merge the PR. PR authors should not self-merge except for when another reviewer explicitly allows it (e.g., "Approve modulo CI passing, may self-merge when green", or "Take or leave the comments. You may self merge".).

Number of commits and squashing

  • Squashing is case-by-case. The balance is between burden on the contributor, keeping a relatively clean history, and keeping a history usable for bisecting. The only time we are really strict about it is to eliminate binary files (ex multiple test image re-generations) and to remove upstream merges.

Branches and backports

Current branches

The current active branches are

main
The current development version. Future meso (v3.N.0) or macro (v4.0.0) will be branched from this.
v3.N.x
Maintenance branch for Matplotlib 3.N. Future micro releases will be tagged from this.
v3.N.M-doc
Documentation for the current micro release. On a micro release, this will be replaced by a properly named branch for the new release.

Branch selection for pull requests

Generally, all pull requests should target the main branch.

Other branches are fed through :ref:`automatic <automated-backports>` or :ref:`manual <manual-backports>`. Directly targeting other branches is only rarely necessary for special maintenance work.

Backport strategy

Backports to the micro release branch (v3.N.x) are the changes that will be included in the next patch (aka bug-fix) release. The goal of the patch releases is to fix bugs without adding any new regressions or behavior changes. We will always attempt to backport:

  • critical bug fixes (segfault, failure to import, things that the user cannot work around)
  • fixes for regressions introduced in the last two meso releases

and may attempt to backport fixes for regressions introduced in older releases.

In the case where the backport is not clean, for example if the bug fix is built on top of other code changes we do not want to backport, balance the effort and risk of re-implementing the bug fix vs the severity of the bug. When in doubt, err on the side of not backporting.

When backporting a Pull Request fails or is declined, re-milestone the original PR to the next meso release and leave a comment explaining why.

The only changes backported to the documentation branch (v3.N.M-doc) are changes to :file:`doc` or :file:`galleries`. Any changes to :file:`lib` or :file:`src`, including docstring-only changes, must not be backported to this branch.

Automated backports

We use MeeseeksDev bot to automatically backport merges to the correct maintenance branch base on the milestone. To work properly the milestone must be set before merging. If you have commit rights, the bot can also be manually triggered after a merge by leaving a message @meeseeksdev backport to BRANCH on the PR. If there are conflicts MeeseeksDev will inform you that the backport needs to be done manually.

The target branch is configured by putting on-merge: backport to TARGETBRANCH in the milestone description on it's own line.

If the bot is not working as expected, please report issues to MeeseeksDev.

Manual backports

When doing backports please copy the form used by MeeseeksDev, Backport PR #XXXX: TITLE OF PR. If you need to manually resolve conflicts make note of them and how you resolved them in the commit message.

We do a backport from main to v2.2.x assuming:

  • matplotlib is a read-only remote branch of the matplotlib/matplotlib repo

The TARGET_SHA is the hash of the merge commit you would like to backport. This can be read off of the GitHub PR page (in the UI with the merge notification) or through the git CLI tools.

Assuming that you already have a local branch v2.2.x (if not, then git checkout -b v2.2.x), and that your remote pointing to https://github.com/matplotlib/matplotlib is called upstream:

git fetch upstream
git checkout v2.2.x  # or include -b if you don't already have this.
git reset --hard upstream/v2.2.x
git cherry-pick -m 1 TARGET_SHA
# resolve conflicts and commit if required

Files with conflicts can be listed by git status, and will have to be fixed by hand (search on >>>>>). Once the conflict is resolved, you will have to re-add the file(s) to the branch and then continue the cherry pick:

git add lib/matplotlib/conflicted_file.py
git add lib/matplotlib/conflicted_file2.py
git cherry-pick --continue

Use your discretion to push directly to upstream or to open a PR; be sure to push or PR against the v2.2.x upstream branch, not main!