Maintainer/Core-Developer Information#


This section is about preparing a major release, incrementing the minor version, or a bug fix release incrementing the patch version. Our convention is that we release one or more release candidates (0.RRrcN) before releasing the final distributions. We follow the PEP101 to indicate release candidates, post, and minor releases.

Before a release#

  1. Update authors table:

    Create a classic token on GitHub with the read:org following permission.

    Run the following script, entering the token in:

    cd build_tools; make authors; cd ..

    and commit. This is only needed if the authors have changed since the last release. This step is sometimes done independent of the release. This updates the maintainer list and is not the contributor list for the release.

  2. Confirm any blockers tagged for the milestone are resolved, and that other issues tagged for the milestone can be postponed.

  3. Ensure the change log and commits correspond (within reason!), and that the change log is reasonably well curated. Some tools for these tasks include:

    • maint_tools/ can put what’s new entries into sections. It’s not perfect, and requires manual checking of the changes. If the what’s new list is well curated, it may not be necessary.

    • The maint_tools/ script may be used to identify pull requests that were merged but likely missing from What’s New.

  4. Make sure the deprecations, FIXME and TODOs tagged for the release have been taken care of.


The release manager must be a maintainer of the scikit-learn/scikit-learn repository to be able to publish on and (via a manual trigger of a dedicated Github Actions workflow).

The release manager does not need extra permissions on to publish a release in particular.

The release manager must be a maintainer of the conda-forge/scikit-learn-feedstock repository. This can be changed by editing the recipe/meta.yaml file in the first release pull-request.

Preparing a release PR#

Major version release#

Prior to branching please do not forget to prepare a Release Highlights page as a runnable example and check that its HTML rendering looks correct. These release highlights should be linked from the doc/whats_new/v0.99.rst file for the new version of scikit-learn.

Releasing the first RC of e.g. version 0.99.0 involves creating the release branch 0.99.X directly on the main repo, where X really is the letter X, not a placeholder. The development for the major and minor releases of 0.99 should also happen under 0.99.X. Each release (rc, major, or minor) is a tag under that branch.

This is done only once, as the major and minor releases happen on the same branch:

# Assuming upstream is an alias for the main scikit-learn repo:
git fetch upstream main
git checkout upstream/main
git checkout -b 0.99.X
git push --set-upstream upstream 0.99.X

Again, X is literal here, and 99 is replaced by the release number. The branches are called 0.19.X, 0.20.X, etc.

In terms of including changes, the first RC ideally counts as a feature freeze. Each coming release candidate and the final release afterwards will include only minor documentation changes and bug fixes. Any major enhancement or feature should be excluded.

Then you can prepare a local branch for the release itself, for instance: release-0.99.0rc1, push it to your github fork and open a PR to the scikit-learn/0.99.X branch. Copy the Release checklist templates in the description of the Pull Request to track progress.

This PR will be used to push commits related to the release as explained in Making a release.

You can also create a second PR from main and targeting main to increment the __version__ variable in sklearn/ and in pyproject.toml to increment the dev version. This means while we’re in the release candidate period, the latest stable is two versions behind the main branch, instead of one. In this PR targeting main you should also include a new file for the matching version under the doc/whats_new/ folder so PRs that target the next version can contribute their changelog entries to this file in parallel to the release process.

Minor version release (also known as bug-fix release)#

The minor releases should include bug fixes and some relevant documentation changes only. Any PR resulting in a behavior change which is not a bug fix should be excluded. As an example, instructions are given for the 1.2.2 release.

  • Create a branch, on your own fork (here referred to as fork) for the release from upstream/main.

    git fetch upstream/main
    git checkout -b release-1.2.2 upstream/main
    git push -u fork release-1.2.2:release-1.2.2
  • Create a draft PR to the upstream/1.2.X branch (not to upstream/main) with all the desired changes.

  • Do not push anything on that branch yet.

  • Locally rebase release-1.2.2 from the upstream/1.2.X branch using:

    git rebase -i upstream/1.2.X

    This will open an interactive rebase with the git-rebase-todo containing all the latest commit on main. At this stage, you have to perform this interactive rebase with at least someone else (being three people rebasing is better not to forget something and to avoid any doubt).

    • Do not remove lines but drop commit by replace pick with drop

    • Commits to pick for bug-fix release generally are prefixed with: FIX, CI, DOC. They should at least include all the commits of the merged PRs that were milestoned for this release on GitHub and/or documented as such in the changelog. It’s likely that some bugfixes were documented in the changelog of the main major release instead of the next bugfix release, in which case, the matching changelog entries will need to be moved, first in the main branch then backported in the release PR.

    • Commits to drop for bug-fix release generally are prefixed with: FEAT, MAINT, ENH, API. Reasons for not including them is to prevent change of behavior (which only must feature in breaking or major releases).

    • After having dropped or picked commit, do no exit but paste the content of the git-rebase-todo message in the PR. This file is located at .git/rebase-merge/git-rebase-todo.

    • Save and exit, starting the interactive rebase.

    • Resolve merge conflicts when they happen.

  • Force push the result of the rebase and the extra release commits to the release PR:

    git push -f fork release-1.2.2:release-1.2.2
  • Copy the Release checklist template and paste it in the description of the Pull Request to track progress.

  • Review all the commits included in the release to make sure that they do not introduce any new feature. We should not blindly trust the commit message prefixes.

  • Remove the draft status of the release PR and invite other maintainers to review the list of included commits.

Making a release#

  1. Ensure that you have checked out the branch of the release PR as explained in Preparing a release PR above.

  2. Update docs. Note that this is for the final release, not necessarily for the RC releases. These changes should be made in main and cherry-picked into the release branch, only before the final release.

    • Edit the doc/whats_new/v0.99.rst file to add release title and list of contributors. You can retrieve the list of contributor names with:

      $ git shortlog -s 0.98.33.. | cut -f2- | sort --ignore-case | tr '\n' ';' | sed 's/;/, /g;s/, $//' | fold -s
      • For major releases, link the release highlights example from the doc/whats_new/v0.99.rst file.

    • Update the release date in whats_new.rst

    • Edit the doc/templates/index.html to change the ‘News’ entry of the front page (with the release month as well). Do not forget to remove the old entries (two years or three releases are typically good enough) and to update the on-going development entry.

  3. On the branch for releasing, update the version number in sklearn/, the __version__ variable, and in pyproject.toml.

    For major releases, please add a 0 at the end: 0.99.0 instead of 0.99.

    For the first release candidate, use the rc1 suffix on the expected final release number: 0.99.0rc1.

  4. Trigger the wheel builder with the [cd build] commit marker using the command:

    git commit --allow-empty -m "Trigger wheel builder workflow: [cd build]"

    The wheel building workflow is managed by GitHub Actions and the results be browsed at: scikit-learn/scikit-learn


Before building the wheels, make sure that the pyproject.toml file is up to date and using the oldest version of numpy for each Python version to avoid ABI incompatibility issues. Moreover, a new line have to be included in the pyproject.toml file for each new supported version of Python.


The acronym CD in [cd build] stands for Continuous Delivery and refers to the automation used to generate the release artifacts (binary and source packages). This can be seen as an extension to CI which stands for Continuous Integration. The CD workflow on GitHub Actions is also used to automatically create nightly builds and publish packages for the development branch of scikit-learn. See Installing nightly builds.

  1. Once all the CD jobs have completed successfully in the PR, merge it, again with the [cd build] marker in the commit message. This time the results will be uploaded to the staging area.

    You should then be able to upload the generated artifacts (.tar.gz and .whl files) to using the “Run workflow” form for the following GitHub Actions workflow:


  2. If this went fine, you can proceed with tagging. Proceed with caution. Ideally, tags should be created when you’re almost certain that the release is ready, since adding a tag to the main repo can trigger certain automated processes.

    Create the tag and push it (if it’s an RC, it can be 0.xx.0rc1 for instance):

    git tag -a 0.99.0  # in the 0.99.X branch
    git push 0.99.0
  3. Confirm that the bot has detected the tag on the conda-forge feedstock repo: conda-forge/scikit-learn-feedstock. If not, submit a PR for the release. If you want to publish an RC release on conda-forge, the PR should target the rc branch as opposed to the main branch. The two branches need to be kept sync together otherwise.

  4. Trigger the GitHub Actions workflow again but this time to upload the artifacts to the real (replace “testpypi” by “pypi” in the “Run workflow” form).

  5. Alternative to step 7: it’s possible to collect locally the generated binary wheel packages and source tarball and upload them all to PyPI by running the following commands in the scikit-learn source folder (checked out at the release tag):

    rm -r dist
    pip install -U wheelhouse_uploader twine
    python -m wheelhouse_uploader fetch \
      --version 0.99.0 \
      --local-folder dist \
      scikit-learn \

    This command will download all the binary packages accumulated in the staging area on the hosting service and put them in your local ./dist folder.

    Check the content of the ./dist folder: it should contain all the wheels along with the source tarball (“scikit-learn-RRR.tar.gz”).

    Make sure that you do not have developer versions or older versions of the scikit-learn package in that folder.

    Before uploading to pypi, you can test upload to

    twine upload --verbose --repository-url dist/*

    Upload everything at once to

    twine upload dist/*
  6. For major/minor (not bug-fix release or release candidates), update the symlink for stable and the latestStable variable in scikit-learn/

    cd /tmp
    git clone --depth 1 --no-checkout
    echo stable > .git/info/sparse-checkout
    git checkout main
    rm stable
    ln -s 0.999 stable
    sed -i "s/latestStable = '.*/latestStable = '0.999';/" versionwarning.js
    git add stable versionwarning.js
    git commit -m "Update stable to point to 0.999"
    git push origin main
  7. Update to reflect the latest supported version.

Release checklist#

The following GitHub checklist might be helpful in a release PR:

* [ ] update news and what's new date in release branch
* [ ] update news and what's new date and sklearn dev0 version in main branch
* [ ] check that the wheels for the release can be built successfully
* [ ] merge the PR with `[cd build]` commit message to upload wheels to the staging repo
* [ ] upload the wheels and source tarball to
* [ ] create tag on the main github repo
* [ ] confirm bot detected at and wait for merge
* [ ] upload the wheels and source tarball to PyPI
* [ ] publish (except for RC)
* [ ] announce on mailing list and on Twitter, and LinkedIn
* [ ] update symlink for stable in (only major/minor)
* [ ] update in main branch (except for RC)

Merging Pull Requests#

Individual commits are squashed when a Pull Request (PR) is merged on Github. Before merging,

  • the resulting commit title can be edited if necessary. Note that this will rename the PR title by default.

  • the detailed description, containing the titles of all the commits, can be edited or deleted.

  • for PRs with multiple code contributors care must be taken to keep the Co-authored-by: name <> tags in the detailed description. This will mark the PR as having multiple co-authors. Whether code contributions are significantly enough to merit co-authorship is left to the maintainer’s discretion, same as for the “what’s new” entry.

The web site#

The scikit-learn web site ( is hosted at GitHub, but should rarely be updated manually by pushing to the scikit-learn/ repository. Most updates can be made by pushing to master (for /dev) or a release branch like 0.99.X, from which Circle CI builds and uploads the documentation automatically.

Experimental features#

The sklearn.experimental module was introduced in 0.21 and contains experimental features / estimators that are subject to change without deprecation cycle.

To create an experimental module, you can just copy and modify the content of, or


These are permalink as in 0.24, where these estimators are still experimental. They might be stable at the time of reading - hence the permalink. See below for instructions on the transition from experimental to stable.

Note that the public import path must be to a public subpackage (like sklearn/ensemble or sklearn/impute), not just a .py module. Also, the (private) experimental features that are imported must be in a submodule/subpackage of the public subpackage, e.g. sklearn/ensemble/_hist_gradient_boosting/ or sklearn/impute/ This is needed so that pickles still work in the future when the features aren’t experimental anymore.

To avoid type checker (e.g. mypy) errors a direct import of experimental estimators should be done in the parent module, protected by the if typing.TYPE_CHECKING check. See sklearn/ensemble/, or sklearn/impute/ for an example.

Please also write basic tests following those in

Make sure every user-facing code you write explicitly mentions that the feature is experimental, and add a # noqa comment to avoid pep8-related warnings:

# To use this experimental feature, we need to explicitly ask for it:
from sklearn.experimental import enable_hist_gradient_boosting  # noqa
from sklearn.ensemble import HistGradientBoostingRegressor

For the docs to render properly, please also import enable_my_experimental_feature in doc/, else sphinx won’t be able to import the corresponding modules. Note that using from sklearn.experimental import * does not work.

Note that some experimental classes / functions are not included in the sklearn.experimental module: sklearn.datasets.fetch_openml.

Once the feature become stable, remove all enable_my_experimental_feature in the scikit-learn code (even feature highlights etc.) and make the enable_my_experimental_feature a no-op that just raises a warning: The file should stay there indefinitely as we don’t want to break users code: we just incentivize them to remove that import with the warning.

Also update the tests accordingly: