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doc/developers/contributing.rst
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.. _contributing:

============
Contributing
============

.. currentmodule:: sklearn

This project is a community effort, and everyone is welcome to
contribute. It is hosted on https://github.com/scikit-learn/scikit-learn.
The decision making process and governance structure of scikit-learn is laid
out in :ref:`governance`.

Scikit-learn is somewhat :ref:`selective <selectiveness>` when it comes to
adding new algorithms, and the best way to contribute and to help the project
is to start working on known issues.
See :ref:`new_contributors` to get started.

.. topic:: **Our community, our values**

    We are a community based on openness and friendly, didactic,
    discussions.

    We aspire to treat everybody equally, and value their contributions.  We
    are particularly seeking people from underrepresented backgrounds in Open
    Source Software and scikit-learn in particular to participate and
    contribute their expertise and experience.

    Decisions are made based on technical merit and consensus.

    Code is not the only way to help the project. Reviewing pull
    requests, answering questions to help others on mailing lists or
    issues, organizing and teaching tutorials, working on the website,
    improving the documentation, are all priceless contributions.

    We abide by the principles of openness, respect, and consideration of
    others of the Python Software Foundation:
    https://www.python.org/psf/codeofconduct/


In case you experience issues using this package, do not hesitate to submit a
ticket to the
`GitHub issue tracker
<https://github.com/scikit-learn/scikit-learn/issues>`_. You are also
welcome to post feature requests or pull requests.

Ways to contribute
==================

There are many ways to contribute to scikit-learn, with the most common ones
being contribution of code or documentation to the project. Improving the
documentation is no less important than improving the library itself.  If you
find a typo in the documentation, or have made improvements, do not hesitate to
create a GitHub issue or preferably submit a GitHub pull request.
Full documentation can be found under the doc/ directory.

But there are many other ways to help. In particular helping to
:ref:`improve, triage, and investigate issues <bug_triaging>` and
:ref:`reviewing other developers' pull requests <code_review>` are very
valuable contributions that decrease the burden on the project
maintainers.

Another way to contribute is to report issues you're facing, and give a "thumbs
up" on issues that others reported and that are relevant to you.  It also helps
us if you spread the word: reference the project from your blog and articles,
link to it from your website, or simply star to say "I use it":

.. raw:: html

  <p>
    <object
      data="https://img.shields.io/github/stars/scikit-learn/scikit-learn?style=for-the-badge&logo=github"
      type="image/svg+xml">
    </object>
  </p>

In case a contribution/issue involves changes to the API principles
or changes to dependencies or supported versions, it must be backed by a
:ref:`slep`, where a SLEP must be submitted as a pull-request to
`enhancement proposals <https://scikit-learn-enhancement-proposals.readthedocs.io>`_
using the `SLEP template <https://scikit-learn-enhancement-proposals.readthedocs.io/en/latest/slep_template.html>`_
and follows the decision-making process outlined in :ref:`governance`.

.. dropdown:: Contributing to related projects

  Scikit-learn thrives in an ecosystem of several related projects, which also
  may have relevant issues to work on, including smaller projects such as:

  * `scikit-learn-contrib <https://github.com/search?q=org%3Ascikit-learn-contrib+is%3Aissue+is%3Aopen+sort%3Aupdated-desc&type=Issues>`__
  * `joblib <https://github.com/joblib/joblib/issues>`__
  * `sphinx-gallery <https://github.com/sphinx-gallery/sphinx-gallery/issues>`__
  * `numpydoc <https://github.com/numpy/numpydoc/issues>`__
  * `liac-arff <https://github.com/renatopp/liac-arff/issues>`__

  and larger projects:

  * `numpy <https://github.com/numpy/numpy/issues>`__
  * `scipy <https://github.com/scipy/scipy/issues>`__
  * `matplotlib <https://github.com/matplotlib/matplotlib/issues>`__
  * and so on.

  Look for issues marked "help wanted" or similar. Helping these projects may help
  scikit-learn too. See also :ref:`related_projects`.

Automated Contributions Policy
==============================

Please refrain from submitting issues or pull requests generated by
fully-automated tools. Maintainers reserve the right, at their sole discretion,
to close such submissions and to block any account responsible for them.

Ideally, contributions should follow from a human-to-human discussion in the
form of an issue.

Submitting a bug report or a feature request
============================================

We use GitHub issues to track all bugs and feature requests; feel free to open
an issue if you have found a bug or wish to see a feature implemented.

In case you experience issues using this package, do not hesitate to submit a
ticket to the
`Bug Tracker <https://github.com/scikit-learn/scikit-learn/issues>`_. You are
also welcome to post feature requests or pull requests.

It is recommended to check that your issue complies with the
following rules before submitting:

-  Verify that your issue is not being currently addressed by other
   `issues <https://github.com/scikit-learn/scikit-learn/issues?q=>`_
   or `pull requests <https://github.com/scikit-learn/scikit-learn/pulls?q=>`_.

-  If you are submitting an algorithm or feature request, please verify that
   the algorithm fulfills our
   `new algorithm requirements
   <https://scikit-learn.org/stable/faq.html#what-are-the-inclusion-criteria-for-new-algorithms>`_.

-  If you are submitting a bug report, we strongly encourage you to follow the guidelines in
   :ref:`filing_bugs`.

.. _filing_bugs:

How to make a good bug report
-----------------------------

When you submit an issue to `GitHub
<https://github.com/scikit-learn/scikit-learn/issues>`__, please do your best to
follow these guidelines! This will make it a lot easier to provide you with good
feedback:

- The ideal bug report contains a :ref:`short reproducible code snippet
  <minimal_reproducer>`, this way anyone can try to reproduce the bug easily. If your
  snippet is longer than around 50 lines, please link to a `Gist
  <https://gist.github.com>`_ or a GitHub repo.

- If not feasible to include a reproducible snippet, please be specific about
  what **estimators and/or functions are involved and the shape of the data**.

- If an exception is raised, please **provide the full traceback**.

- Please include your **operating system type and version number**, as well as
  your **Python, scikit-learn, numpy, and scipy versions**. This information
  can be found by running:

  .. prompt:: bash

    python -c "import sklearn; sklearn.show_versions()"

- Please ensure all **code snippets and error messages are formatted in
  appropriate code blocks**.  See `Creating and highlighting code blocks
  <https://help.github.com/articles/creating-and-highlighting-code-blocks>`_
  for more details.

If you want to help curate issues, read about :ref:`bug_triaging`.

Contributing code
=================

.. note::

  To avoid duplicating work, it is highly advised that you search through the
  `issue tracker <https://github.com/scikit-learn/scikit-learn/issues>`_ and
  the `PR list <https://github.com/scikit-learn/scikit-learn/pulls>`_.
  If in doubt about duplicated work, or if you want to work on a non-trivial
  feature, it's recommended to first open an issue in
  the `issue tracker <https://github.com/scikit-learn/scikit-learn/issues>`_
  to get some feedback from core developers.

  One easy way to find an issue to work on is by applying the "help wanted"
  label in your search. This lists all the issues that have been unclaimed
  so far. In order to claim an issue for yourself, please comment exactly
  ``/take`` on it for the CI to automatically assign the issue to you.

To maintain the quality of the codebase and ease the review process, any
contribution must conform to the project's :ref:`coding guidelines
<coding-guidelines>`, in particular:

- Don't modify unrelated lines to keep the PR focused on the scope stated in its
  description or issue.
- Only write inline comments that add value and avoid stating the obvious: explain
  the "why" rather than the "what".
- **Most importantly**: Do not contribute code that you don't understand.

Video resources
---------------
These videos are step-by-step introductions on how to contribute to
scikit-learn, and are a great companion to the following text guidelines.
Please make sure to still check our guidelines below, since they describe our
latest up-to-date workflow.

- Crash Course in Contributing to Scikit-Learn & Open Source Projects:
  `Video <https://youtu.be/5OL8XoMMOfA>`__,
  `Transcript
  <https://github.com/data-umbrella/event-transcripts/blob/main/2020/05-andreas-mueller-contributing.md>`__

- Example of Submitting a Pull Request to scikit-learn:
  `Video <https://youtu.be/PU1WyDPGePI>`__,
  `Transcript
  <https://github.com/data-umbrella/event-transcripts/blob/main/2020/06-reshama-shaikh-sklearn-pr.md>`__

- Sprint-specific instructions and practical tips:
  `Video <https://youtu.be/p_2Uw2BxdhA>`__,
  `Transcript
  <https://github.com/data-umbrella/data-umbrella-scikit-learn-sprint/blob/master/3_transcript_ACM_video_vol2.md>`__

- 3 Components of Reviewing a Pull Request:
  `Video <https://youtu.be/dyxS9KKCNzA>`__,
  `Transcript
  <https://github.com/data-umbrella/event-transcripts/blob/main/2021/27-thomas-pr.md>`__

.. note::
  In January 2021, the default branch name changed from ``master`` to ``main``
  for the scikit-learn GitHub repository to use more inclusive terms.
  These videos were created prior to the renaming of the branch.
  For contributors who are viewing these videos to set up their
  working environment and submitting a PR, ``master`` should be replaced to ``main``.

How to contribute
-----------------

The preferred way to contribute to scikit-learn is to fork the `main
repository <https://github.com/scikit-learn/scikit-learn/>`__ on GitHub,
then submit a "pull request" (PR).

In the first few steps, we explain how to locally install scikit-learn, and
how to set up your git repository:

1. `Create an account <https://github.com/join>`_ on
   GitHub if you do not already have one.

2. Fork the `project repository
   <https://github.com/scikit-learn/scikit-learn>`__: click on the 'Fork'
   button near the top of the page. This creates a copy of the code under your
   account on the GitHub user account. For more details on how to fork a
   repository see `this guide <https://help.github.com/articles/fork-a-repo/>`_.

3. Clone your fork of the scikit-learn repo from your GitHub account to your
   local disk:

   .. prompt:: bash

      git clone git@github.com:YourLogin/scikit-learn.git  # add --depth 1 if your connection is slow
      cd scikit-learn

4. Follow steps 2-6 in :ref:`install_bleeding_edge` to build scikit-learn in
   development mode and return to this document.

5. Install the development dependencies:

   .. prompt:: bash

        pip install pytest pytest-cov ruff==0.11.2 mypy numpydoc

.. _upstream:

6. Add the ``upstream`` remote. This saves a reference to the main
   scikit-learn repository, which you can use to keep your repository
   synchronized with the latest changes:

   .. prompt:: bash

        git remote add upstream git@github.com:scikit-learn/scikit-learn.git

7. Check that the `upstream` and `origin` remote aliases are configured correctly
   by running:

   .. prompt:: bash

        git remote -v

   This should display:

   .. code-block:: text

        origin    git@github.com:YourLogin/scikit-learn.git (fetch)
        origin    git@github.com:YourLogin/scikit-learn.git (push)
        upstream  git@github.com:scikit-learn/scikit-learn.git (fetch)
        upstream  git@github.com:scikit-learn/scikit-learn.git (push)

You should now have a working installation of scikit-learn, and your git repository
properly configured. It could be useful to run some test to verify your installation.
Please refer to :ref:`pytest_tips` for examples.

The next steps now describe the process of modifying code and submitting a PR:

8. Synchronize your ``main`` branch with the ``upstream/main`` branch,
   more details on `GitHub Docs <https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/syncing-a-fork>`_:

   .. prompt:: bash

        git checkout main
        git fetch upstream
        git merge upstream/main

9. Create a feature branch to hold your development changes:

   .. prompt:: bash

        git checkout -b my_feature

   and start making changes. Always use a feature branch. It's good
   practice to never work on the ``main`` branch!

10. (**Optional**) Install `pre-commit <https://pre-commit.com/#install>`_ to
    run code style checks before each commit:

    .. prompt:: bash

          pip install pre-commit
          pre-commit install

    pre-commit checks can be disabled for a particular commit with
    `git commit -n`.

11. Develop the feature on your feature branch on your computer, using Git to
    do the version control. When you're done editing, add changed files using
    ``git add`` and then ``git commit``:

    .. prompt:: bash

        git add modified_files
        git commit

    to record your changes in Git, then push the changes to your GitHub
    account with:

    .. prompt:: bash

       git push -u origin my_feature

12. Follow `these
    <https://help.github.com/articles/creating-a-pull-request-from-a-fork>`_
    instructions to create a pull request from your fork. This will send a
    notification to potential reviewers. You may want to consider sending a message to
    the `discord <https://discord.com/invite/h9qyrK8Jc8>`_ in the development
    channel for more visibility if your pull request does not receive attention after
    a couple of days (instant replies are not guaranteed though).

It is often helpful to keep your local feature branch synchronized with the
latest changes of the main scikit-learn repository:

.. prompt:: bash

    git fetch upstream
    git merge upstream/main

Subsequently, you might need to solve the conflicts. You can refer to the
`Git documentation related to resolving merge conflict using the command
line
<https://help.github.com/articles/resolving-a-merge-conflict-using-the-command-line/>`_.

.. topic:: Learning Git

    The `Git documentation <https://git-scm.com/doc>`_ and
    http://try.github.io are excellent resources to get started with git,
    and understanding all of the commands shown here.

.. _pr_checklist:

Pull request checklist
----------------------

Before a PR can be merged, it needs to be approved by two core developers.
An incomplete contribution -- where you expect to do more work before receiving
a full review -- should be marked as a `draft pull request
<https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/changing-the-stage-of-a-pull-request>`__
and changed to "ready for review" when it matures. Draft PRs may be useful to:
indicate you are working on something to avoid duplicated work, request
broad review of functionality or API, or seek collaborators. Draft PRs often
benefit from the inclusion of a `task list
<https://github.com/blog/1375-task-lists-in-gfm-issues-pulls-comments>`_ in
the PR description.

In order to ease the reviewing process, we recommend that your contribution
complies with the following rules before marking a PR as "ready for review". The
**bolded** ones are especially important:

1. **Give your pull request a helpful title** that summarizes what your
   contribution does. This title will often become the commit message once
   merged so it should summarize your contribution for posterity. In some
   cases "Fix <ISSUE TITLE>" is enough. "Fix #<ISSUE NUMBER>" is never a
   good title.

2. **Make sure your code passes the tests**. The whole test suite can be run
   with `pytest`, but it is usually not recommended since it takes a long
   time. It is often enough to only run the test related to your changes:
   for example, if you changed something in
   `sklearn/linear_model/_logistic.py`, running the following commands will
   usually be enough:

   - `pytest sklearn/linear_model/_logistic.py` to make sure the doctest
     examples are correct
   - `pytest sklearn/linear_model/tests/test_logistic.py` to run the tests
     specific to the file
   - `pytest sklearn/linear_model` to test the whole
     :mod:`~sklearn.linear_model` module
   - `pytest doc/modules/linear_model.rst` to make sure the user guide
     examples are correct.
   - `pytest sklearn/tests/test_common.py -k LogisticRegression` to run all our
     estimator checks (specifically for `LogisticRegression`, if that's the
     estimator you changed).

   There may be other failing tests, but they will be caught by the CI so
   you don't need to run the whole test suite locally. For guidelines on how
   to use ``pytest`` efficiently, see the :ref:`pytest_tips`.

3. **Make sure your code is properly commented and documented**, and **make
   sure the documentation renders properly**. To build the documentation, please
   refer to our :ref:`contribute_documentation` guidelines. The CI will also
   build the docs: please refer to :ref:`generated_doc_CI`.

4. **Tests are necessary for enhancements to be
   accepted**. Bug-fixes or new features should be provided with
   `non-regression tests
   <https://en.wikipedia.org/wiki/Non-regression_testing>`_. These tests
   verify the correct behavior of the fix or feature. In this manner, further
   modifications on the code base are granted to be consistent with the
   desired behavior. In the case of bug fixes, at the time of the PR, the
   non-regression tests should fail for the code base in the ``main`` branch
   and pass for the PR code.

5. If your PR is likely to affect users, you need to add a changelog entry describing
   your PR changes. See the
   `README <https://github.com/scikit-learn/scikit-learn/blob/main/doc/whats_new/upcoming_changes/README.md>`_
   for more details.

6. Follow the :ref:`coding-guidelines`.

7. When applicable, use the validation tools and scripts in the :mod:`sklearn.utils`
   module. A list of utility routines available for developers can be found in the
   :ref:`developers-utils` page.

8. Often pull requests resolve one or more other issues (or pull requests).
   If merging your pull request means that some other issues/PRs should
   be closed, you should `use keywords to create link to them
   <https://github.com/blog/1506-closing-issues-via-pull-requests/>`_
   (e.g., ``Fixes #1234``; multiple issues/PRs are allowed as long as each
   one is preceded by a keyword). Upon merging, those issues/PRs will
   automatically be closed by GitHub. If your pull request is simply
   related to some other issues/PRs, or it only partially resolves the target
   issue, create a link to them without using the keywords (e.g., ``Towards #1234``).

9. PRs should often substantiate the change, through benchmarks of
   performance and efficiency (see :ref:`monitoring_performances`) or through
   examples of usage. Examples also illustrate the features and intricacies of
   the library to users. Have a look at other examples in the `examples/
   <https://github.com/scikit-learn/scikit-learn/tree/main/examples>`_
   directory for reference. Examples should demonstrate why the new
   functionality is useful in practice and, if possible, compare it to other
   methods available in scikit-learn.

10. New features have some maintenance overhead. We expect PR authors
    to take part in the maintenance for the code they submit, at least
    initially. New features need to be illustrated with narrative
    documentation in the user guide, with small code snippets.
    If relevant, please also add references in the literature, with PDF links
    when possible.

11. The user guide should also include expected time and space complexity
    of the algorithm and scalability, e.g. "this algorithm can scale to a
    large number of samples > 100000, but does not scale in dimensionality:
    `n_features` is expected to be lower than 100".

You can also check our :ref:`code_review` to get an idea of what reviewers
will expect.

You can check for common programming errors with the following tools:

* Code with a good unit test coverage (at least 80%, better 100%), check with:

  .. prompt:: bash

    pip install pytest pytest-cov
    pytest --cov sklearn path/to/tests

  See also :ref:`testing_coverage`.

* Run static analysis with `mypy`:

  .. prompt:: bash

      mypy sklearn

  This must not produce new errors in your pull request. Using `# type: ignore`
  annotation can be a workaround for a few cases that are not supported by
  mypy, in particular,

  - when importing C or Cython modules,
  - on properties with decorators.

Bonus points for contributions that include a performance analysis with
a benchmark script and profiling output (see :ref:`monitoring_performances`).
Also check out the :ref:`performance-howto` guide for more details on
profiling and Cython optimizations.

.. note::

  The current state of the scikit-learn code base is not compliant with
  all of those guidelines, but we expect that enforcing those constraints
  on all new contributions will get the overall code base quality in the
  right direction.

.. seealso::

   For two very well documented and more detailed guides on development
   workflow, please pay a visit to the `Scipy Development Workflow
   <http://scipy.github.io/devdocs/dev/dev_quickstart.html>`_ -
   and the `Astropy Workflow for Developers
   <https://astropy.readthedocs.io/en/latest/development/workflow/development_workflow.html>`_
   sections.

Continuous Integration (CI)
---------------------------

* Azure pipelines are used for testing scikit-learn on Linux, Mac and Windows,
  with different dependencies and settings.
* CircleCI is used to build the docs for viewing.
* Github Actions are used for various tasks, including building wheels and
  source distributions.

.. _commit_markers:

Commit message markers
^^^^^^^^^^^^^^^^^^^^^^

Please note that if one of the following markers appears in the latest commit
message, the following actions are taken.

====================== ===================
Commit Message Marker  Action Taken by CI
====================== ===================
[ci skip]              CI is skipped completely
[cd build]             CD is run (wheels and source distribution are built)
[lint skip]            Azure pipeline skips linting
[scipy-dev]            Build & test with our dependencies (numpy, scipy, etc.) development builds
[free-threaded]        Build & test with CPython 3.13 free-threaded
[pyodide]              Build & test with Pyodide
[azure parallel]       Run Azure CI jobs in parallel
[float32]              Run float32 tests by setting `SKLEARN_RUN_FLOAT32_TESTS=1`. See :ref:`environment_variable` for more details
[doc skip]             Docs are not built
[doc quick]            Docs built, but excludes example gallery plots
[doc build]            Docs built including example gallery plots (very long)
====================== ===================

Note that, by default, the documentation is built but only the examples
that are directly modified by the pull request are executed.

Resolve conflicts in lock files
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Here is a bash snippet that helps resolving conflicts in environment and lock files:

.. prompt:: bash

  # pull latest upstream/main
  git pull upstream main --no-rebase
  # resolve conflicts - keeping the upstream/main version for specific files
  git checkout --theirs  build_tools/*/*.lock build_tools/*/*environment.yml \
      build_tools/*/*lock.txt build_tools/*/*requirements.txt
  git add build_tools/*/*.lock build_tools/*/*environment.yml \
      build_tools/*/*lock.txt build_tools/*/*requirements.txt
  git merge --continue

This will merge `upstream/main` into our branch, automatically prioritising the
`upstream/main` for conflicting environment and lock files (this is good enough, because
we will re-generate the lock files afterwards).

Note that this only fixes conflicts in environment and lock files and you might have
other conflicts to resolve.

Finally, we have to re-generate the environment and lock files for the CIs by running:

.. prompt:: bash

  python build_tools/update_environments_and_lock_files.py

.. _stalled_pull_request:

Stalled pull requests
---------------------

As contributing a feature can be a lengthy process, some
pull requests appear inactive but unfinished. In such a case, taking
them over is a great service for the project. A good etiquette to take over is:

* **Determine if a PR is stalled**

  * A pull request may have the label "stalled" or "help wanted" if we
    have already identified it as a candidate for other contributors.

  * To decide whether an inactive PR is stalled, ask the contributor if
    she/he plans to continue working on the PR in the near future.
    Failure to respond within 2 weeks with an activity that moves the PR
    forward suggests that the PR is stalled and will result in tagging
    that PR with "help wanted".

    Note that if a PR has received earlier comments on the contribution
    that have had no reply in a month, it is safe to assume that the PR
    is stalled and to shorten the wait time to one day.

    After a sprint, follow-up for un-merged PRs opened during sprint will
    be communicated to participants at the sprint, and those PRs will be
    tagged "sprint". PRs tagged with "sprint" can be reassigned or
    declared stalled by sprint leaders.

* **Taking over a stalled PR**: To take over a PR, it is important to
  comment on the stalled PR that you are taking over and to link from the
  new PR to the old one. The new PR should be created by pulling from the
  old one.

Stalled and Unclaimed Issues
----------------------------

Generally speaking, issues which are up for grabs will have a
`"help wanted" <https://github.com/scikit-learn/scikit-learn/labels/help%20wanted>`_.
tag. However, not all issues which need contributors will have this tag,
as the "help wanted" tag is not always up-to-date with the state
of the issue. Contributors can find issues which are still up for grabs
using the following guidelines:

* First, to **determine if an issue is claimed**:

  * Check for linked pull requests
  * Check the conversation to see if anyone has said that they're working on
    creating a pull request

* If a contributor comments on an issue to say they are working on it,
  a pull request is expected within 2 weeks (new contributor) or 4 weeks
  (contributor or core dev), unless a larger time frame is explicitly given.
  Beyond that time, another contributor can take the issue and make a
  pull request for it. We encourage contributors to comment directly on the
  stalled or unclaimed issue to let community members know that they will be
  working on it.

* If the issue is linked to a :ref:`stalled pull request <stalled_pull_request>`,
  we recommend that contributors follow the procedure
  described in the :ref:`stalled_pull_request`
  section rather than working directly on the issue.

.. _new_contributors:

Issues for New Contributors
---------------------------

New contributors should look for the following tags when looking for issues.  We
strongly recommend that new contributors tackle "easy" issues first: this helps
the contributor become familiar with the contribution workflow, and for the core
devs to become acquainted with the contributor; besides which, we frequently
underestimate how easy an issue is to solve!

- **Good first issue tag**

  A great way to start contributing to scikit-learn is to pick an item from
  the list of `good first issues
  <https://github.com/scikit-learn/scikit-learn/labels/good%20first%20issue>`_
  in the issue tracker. Resolving these issues allows you to start contributing
  to the project without much prior knowledge. If you have already contributed
  to scikit-learn, you should look at Easy issues instead.

- **Easy tag**

  If you have already contributed to scikit-learn, another great way to contribute
  to scikit-learn is to pick an item from the list of `Easy issues
  <https://github.com/scikit-learn/scikit-learn/labels/Easy>`_ in the issue
  tracker. Your assistance in this area will be greatly appreciated by the
  more experienced developers as it helps free up their time to concentrate on
  other issues.

- **Help wanted tag**

  We often use the help wanted tag to mark issues regardless of difficulty.
  Additionally, we use the help wanted tag to mark Pull Requests which have been
  abandoned by their original contributor and are available for someone to pick up where
  the original contributor left off. The list of issues with the help wanted tag can be
  found `here <https://github.com/scikit-learn/scikit-learn/labels/help%20wanted>`_.
  Note that not all issues which need contributors will have this tag.

.. _contribute_documentation:

Documentation
=============

We are glad to accept any sort of documentation:

* **Function/method/class docstrings:** Also known as "API documentation", these
  describe what the object does and detail any parameters, attributes and
  methods. Docstrings live alongside the code in `sklearn/
  <https://github.com/scikit-learn/scikit-learn/tree/main/sklearn>`_, and are
  generated according to `doc/api_reference.py
  <https://github.com/scikit-learn/scikit-learn/blob/main/doc/api_reference.py>`_. To
  add, update, remove, or deprecate a public API that is listed in :ref:`api_ref`, this
  is the place to look at.
* **User guide:** These provide more detailed information about the algorithms
  implemented in scikit-learn and generally live in the root
  `doc/ <https://github.com/scikit-learn/scikit-learn/tree/main/doc>`_ directory
  and
  `doc/modules/ <https://github.com/scikit-learn/scikit-learn/tree/main/doc/modules>`_.
* **Examples:** These provide full code examples that may demonstrate the use
  of scikit-learn modules, compare different algorithms or discuss their
  interpretation, etc. Examples live in
  `examples/ <https://github.com/scikit-learn/scikit-learn/tree/main/examples>`_.
* **Other reStructuredText documents:** These provide various other useful information
  (e.g., the :ref:`contributing` guide) and live in
  `doc/ <https://github.com/scikit-learn/scikit-learn/tree/main/doc>`_.


.. dropdown:: Guidelines for writing docstrings

  * You can use `pytest` to test docstrings, e.g. assuming the
    `RandomForestClassifier` docstring has been modified, the following command
    would test its docstring compliance:

    .. prompt:: bash

      pytest --doctest-modules sklearn/ensemble/_forest.py -k RandomForestClassifier

  * The correct order of sections is: Parameters, Returns, See Also, Notes, Examples.
    See the `numpydoc documentation
    <https://numpydoc.readthedocs.io/en/latest/format.html#sections>`_ for
    information on other possible sections.

  * When documenting the parameters and attributes, here is a list of some
    well-formatted examples

    .. code-block:: text

      n_clusters : int, default=3
          The number of clusters detected by the algorithm.

      some_param : {"hello", "goodbye"}, bool or int, default=True
          The parameter description goes here, which can be either a string
          literal (either `hello` or `goodbye`), a bool, or an int. The default
          value is True.

      array_parameter : {array-like, sparse matrix} of shape (n_samples, n_features) \
          or (n_samples,)
          This parameter accepts data in either of the mentioned forms, with one
          of the mentioned shapes. The default value is `np.ones(shape=(n_samples,))`.

      list_param : list of int

      typed_ndarray : ndarray of shape (n_samples,), dtype=np.int32

      sample_weight : array-like of shape (n_samples,), default=None

      multioutput_array : ndarray of shape (n_samples, n_classes) or list of such arrays

    In general have the following in mind:

    * Use Python basic types. (``bool`` instead of ``boolean``)
    * Use parenthesis for defining shapes: ``array-like of shape (n_samples,)``
      or ``array-like of shape (n_samples, n_features)``
    * For strings with multiple options, use brackets: ``input: {'log',
      'squared', 'multinomial'}``
    * 1D or 2D data can be a subset of ``{array-like, ndarray, sparse matrix,
      dataframe}``. Note that ``array-like`` can also be a ``list``, while
      ``ndarray`` is explicitly only a ``numpy.ndarray``.
    * Specify ``dataframe`` when "frame-like" features are being used, such as
      the column names.
    * When specifying the data type of a list, use ``of`` as a delimiter: ``list
      of int``. When the parameter supports arrays giving details about the
      shape and/or data type and a list of such arrays, you can use one of
      ``array-like of shape (n_samples,) or list of such arrays``.
    * When specifying the dtype of an ndarray, use e.g. ``dtype=np.int32`` after
      defining the shape: ``ndarray of shape (n_samples,), dtype=np.int32``. You
      can specify multiple dtype as a set: ``array-like of shape (n_samples,),
      dtype={np.float64, np.float32}``. If one wants to mention arbitrary
      precision, use `integral` and `floating` rather than the Python dtype
      `int` and `float`. When both `int` and `floating` are supported, there is
      no need to specify the dtype.
    * When the default is ``None``, ``None`` only needs to be specified at the
      end with ``default=None``. Be sure to include in the docstring, what it
      means for the parameter or attribute to be ``None``.

  * Add "See Also" in docstrings for related classes/functions.

  * "See Also" in docstrings should be one line per reference, with a colon and an
    explanation, for example:

    .. code-block:: text

      See Also
      --------
      SelectKBest : Select features based on the k highest scores.
      SelectFpr : Select features based on a false positive rate test.

  * The "Notes" section is optional. It is meant to provide information on
    specific behavior of a function/class/classmethod/method.

  * A `Note` can also be added to an attribute, but in that case it requires
    using the `.. rubric:: Note` directive.

  * Add one or two **snippets** of code in "Example" section to show how it can
    be used. The code should be runable as is, i.e. it should include all
    required imports. Keep this section as brief as possible.


.. dropdown:: Guidelines for writing the user guide and other reStructuredText documents

  It is important to keep a good compromise between mathematical and algorithmic
  details, and give intuition to the reader on what the algorithm does.

  * Begin with a concise, hand-waving explanation of what the algorithm/code does on
    the data.

  * Highlight the usefulness of the feature and its recommended application.
    Consider including the algorithm's complexity
    (:math:`O\left(g\left(n\right)\right)`) if available, as "rules of thumb" can
    be very machine-dependent. Only if those complexities are not available, then
    rules of thumb may be provided instead.

  * Incorporate a relevant figure (generated from an example) to provide intuitions.

  * Include one or two short code examples to demonstrate the feature's usage.

  * Introduce any necessary mathematical equations, followed by references. By
    deferring the mathematical aspects, the documentation becomes more accessible
    to users primarily interested in understanding the feature's practical
    implications rather than its underlying mechanics.

  * When editing reStructuredText (``.rst``) files, try to keep line length under
    88 characters when possible (exceptions include links and tables).

  * In scikit-learn reStructuredText files both single and double backticks
    surrounding text will render as inline literal (often used for code, e.g.,
    `list`). This is due to specific configurations we have set. Single
    backticks should be used nowadays.

  * Too much information makes it difficult for users to access the content they
    are interested in. Use dropdowns to factorize it by using the following syntax

    .. code-block:: rst

      .. dropdown:: Dropdown title

        Dropdown content.

    The snippet above will result in the following dropdown:

    .. dropdown:: Dropdown title

      Dropdown content.

  * Information that can be hidden by default using dropdowns is:

    * low hierarchy sections such as `References`, `Properties`, etc. (see for
      instance the subsections in :ref:`det_curve`);

    * in-depth mathematical details;

    * narrative that is use-case specific;

    * in general, narrative that may only interest users that want to go beyond
      the pragmatics of a given tool.

  * Do not use dropdowns for the low level section `Examples`, as it should stay
    visible to all users. Make sure that the `Examples` section comes right after
    the main discussion with the least possible folded section in-between.

  * Be aware that dropdowns break cross-references. If that makes sense, hide the
    reference along with the text mentioning it. Else, do not use dropdown.


.. dropdown:: Guidelines for writing references

  * When bibliographic references are available with `arxiv <https://arxiv.org/>`_
    or `Digital Object Identifier <https://www.doi.org/>`_ identification numbers,
    use the sphinx directives `:arxiv:` or `:doi:`. For example, see references in
    :ref:`Spectral Clustering Graphs <spectral_clustering_graph>`.

  * For the "References" section in docstrings, see
    :func:`sklearn.metrics.silhouette_score` as an example.

  * To cross-reference to other pages in the scikit-learn documentation use the
    reStructuredText cross-referencing syntax:

    * **Section:** to link to an arbitrary section in the documentation, use
      reference labels (see `Sphinx docs
      <https://www.sphinx-doc.org/en/master/usage/restructuredtext/roles.html#ref-role>`_).
      For example:

      .. code-block:: rst

          .. _my-section:

          My section
          ----------

          This is the text of the section.

          To refer to itself use :ref:`my-section`.

      You should not modify existing sphinx reference labels as this would break
      existing cross references and external links pointing to specific sections
      in the scikit-learn documentation.

    * **Glossary:** linking to a term in the :ref:`glossary`:

      .. code-block:: rst

          :term:`cross_validation`

    * **Function:** to link to the documentation of a function, use the full import
      path to the function:

      .. code-block:: rst

          :func:`~sklearn.model_selection.cross_val_score`

      However, if there is a `.. currentmodule::` directive above you in the document,
      you will only need to use the path to the function succeeding the current
      module specified. For example:

      .. code-block:: rst

          .. currentmodule:: sklearn.model_selection

          :func:`cross_val_score`

    * **Class:** to link to documentation of a class, use the full import path to the
      class, unless there is a `.. currentmodule::` directive in the document above
      (see above):

      .. code-block:: rst

          :class:`~sklearn.preprocessing.StandardScaler`

You can edit the documentation using any text editor, and then generate the
HTML output by following :ref:`building_documentation`. The resulting HTML files
will be placed in ``_build/html/`` and are viewable in a web browser, for instance by
opening the local ``_build/html/index.html`` file or by running a local server

.. prompt:: bash

  python -m http.server -d _build/html


.. _building_documentation:

Building the documentation
--------------------------

**Before submitting a pull request check if your modifications have introduced
new sphinx warnings by building the documentation locally and try to fix them.**

First, make sure you have :ref:`properly installed <install_bleeding_edge>` the
development version. On top of that, building the documentation requires installing some
additional packages:

..
    packaging is not needed once setuptools starts shipping packaging>=17.0

.. prompt:: bash

    pip install sphinx sphinx-gallery numpydoc matplotlib Pillow pandas \
                polars scikit-image packaging seaborn sphinx-prompt \
                sphinxext-opengraph sphinx-copybutton plotly pooch \
                pydata-sphinx-theme sphinxcontrib-sass sphinx-design \
                sphinx-remove-toctrees

To build the documentation, you need to be in the ``doc`` folder:

.. prompt:: bash

    cd doc

In the vast majority of cases, you only need to generate the web site without
the example gallery:

.. prompt:: bash

    make

The documentation will be generated in the ``_build/html/stable`` directory
and are viewable in a web browser, for instance by opening the local
``_build/html/stable/index.html`` file.
To also generate the example gallery you can use:

.. prompt:: bash

    make html

This will run all the examples, which takes a while. You can also run only a few examples based on their file names.
Here is a way to run all examples with filenames containing `plot_calibration`:

.. prompt:: bash

    EXAMPLES_PATTERN="plot_calibration" make html

You can use regular expressions for more advanced use cases.

Set the environment variable `NO_MATHJAX=1` if you intend to view the documentation in
an offline setting. To build the PDF manual, run:

.. prompt:: bash

    make latexpdf

.. admonition:: Sphinx version
   :class: warning

   While we do our best to have the documentation build under as many
   versions of Sphinx as possible, the different versions tend to
   behave slightly differently. To get the best results, you should
   use the same version as the one we used on CircleCI. Look at this
   `GitHub search <https://github.com/search?q=repo%3Ascikit-learn%2Fscikit-learn+%2F%5C%2Fsphinx-%5B0-9.%5D%2B%2F+path%3Abuild_tools%2Fcircle%2Fdoc_linux-64_conda.lock&type=code>`_
   to know the exact version.


.. _generated_doc_CI:

Generated documentation on GitHub Actions
-----------------------------------------

When you change the documentation in a pull request, GitHub Actions automatically
builds it. To view the documentation generated by GitHub Actions, simply go to the
bottom of your PR page, look for the item "Check the rendered docs here!" and
click on 'details' next to it:

.. image:: ../images/generated-doc-ci.png
   :align: center

.. _testing_coverage:

Testing and improving test coverage
===================================

High-quality `unit testing <https://en.wikipedia.org/wiki/Unit_testing>`_
is a corner-stone of the scikit-learn development process. For this
purpose, we use the `pytest <https://docs.pytest.org>`_
package. The tests are functions appropriately named, located in `tests`
subdirectories, that check the validity of the algorithms and the
different options of the code.

Running `pytest` in a folder will run all the tests of the corresponding
subpackages. For a more detailed `pytest` workflow, please refer to the
:ref:`pr_checklist`.

We expect code coverage of new features to be at least around 90%.

.. dropdown:: Writing matplotlib-related tests

  Test fixtures ensure that a set of tests will be executing with the appropriate
  initialization and cleanup. The scikit-learn test suite implements a ``pyplot``
  fixture which can be used with ``matplotlib``.

  The ``pyplot`` fixture should be used when a test function is dealing with
  ``matplotlib``. ``matplotlib`` is a soft dependency and is not required.
  This fixture is in charge of skipping the tests if ``matplotlib`` is not
  installed. In addition, figures created during the tests will be
  automatically closed once the test function has been executed.

  To use this fixture in a test function, one needs to pass it as an
  argument::

      def test_requiring_mpl_fixture(pyplot):
          # you can now safely use matplotlib

.. dropdown:: Workflow to improve test coverage

  To test code coverage, you need to install the `coverage
  <https://pypi.org/project/coverage/>`_ package in addition to `pytest`.

  1. Run `pytest --cov sklearn /path/to/tests`. The output lists for each file the line
     numbers that are not tested.

  2. Find a low hanging fruit, looking at which lines are not tested,
     write or adapt a test specifically for these lines.

  3. Loop.

.. _monitoring_performances:

Monitoring performance
======================

*This section is heavily inspired from the* `pandas documentation
<https://pandas.pydata.org/docs/development/contributing_codebase.html#running-the-performance-test-suite>`_.

When proposing changes to the existing code base, it's important to make sure
that they don't introduce performance regressions. Scikit-learn uses
`asv benchmarks <https://github.com/airspeed-velocity/asv>`_ to monitor the
performance of a selection of common estimators and functions. You can view
these benchmarks on the `scikit-learn benchmark page
<https://scikit-learn.org/scikit-learn-benchmarks>`_.
The corresponding benchmark suite can be found in the `asv_benchmarks/` directory.

To use all features of asv, you will need either `conda` or `virtualenv`. For
more details please check the `asv installation webpage
<https://asv.readthedocs.io/en/latest/installing.html>`_.

First of all you need to install the development version of asv:

.. prompt:: bash

    pip install git+https://github.com/airspeed-velocity/asv

and change your directory to `asv_benchmarks/`:

.. prompt:: bash

  cd asv_benchmarks

The benchmark suite is configured to run against your local clone of
scikit-learn. Make sure it is up to date:

.. prompt:: bash

  git fetch upstream

In the benchmark suite, the benchmarks are organized following the same
structure as scikit-learn. For example, you can compare the performance of a
specific estimator between ``upstream/main`` and the branch you are working on:

.. prompt:: bash

  asv continuous -b LogisticRegression upstream/main HEAD

The command uses conda by default for creating the benchmark environments. If
you want to use virtualenv instead, use the `-E` flag:

.. prompt:: bash

  asv continuous -E virtualenv -b LogisticRegression upstream/main HEAD

You can also specify a whole module to benchmark:

.. prompt:: bash

  asv continuous -b linear_model upstream/main HEAD

You can replace `HEAD` by any local branch. By default it will only report the
benchmarks that have changed by at least 10%. You can control this ratio with
the `-f` flag.

To run the full benchmark suite, simply remove the `-b` flag :

.. prompt:: bash

  asv continuous upstream/main HEAD

However this can take up to two hours. The `-b` flag also accepts a regular
expression for a more complex subset of benchmarks to run.

To run the benchmarks without comparing to another branch, use the `run`
command:

.. prompt:: bash

  asv run -b linear_model HEAD^!

You can also run the benchmark suite using the version of scikit-learn already
installed in your current Python environment:

.. prompt:: bash

  asv run --python=same

It's particularly useful when you installed scikit-learn in editable mode to
avoid creating a new environment each time you run the benchmarks. By default
the results are not saved when using an existing installation. To save the
results you must specify a commit hash:

.. prompt:: bash

  asv run --python=same --set-commit-hash=<commit hash>

Benchmarks are saved and organized by machine, environment and commit. To see
the list of all saved benchmarks:

.. prompt:: bash

  asv show

and to see the report of a specific run:

.. prompt:: bash

  asv show <commit hash>

When running benchmarks for a pull request you're working on please report the
results on github.

The benchmark suite supports additional configurable options which can be set
in the `benchmarks/config.json` configuration file. For example, the benchmarks
can run for a provided list of values for the `n_jobs` parameter.

More information on how to write a benchmark and how to use asv can be found in
the `asv documentation <https://asv.readthedocs.io/en/latest/index.html>`_.

.. _issue_tracker_tags:

Issue Tracker Tags
==================

All issues and pull requests on the
`GitHub issue tracker <https://github.com/scikit-learn/scikit-learn/issues>`_
should have (at least) one of the following tags:

:Bug:
    Something is happening that clearly shouldn't happen.
    Wrong results as well as unexpected errors from estimators go here.

:Enhancement:
    Improving performance, usability, consistency.

:Documentation:
    Missing, incorrect or sub-standard documentations and examples.

:New Feature:
    Feature requests and pull requests implementing a new feature.

There are four other tags to help new contributors:

:Good first issue:
    This issue is ideal for a first contribution to scikit-learn. Ask for help
    if the formulation is unclear. If you have already contributed to
    scikit-learn, look at Easy issues instead.

:Easy:
    This issue can be tackled without much prior experience.

:Moderate:
    Might need some knowledge of machine learning or the package,
    but is still approachable for someone new to the project.

:Help wanted:
    This tag marks an issue which currently lacks a contributor or a
    PR that needs another contributor to take over the work. These
    issues can range in difficulty, and may not be approachable
    for new contributors. Note that not all issues which need
    contributors will have this tag.

.. _backwards-compatibility:

Maintaining backwards compatibility
===================================

.. _contributing_deprecation:

Deprecation
-----------

If any publicly accessible class, function, method, attribute or parameter is renamed,
we still support the old one for two releases and issue a deprecation warning when it is
called, passed, or accessed.

.. rubric:: Deprecating a class or a function

Suppose the function ``zero_one`` is renamed to ``zero_one_loss``, we add the decorator
:class:`utils.deprecated` to ``zero_one`` and call ``zero_one_loss`` from that
function::

    from ..utils import deprecated

    def zero_one_loss(y_true, y_pred, normalize=True):
        # actual implementation
        pass

    @deprecated(
        "Function `zero_one` was renamed to `zero_one_loss` in 0.13 and will be "
        "removed in 0.15. Default behavior is changed from `normalize=False` to "
        "`normalize=True`"
    )
    def zero_one(y_true, y_pred, normalize=False):
        return zero_one_loss(y_true, y_pred, normalize)

One also needs to move ``zero_one`` from ``API_REFERENCE`` to
``DEPRECATED_API_REFERENCE`` and add ``zero_one_loss`` to ``API_REFERENCE`` in the
``doc/api_reference.py`` file to reflect the changes in :ref:`api_ref`.

.. rubric:: Deprecating an attribute or a method

If an attribute or a method is to be deprecated, use the decorator
:class:`~utils.deprecated` on the property. Please note that the
:class:`~utils.deprecated` decorator should be placed before the ``property`` decorator
if there is one, so that the docstrings can be rendered properly. For instance, renaming
an attribute ``labels_`` to ``classes_`` can be done as::

    @deprecated(
        "Attribute `labels_` was deprecated in 0.13 and will be removed in 0.15. Use "
        "`classes_` instead"
    )
    @property
    def labels_(self):
        return self.classes_

.. rubric:: Deprecating a parameter

If a parameter has to be deprecated, a ``FutureWarning`` warning must be raised
manually. In the following example, ``k`` is deprecated and renamed to n_clusters::

    import warnings

    def example_function(n_clusters=8, k="deprecated"):
        if k != "deprecated":
            warnings.warn(
                "`k` was renamed to `n_clusters` in 0.13 and will be removed in 0.15",
                FutureWarning,
            )
            n_clusters = k

When the change is in a class, we validate and raise warning in ``fit``::

  import warnings

  class ExampleEstimator(BaseEstimator):
      def __init__(self, n_clusters=8, k='deprecated'):
          self.n_clusters = n_clusters
          self.k = k

      def fit(self, X, y):
          if self.k != "deprecated":
              warnings.warn(
                  "`k` was renamed to `n_clusters` in 0.13 and will be removed in 0.15.",
                  FutureWarning,
              )
              self._n_clusters = self.k
          else:
              self._n_clusters = self.n_clusters

As in these examples, the warning message should always give both the
version in which the deprecation happened and the version in which the
old behavior will be removed. If the deprecation happened in version
0.x-dev, the message should say deprecation occurred in version 0.x and
the removal will be in 0.(x+2), so that users will have enough time to
adapt their code to the new behaviour. For example, if the deprecation happened
in version 0.18-dev, the message should say it happened in version 0.18
and the old behavior will be removed in version 0.20.

The warning message should also include a brief explanation of the change and point
users to an alternative.

In addition, a deprecation note should be added in the docstring, recalling the
same information as the deprecation warning as explained above. Use the
``.. deprecated::`` directive:

.. code-block:: rst

  .. deprecated:: 0.13
     ``k`` was renamed to ``n_clusters`` in version 0.13 and will be removed
     in 0.15.

What's more, a deprecation requires a test which ensures that the warning is
raised in relevant cases but not in other cases. The warning should be caught
in all other tests (using e.g., ``@pytest.mark.filterwarnings``),
and there should be no warning in the examples.


Change the default value of a parameter
---------------------------------------

If the default value of a parameter needs to be changed, please replace the
default value with a specific value (e.g., ``"warn"``) and raise
``FutureWarning`` when users are using the default value. The following
example assumes that the current version is 0.20 and that we change the
default value of ``n_clusters`` from 5 (old default for 0.20) to 10
(new default for 0.22)::

    import warnings

    def example_function(n_clusters="warn"):
        if n_clusters == "warn":
            warnings.warn(
                "The default value of `n_clusters` will change from 5 to 10 in 0.22.",
                FutureWarning,
            )
            n_clusters = 5

When the change is in a class, we validate and raise warning in ``fit``::

  import warnings

  class ExampleEstimator:
      def __init__(self, n_clusters="warn"):
          self.n_clusters = n_clusters

      def fit(self, X, y):
          if self.n_clusters == "warn":
              warnings.warn(
                  "The default value of `n_clusters` will change from 5 to 10 in 0.22.",
                  FutureWarning,
              )
              self._n_clusters = 5

Similar to deprecations, the warning message should always give both the
version in which the change happened and the version in which the old behavior
will be removed.

The parameter description in the docstring needs to be updated accordingly by adding
a ``versionchanged`` directive with the old and new default value, pointing to the
version when the change will be effective:

.. code-block:: rst

    .. versionchanged:: 0.22
       The default value for `n_clusters` will change from 5 to 10 in version 0.22.

Finally, we need a test which ensures that the warning is raised in relevant cases but
not in other cases. The warning should be caught in all other tests
(using e.g., ``@pytest.mark.filterwarnings``), and there should be no warning
in the examples.

.. _code_review:

Code Review Guidelines
======================

Reviewing code contributed to the project as PRs is a crucial component of
scikit-learn development. We encourage anyone to start reviewing code of other
developers. The code review process is often highly educational for everybody
involved. This is particularly appropriate if it is a feature you would like to
use, and so can respond critically about whether the PR meets your needs. While
each pull request needs to be signed off by two core developers, you can speed
up this process by providing your feedback.

.. note::

  The difference between an objective improvement and a subjective nit isn't
  always clear. Reviewers should recall that code review is primarily about
  reducing risk in the project. When reviewing code, one should aim at
  preventing situations which may require a bug fix, a deprecation, or a
  retraction. Regarding docs: typos, grammar issues and disambiguations are
  better addressed immediately.

.. dropdown:: Important aspects to be covered in any code review

  Here are a few important aspects that need to be covered in any code review,
  from high-level questions to a more detailed check-list.

  - Do we want this in the library? Is it likely to be used? Do you, as
    a scikit-learn user, like the change and intend to use it? Is it in
    the scope of scikit-learn? Will the cost of maintaining a new
    feature be worth its benefits?

  - Is the code consistent with the API of scikit-learn? Are public
    functions/classes/parameters well named and intuitively designed?

  - Are all public functions/classes and their parameters, return types, and
    stored attributes named according to scikit-learn conventions and documented clearly?

  - Is any new functionality described in the user-guide and illustrated with examples?

  - Is every public function/class tested? Are a reasonable set of
    parameters, their values, value types, and combinations tested? Do
    the tests validate that the code is correct, i.e. doing what the
    documentation says it does? If the change is a bug-fix, is a
    non-regression test included? Look at `this
    <https://jeffknupp.com/blog/2013/12/09/improve-your-python-understanding-unit-testing>`__
    to get started with testing in Python.

  - Do the tests pass in the continuous integration build? If
    appropriate, help the contributor understand why tests failed.

  - Do the tests cover every line of code (see the coverage report in the build
    log)? If not, are the lines missing coverage good exceptions?

  - Is the code easy to read and low on redundancy? Should variable names be
    improved for clarity or consistency? Should comments be added? Should comments
    be removed as unhelpful or extraneous?

  - Could the code easily be rewritten to run much more efficiently for
    relevant settings?

  - Is the code backwards compatible with previous versions? (or is a
    deprecation cycle necessary?)

  - Will the new code add any dependencies on other libraries? (this is
    unlikely to be accepted)

  - Does the documentation render properly (see the
    :ref:`contribute_documentation` section for more details), and are the plots
    instructive?

  :ref:`saved_replies` includes some frequent comments that reviewers may make.

.. _communication:

.. dropdown:: Communication Guidelines

  Reviewing open pull requests (PRs) helps move the project forward. It is a
  great way to get familiar with the codebase and should motivate the
  contributor to keep involved in the project. [1]_

  - Every PR, good or bad, is an act of generosity. Opening with a positive
    comment will help the author feel rewarded, and your subsequent remarks may
    be heard more clearly. You may feel good also.
  - Begin if possible with the large issues, so the author knows they've been
    understood. Resist the temptation to immediately go line by line, or to open
    with small pervasive issues.
  - Do not let perfect be the enemy of the good. If you find yourself making
    many small suggestions that don't fall into the :ref:`code_review`, consider
    the following approaches:

    - refrain from submitting these;
    - prefix them as "Nit" so that the contributor knows it's OK not to address;
    - follow up in a subsequent PR, out of courtesy, you may want to let the
      original contributor know.

  - Do not rush, take the time to make your comments clear and justify your
    suggestions.
  - You are the face of the project. Bad days occur to everyone, in that
    occasion you deserve a break: try to take your time and stay offline.

  .. [1] Adapted from the numpy `communication guidelines
        <https://numpy.org/devdocs/dev/reviewer_guidelines.html#communication-guidelines>`_.

Reading the existing code base
==============================

Reading and digesting an existing code base is always a difficult exercise
that takes time and experience to master. Even though we try to write simple
code in general, understanding the code can seem overwhelming at first,
given the sheer size of the project. Here is a list of tips that may help
make this task easier and faster (in no particular order).

- Get acquainted with the :ref:`api_overview`: understand what :term:`fit`,
  :term:`predict`, :term:`transform`, etc. are used for.
- Before diving into reading the code of a function / class, go through the
  docstrings first and try to get an idea of what each parameter / attribute
  is doing. It may also help to stop a minute and think *how would I do this
  myself if I had to?*
- The trickiest thing is often to identify which portions of the code are
  relevant, and which are not. In scikit-learn **a lot** of input checking
  is performed, especially at the beginning of the :term:`fit` methods.
  Sometimes, only a very small portion of the code is doing the actual job.
  For example looking at the :meth:`~linear_model.LinearRegression.fit` method of
  :class:`~linear_model.LinearRegression`, what you're looking for
  might just be the call the :func:`scipy.linalg.lstsq`, but it is buried into
  multiple lines of input checking and the handling of different kinds of
  parameters.
- Due to the use of `Inheritance
  <https://en.wikipedia.org/wiki/Inheritance_(object-oriented_programming)>`_,
  some methods may be implemented in parent classes. All estimators inherit
  at least from :class:`~base.BaseEstimator`, and
  from a ``Mixin`` class (e.g. :class:`~base.ClassifierMixin`) that enables default
  behaviour depending on the nature of the estimator (classifier, regressor,
  transformer, etc.).
- Sometimes, reading the tests for a given function will give you an idea of
  what its intended purpose is. You can use ``git grep`` (see below) to find
  all the tests written for a function. Most tests for a specific
  function/class are placed under the ``tests/`` folder of the module
- You'll often see code looking like this:
  ``out = Parallel(...)(delayed(some_function)(param) for param in
  some_iterable)``. This runs ``some_function`` in parallel using `Joblib
  <https://joblib.readthedocs.io/>`_. ``out`` is then an iterable containing
  the values returned by ``some_function`` for each call.
- We use `Cython <https://cython.org/>`_ to write fast code. Cython code is
  located in ``.pyx`` and ``.pxd`` files. Cython code has a more C-like flavor:
  we use pointers, perform manual memory allocation, etc. Having some minimal
  experience in C / C++ is pretty much mandatory here. For more information see
  :ref:`cython`.
- Master your tools.

  - With such a big project, being efficient with your favorite editor or
    IDE goes a long way towards digesting the code base. Being able to quickly
    jump (or *peek*) to a function/class/attribute definition helps a lot.
    So does being able to quickly see where a given name is used in a file.
  - `Git <https://git-scm.com/book/en>`_ also has some built-in killer
    features. It is often useful to understand how a file changed over time,
    using e.g. ``git blame`` (`manual
    <https://git-scm.com/docs/git-blame>`_). This can also be done directly
    on GitHub. ``git grep`` (`examples
    <https://git-scm.com/docs/git-grep#_examples>`_) is also extremely
    useful to see every occurrence of a pattern (e.g. a function call or a
    variable) in the code base.

- Configure `git blame` to ignore the commit that migrated the code style to
  `black` and then `ruff`.

  .. prompt:: bash

      git config blame.ignoreRevsFile .git-blame-ignore-revs

  Find out more information in black's
  `documentation for avoiding ruining git blame <https://black.readthedocs.io/en/stable/guides/introducing_black_to_your_project.html#avoiding-ruining-git-blame>`_.