.. include:: _contributors.rst

.. currentmodule:: sklearn

.. _changes_0_21_3:

Version 0.21.3
==============

.. include:: changelog_legend.inc

**July 30, 2019**

Changed models
--------------

The following estimators and functions, when fit with the same data and
parameters, may produce different models from the previous version. This often
occurs due to changes in the modelling logic (bug fixes or enhancements), or in
random sampling procedures.

- The v0.20.0 release notes failed to mention a backwards incompatibility in
  :func:`metrics.make_scorer` when `needs_proba=True` and `y_true` is binary.
  Now, the scorer function is supposed to accept a 1D `y_pred` (i.e.,
  probability of the positive class, shape `(n_samples,)`), instead of a 2D
  `y_pred` (i.e., shape `(n_samples, 2)`).

Changelog
---------

:mod:`sklearn.cluster`
......................

- |Fix| Fixed a bug in :class:`cluster.KMeans` where computation with
  `init='random'` was single threaded for `n_jobs > 1` or `n_jobs = -1`.
  :pr:`12955` by :user:`Prabakaran Kumaresshan <nixphix>`.

- |Fix| Fixed a bug in :class:`cluster.OPTICS` where users were unable to pass
  float `min_samples` and `min_cluster_size`. :pr:`14496` by
  :user:`Fabian Klopfer <someusername1>`
  and :user:`Hanmin Qin <qinhanmin2014>`.

- |Fix| Fixed a bug in :class:`cluster.KMeans` where KMeans++ initialisation
  could rarely result in an IndexError. :issue:`11756` by `Joel Nothman`_.

:mod:`sklearn.compose`
......................

- |Fix| Fixed an issue in :class:`compose.ColumnTransformer` where using
  DataFrames whose column order differs between :func:``fit`` and
  :func:``transform`` could lead to silently passing incorrect columns to the
  ``remainder`` transformer.
  :pr:`14237` by `Andreas Schuderer <schuderer>`.

:mod:`sklearn.datasets`
.......................

- |Fix| :func:`datasets.fetch_california_housing`,
  :func:`datasets.fetch_covtype`,
  :func:`datasets.fetch_kddcup99`, :func:`datasets.fetch_olivetti_faces`,
  :func:`datasets.fetch_rcv1`, and :func:`datasets.fetch_species_distributions`
  try to persist the previously cache using the new ``joblib`` if the cached
  data was persisted using the deprecated ``sklearn.externals.joblib``. This
  behavior is set to be deprecated and removed in v0.23.
  :pr:`14197` by `Adrin Jalali`_.

:mod:`sklearn.ensemble`
.......................

- |Fix| Fix zero division error in :func:`HistGradientBoostingClassifier` and
  :func:`HistGradientBoostingRegressor`.
  :pr:`14024` by `Nicolas Hug <NicolasHug>`.

:mod:`sklearn.impute`
.....................

- |Fix| Fixed a bug in :class:`impute.SimpleImputer` and
  :class:`impute.IterativeImputer` so that no errors are thrown when there are
  missing values in training data. :pr:`13974` by `Frank Hoang <fhoang7>`.

:mod:`sklearn.inspection`
.........................

- |Fix| Fixed a bug in :func:`inspection.plot_partial_dependence` where 
  ``target`` parameter was not being taken into account for multiclass problems.
  :pr:`14393` by :user:`Guillem G. Subies <guillemgsubies>`.

:mod:`sklearn.linear_model`
...........................

- |Fix| Fixed a bug in :class:`linear_model.LogisticRegressionCV` where
  ``refit=False`` would fail depending on the ``'multiclass'`` and
  ``'penalty'`` parameters (regression introduced in 0.21). :pr:`14087` by
  `Nicolas Hug`_.

- |Fix| Compatibility fix for :class:`linear_model.ARDRegression` and
  Scipy>=1.3.0. Adapts to upstream changes to the default `pinvh` cutoff
  threshold which otherwise results in poor accuracy in some cases.
  :pr:`14067` by :user:`Tim Staley <timstaley>`.

:mod:`sklearn.neighbors`
........................

- |Fix| Fixed a bug in :class:`neighbors.NeighborhoodComponentsAnalysis` where
  the validation of initial parameters ``n_components``, ``max_iter`` and
  ``tol`` required too strict types. :pr:`14092` by
  :user:`Jérémie du Boisberranger <jeremiedbb>`.

:mod:`sklearn.tree`
...................

- |Fix| Fixed bug in :func:`tree.export_text` when the tree has one feature and 
  a single feature name is passed in. :pr:`14053` by `Thomas Fan`.

- |Fix| Fixed an issue with :func:`plot_tree` where it displayed
  entropy calculations even for `gini` criterion in DecisionTreeClassifiers.
  :pr:`13947` by :user:`Frank Hoang <fhoang7>`.

.. _changes_0_21_2:

Version 0.21.2
==============

**24 May 2019**

Changelog
---------

:mod:`sklearn.decomposition`
............................

- |Fix| Fixed a bug in :class:`cross_decomposition.CCA` improving numerical 
  stability when `Y` is close to zero. :pr:`13903` by `Thomas Fan`_.

:mod:`sklearn.metrics`
......................

- |Fix| Fixed a bug in :func:`metrics.pairwise.euclidean_distances` where a
  part of the distance matrix was left un-instanciated for sufficiently large
  float32 datasets (regression introduced in 0.21). :pr:`13910` by
  :user:`Jérémie du Boisberranger <jeremiedbb>`.

:mod:`sklearn.preprocessing`
............................

- |Fix| Fixed a bug in :class:`preprocessing.OneHotEncoder` where the new
  `drop` parameter was not reflected in `get_feature_names`. :pr:`13894`
  by :user:`James Myatt <jamesmyatt>`.


:mod:`sklearn.utils.sparsefuncs`
................................

- |Fix| Fixed a bug where :func:`min_max_axis` would fail on 32-bit systems
  for certain large inputs. This affects :class:`preprocessing.MaxAbsScaler`, 
  :func:`preprocessing.normalize` and :class:`preprocessing.LabelBinarizer`.
  :pr:`13741` by :user:`Roddy MacSween <rlms>`.

.. _changes_0_21_1:

Version 0.21.1
==============

**17 May 2019**

This is a bug-fix release to primarily resolve some packaging issues in version
0.21.0. It also includes minor documentation improvements and some bug fixes.

Changelog
---------

:mod:`sklearn.inspection`
.........................

- |Fix| Fixed a bug in :func:`inspection.partial_dependence` to only check
  classifier and not regressor for the multiclass-multioutput case.
  :pr:`14309` by :user:`Guillaume Lemaitre <glemaitre>`.

:mod:`sklearn.metrics`
......................

- |Fix| Fixed a bug in :class:`metrics.pairwise_distances` where it would raise
  ``AttributeError`` for boolean metrics when ``X`` had a boolean dtype and
  ``Y == None``.
  :issue:`13864` by :user:`Paresh Mathur <rick2047>`.

- |Fix| Fixed two bugs in :class:`metrics.pairwise_distances` when
  ``n_jobs > 1``. First it used to return a distance matrix with same dtype as
  input, even for integer dtype. Then the diagonal was not zeros for euclidean
  metric when ``Y`` is ``X``. :issue:`13877` by
  :user:`Jérémie du Boisberranger <jeremiedbb>`.

:mod:`sklearn.neighbors`
........................

- |Fix| Fixed a bug in :class:`neighbors.KernelDensity` which could not be
  restored from a pickle if ``sample_weight`` had been used.
  :issue:`13772` by :user:`Aditya Vyas <aditya1702>`.


.. _changes_0_21:

Version 0.21.0
==============

**May 2019**

Changed models
--------------

The following estimators and functions, when fit with the same data and
parameters, may produce different models from the previous version. This often
occurs due to changes in the modelling logic (bug fixes or enhancements), or in
random sampling procedures.

- :class:`discriminant_analysis.LinearDiscriminantAnalysis` for multiclass
  classification. |Fix|
- :class:`discriminant_analysis.LinearDiscriminantAnalysis` with 'eigen'
  solver. |Fix|
- :class:`linear_model.BayesianRidge` |Fix|
- Decision trees and derived ensembles when both `max_depth` and
  `max_leaf_nodes` are set. |Fix|
- :class:`linear_model.LogisticRegression` and
  :class:`linear_model.LogisticRegressionCV` with 'saga' solver. |Fix|
- :class:`ensemble.GradientBoostingClassifier` |Fix|
- :class:`sklearn.feature_extraction.text.HashingVectorizer`,
  :class:`sklearn.feature_extraction.text.TfidfVectorizer`, and
  :class:`sklearn.feature_extraction.text.CountVectorizer` |Fix|
- :class:`neural_network.MLPClassifier` |Fix|
- :func:`svm.SVC.decision_function` and
  :func:`multiclass.OneVsOneClassifier.decision_function`. |Fix|
- :class:`linear_model.SGDClassifier` and any derived classifiers. |Fix|
- Any model using the :func:`linear_model._sag.sag_solver` function with a `0`
  seed, including :class:`linear_model.LogisticRegression`,
  :class:`linear_model.LogisticRegressionCV`, :class:`linear_model.Ridge`,
  and :class:`linear_model.RidgeCV` with 'sag' solver. |Fix|
- :class:`linear_model.RidgeCV` when using leave-one-out cross-validation
  with sparse inputs. |Fix|


Details are listed in the changelog below.

(While we are trying to better inform users by providing this information, we
cannot assure that this list is complete.)

Known Major Bugs
----------------

* The default `max_iter` for :class:`linear_model.LogisticRegression` is too
  small for many solvers given the default `tol`. In particular, we
  accidentally changed the default `max_iter` for the liblinear solver from
  1000 to 100 iterations in :pr:`3591` released in version 0.16.
  In a future release we hope to choose better default `max_iter` and `tol`
  heuristically depending on the solver (see :pr:`13317`).

Changelog
---------

Support for Python 3.4 and below has been officially dropped.

..
    Entries should be grouped by module (in alphabetic order) and prefixed with
    one of the labels: |MajorFeature|, |Feature|, |Efficiency|, |Enhancement|,
    |Fix| or |API| (see whats_new.rst for descriptions).
    Entries should be ordered by those labels (e.g. |Fix| after |Efficiency|).
    Changes not specific to a module should be listed under *Multiple Modules*
    or *Miscellaneous*.
    Entries should end with:
    :pr:`123456` by :user:`Joe Bloggs <joeongithub>`.
    where 123456 is the *pull request* number, not the issue number.

:mod:`sklearn.base`
...................

- |API| The R2 score used when calling ``score`` on a regressor will use
  ``multioutput='uniform_average'`` from version 0.23 to keep consistent with
  :func:`metrics.r2_score`. This will influence the ``score`` method of all
  the multioutput regressors (except for
  :class:`multioutput.MultiOutputRegressor`).
  :pr:`13157` by :user:`Hanmin Qin <qinhanmin2014>`.

:mod:`sklearn.calibration`
..........................

- |Enhancement| Added support to bin the data passed into
  :class:`calibration.calibration_curve` by quantiles instead of uniformly
  between 0 and 1.
  :pr:`13086` by :user:`Scott Cole <srcole>`.

- |Enhancement| Allow n-dimensional arrays as input for
  `calibration.CalibratedClassifierCV`. :pr:`13485` by
  :user:`William de Vazelhes <wdevazelhes>`.

:mod:`sklearn.cluster`
......................

- |MajorFeature| A new clustering algorithm: :class:`cluster.OPTICS`: an
  algorithm related to :class:`cluster.DBSCAN`, that has hyperparameters easier
  to set and that scales better, by :user:`Shane <espg>`,
  `Adrin Jalali`_, :user:`Erich Schubert <kno10>`, `Hanmin Qin`_, and
  :user:`Assia Benbihi <assiaben>`.

- |Fix| Fixed a bug where :class:`cluster.Birch` could occasionally raise an
  AttributeError. :pr:`13651` by `Joel Nothman`_.

- |Fix| Fixed a bug in :class:`cluster.KMeans` where empty clusters weren't
  correctly relocated when using sample weights. :pr:`13486` by
  :user:`Jérémie du Boisberranger <jeremiedbb>`.

- |API| The ``n_components_`` attribute in :class:`cluster.AgglomerativeClustering`
  and :class:`cluster.FeatureAgglomeration` has been renamed to
  ``n_connected_components_``.
  :pr:`13427` by :user:`Stephane Couvreur <scouvreur>`.

- |Enhancement| :class:`cluster.AgglomerativeClustering` and
  :class:`cluster.FeatureAgglomeration` now accept a ``distance_threshold``
  parameter which can be used to find the clusters instead of ``n_clusters``.
  :issue:`9069` by :user:`Vathsala Achar <VathsalaAchar>` and `Adrin Jalali`_.

:mod:`sklearn.compose`
......................

- |API| :class:`compose.ColumnTransformer` is no longer an experimental
  feature. :pr:`13835` by :user:`Hanmin Qin <qinhanmin2014>`.

:mod:`sklearn.datasets`
.......................

- |Fix| Added support for 64-bit group IDs and pointers in SVMLight files.
  :pr:`10727` by :user:`Bryan K Woods <bryan-woods>`.

- |Fix| :func:`datasets.load_sample_images` returns images with a deterministic
  order. :pr:`13250` by :user:`Thomas Fan <thomasjpfan>`.

:mod:`sklearn.decomposition`
............................

- |Enhancement| :class:`decomposition.KernelPCA` now has deterministic output
  (resolved sign ambiguity in eigenvalue decomposition of the kernel matrix).
  :pr:`13241` by :user:`Aurélien Bellet <bellet>`.

- |Fix| Fixed a bug in :class:`decomposition.KernelPCA`, `fit().transform()`
  now produces the correct output (the same as `fit_transform()`) in case
  of non-removed zero eigenvalues (`remove_zero_eig=False`).
  `fit_inverse_transform` was also accelerated by using the same trick as
  `fit_transform` to compute the transform of `X`.
  :pr:`12143` by :user:`Sylvain Marié <smarie>`

- |Fix| Fixed a bug in :class:`decomposition.NMF` where `init = 'nndsvd'`,
  `init = 'nndsvda'`, and `init = 'nndsvdar'` are allowed when
  `n_components < n_features` instead of
  `n_components <= min(n_samples, n_features)`.
  :pr:`11650` by :user:`Hossein Pourbozorg <hossein-pourbozorg>` and
  :user:`Zijie (ZJ) Poh <zjpoh>`.

- |API| The default value of the :code:`init` argument in
  :func:`decomposition.non_negative_factorization` will change from
  :code:`random` to :code:`None` in version 0.23 to make it consistent with
  :class:`decomposition.NMF`. A FutureWarning is raised when
  the default value is used.
  :pr:`12988` by :user:`Zijie (ZJ) Poh <zjpoh>`.

:mod:`sklearn.discriminant_analysis`
....................................

- |Enhancement| :class:`discriminant_analysis.LinearDiscriminantAnalysis` now
  preserves ``float32`` and ``float64`` dtypes. :pr:`8769` and
  :pr:`11000` by :user:`Thibault Sejourne <thibsej>`

- |Fix| A ``ChangedBehaviourWarning`` is now raised when
  :class:`discriminant_analysis.LinearDiscriminantAnalysis` is given as
  parameter ``n_components > min(n_features, n_classes - 1)``, and
  ``n_components`` is changed to ``min(n_features, n_classes - 1)`` if so.
  Previously the change was made, but silently. :pr:`11526` by
  :user:`William de Vazelhes<wdevazelhes>`.

- |Fix| Fixed a bug in :class:`discriminant_analysis.LinearDiscriminantAnalysis`
  where the predicted probabilities would be incorrectly computed in the
  multiclass case. :pr:`6848`, by :user:`Agamemnon Krasoulis
  <agamemnonc>` and `Guillaume Lemaitre <glemaitre>`.

- |Fix| Fixed a bug in :class:`discriminant_analysis.LinearDiscriminantAnalysis`
  where the predicted probabilities would be incorrectly computed with ``eigen``
  solver. :pr:`11727`, by :user:`Agamemnon Krasoulis
  <agamemnonc>`.

:mod:`sklearn.dummy`
....................

- |Fix| Fixed a bug in :class:`dummy.DummyClassifier` where the
  ``predict_proba`` method was returning int32 array instead of
  float64 for the ``stratified`` strategy. :pr:`13266` by
  :user:`Christos Aridas<chkoar>`.

- |Fix| Fixed a bug in :class:`dummy.DummyClassifier` where it was throwing a
  dimension mismatch error in prediction time if a column vector ``y`` with
  ``shape=(n, 1)`` was given at ``fit`` time. :pr:`13545` by :user:`Nick
  Sorros <nsorros>` and `Adrin Jalali`_.

:mod:`sklearn.ensemble`
.......................

- |MajorFeature| Add two new implementations of
  gradient boosting trees: :class:`ensemble.HistGradientBoostingClassifier`
  and :class:`ensemble.HistGradientBoostingRegressor`. The implementation of
  these estimators is inspired by
  `LightGBM <https://github.com/Microsoft/LightGBM>`_ and can be orders of
  magnitude faster than :class:`ensemble.GradientBoostingRegressor` and
  :class:`ensemble.GradientBoostingClassifier` when the number of samples is
  larger than tens of thousands of samples. The API of these new estimators
  is slightly different, and some of the features from
  :class:`ensemble.GradientBoostingClassifier` and
  :class:`ensemble.GradientBoostingRegressor` are not yet supported.

  These new estimators are experimental, which means that their results or
  their API might change without any deprecation cycle. To use them, you
  need to explicitly import ``enable_hist_gradient_boosting``::

    >>> # explicitly require this experimental feature
    >>> from sklearn.experimental import enable_hist_gradient_boosting  # noqa
    >>> # now you can import normally from sklearn.ensemble
    >>> from sklearn.ensemble import HistGradientBoostingClassifier
  
  .. note::
      Update: since version 1.0, these estimators are not experimental
      anymore and you don't need to use `from sklearn.experimental import
      enable_hist_gradient_boosting`.

  :pr:`12807` by :user:`Nicolas Hug<NicolasHug>`.

- |Feature| Add :class:`ensemble.VotingRegressor`
  which provides an equivalent of :class:`ensemble.VotingClassifier`
  for regression problems.
  :pr:`12513` by :user:`Ramil Nugmanov <stsouko>` and
  :user:`Mohamed Ali Jamaoui <mohamed-ali>`.

- |Efficiency| Make :class:`ensemble.IsolationForest` prefer threads over
  processes when running with ``n_jobs > 1`` as the underlying decision tree
  fit calls do release the GIL. This changes reduces memory usage and
  communication overhead. :pr:`12543` by :user:`Isaac Storch <istorch>`
  and `Olivier Grisel`_.

- |Efficiency| Make :class:`ensemble.IsolationForest` more memory efficient
  by avoiding keeping in memory each tree prediction. :pr:`13260` by
  `Nicolas Goix`_.

- |Efficiency| :class:`ensemble.IsolationForest` now uses chunks of data at
  prediction step, thus capping the memory usage. :pr:`13283` by
  `Nicolas Goix`_.

- |Efficiency| :class:`sklearn.ensemble.GradientBoostingClassifier` and
  :class:`sklearn.ensemble.GradientBoostingRegressor` now keep the
  input ``y`` as ``float64`` to avoid it being copied internally by trees.
  :pr:`13524` by `Adrin Jalali`_.

- |Enhancement| Minimized the validation of X in
  :class:`ensemble.AdaBoostClassifier` and :class:`ensemble.AdaBoostRegressor`
  :pr:`13174` by :user:`Christos Aridas <chkoar>`.

- |Enhancement| :class:`ensemble.IsolationForest` now exposes ``warm_start``
  parameter, allowing iterative addition of trees to an isolation
  forest. :pr:`13496` by :user:`Peter Marko <petibear>`.

- |Fix| The values of ``feature_importances_`` in all random forest based
  models (i.e.
  :class:`ensemble.RandomForestClassifier`,
  :class:`ensemble.RandomForestRegressor`,
  :class:`ensemble.ExtraTreesClassifier`,
  :class:`ensemble.ExtraTreesRegressor`,
  :class:`ensemble.RandomTreesEmbedding`,
  :class:`ensemble.GradientBoostingClassifier`, and
  :class:`ensemble.GradientBoostingRegressor`) now:

  - sum up to ``1``
  - all the single node trees in feature importance calculation are ignored
  - in case all trees have only one single node (i.e. a root node),
    feature importances will be an array of all zeros.

  :pr:`13636` and :pr:`13620` by `Adrin Jalali`_.

- |Fix| Fixed a bug in :class:`ensemble.GradientBoostingClassifier` and
  :class:`ensemble.GradientBoostingRegressor`, which didn't support
  scikit-learn estimators as the initial estimator. Also added support of
  initial estimator which does not support sample weights. :pr:`12436` by
  :user:`Jérémie du Boisberranger <jeremiedbb>` and :pr:`12983` by
  :user:`Nicolas Hug<NicolasHug>`.

- |Fix| Fixed the output of the average path length computed in
  :class:`ensemble.IsolationForest` when the input is either 0, 1 or 2.
  :pr:`13251` by :user:`Albert Thomas <albertcthomas>`
  and :user:`joshuakennethjones <joshuakennethjones>`.

- |Fix| Fixed a bug in :class:`ensemble.GradientBoostingClassifier` where
  the gradients would be incorrectly computed in multiclass classification
  problems. :pr:`12715` by :user:`Nicolas Hug<NicolasHug>`.

- |Fix| Fixed a bug in :class:`ensemble.GradientBoostingClassifier` where
  validation sets for early stopping were not sampled with stratification.
  :pr:`13164` by :user:`Nicolas Hug<NicolasHug>`.

- |Fix| Fixed a bug in :class:`ensemble.GradientBoostingClassifier` where
  the default initial prediction of a multiclass classifier would predict the
  classes priors instead of the log of the priors. :pr:`12983` by
  :user:`Nicolas Hug<NicolasHug>`.

- |Fix| Fixed a bug in :class:`ensemble.RandomForestClassifier` where the
  ``predict`` method would error for multiclass multioutput forests models
  if any targets were strings. :pr:`12834` by :user:`Elizabeth Sander
  <elsander>`.

- |Fix| Fixed a bug in :class:`ensemble.gradient_boosting.LossFunction` and
  :class:`ensemble.gradient_boosting.LeastSquaresError` where the default
  value of ``learning_rate`` in ``update_terminal_regions`` is not consistent
  with the document and the caller functions. Note however that directly using
  these loss functions is deprecated.
  :pr:`6463` by :user:`movelikeriver <movelikeriver>`.

- |Fix| :func:`ensemble.partial_dependence` (and consequently the new
  version :func:`sklearn.inspection.partial_dependence`) now takes sample
  weights into account for the partial dependence computation when the
  gradient boosting model has been trained with sample weights.
  :pr:`13193` by :user:`Samuel O. Ronsin <samronsin>`.

- |API| :func:`ensemble.partial_dependence` and
  :func:`ensemble.plot_partial_dependence` are now deprecated in favor of
  :func:`inspection.partial_dependence<sklearn.inspection.partial_dependence>`
  and
  :func:`inspection.plot_partial_dependence<sklearn.inspection.plot_partial_dependence>`.
  :pr:`12599` by :user:`Trevor Stephens<trevorstephens>` and
  :user:`Nicolas Hug<NicolasHug>`.

- |Fix| :class:`ensemble.VotingClassifier` and
  :class:`ensemble.VotingRegressor` were failing during ``fit`` in one
  of the estimators was set to ``None`` and ``sample_weight`` was not ``None``.
  :pr:`13779` by :user:`Guillaume Lemaitre <glemaitre>`.

- |API| :class:`ensemble.VotingClassifier` and
  :class:`ensemble.VotingRegressor` accept ``'drop'`` to disable an estimator
  in addition to ``None`` to be consistent with other estimators (i.e.,
  :class:`pipeline.FeatureUnion` and :class:`compose.ColumnTransformer`).
  :pr:`13780` by :user:`Guillaume Lemaitre <glemaitre>`.

:mod:`sklearn.externals`
........................

- |API| Deprecated :mod:`externals.six` since we have dropped support for
  Python 2.7. :pr:`12916` by :user:`Hanmin Qin <qinhanmin2014>`.

:mod:`sklearn.feature_extraction`
.................................

- |Fix| If ``input='file'`` or ``input='filename'``, and a callable is given as
  the ``analyzer``, :class:`sklearn.feature_extraction.text.HashingVectorizer`,
  :class:`sklearn.feature_extraction.text.TfidfVectorizer`, and
  :class:`sklearn.feature_extraction.text.CountVectorizer` now read the data
  from the file(s) and then pass it to the given ``analyzer``, instead of
  passing the file name(s) or the file object(s) to the analyzer.
  :pr:`13641` by `Adrin Jalali`_.

:mod:`sklearn.impute`
.....................

- |MajorFeature| Added :class:`impute.IterativeImputer`, which is a strategy
  for imputing missing values by modeling each feature with missing values as a
  function of other features in a round-robin fashion. :pr:`8478` and
  :pr:`12177` by :user:`Sergey Feldman <sergeyf>` and :user:`Ben Lawson
  <benlawson>`.

  The API of IterativeImputer is experimental and subject to change without any
  deprecation cycle. To use them, you need to explicitly import
  ``enable_iterative_imputer``::

    >>> from sklearn.experimental import enable_iterative_imputer  # noqa
    >>> # now you can import normally from sklearn.impute
    >>> from sklearn.impute import IterativeImputer


- |Feature| The :class:`impute.SimpleImputer` and
  :class:`impute.IterativeImputer` have a new parameter ``'add_indicator'``,
  which simply stacks a :class:`impute.MissingIndicator` transform into the
  output of the imputer's transform. That allows a predictive estimator to
  account for missingness. :pr:`12583`, :pr:`13601` by :user:`Danylo Baibak
  <DanilBaibak>`.

- |Fix| In :class:`impute.MissingIndicator` avoid implicit densification by
  raising an exception if input is sparse add `missing_values` property
  is set to 0. :pr:`13240` by :user:`Bartosz Telenczuk <btel>`.

- |Fix| Fixed two bugs in :class:`impute.MissingIndicator`. First, when
  ``X`` is sparse, all the non-zero non missing values used to become
  explicit False in the transformed data. Then, when
  ``features='missing-only'``, all features used to be kept if there were no
  missing values at all. :pr:`13562` by :user:`Jérémie du Boisberranger
  <jeremiedbb>`.

:mod:`sklearn.inspection`
.........................

(new subpackage)

- |Feature| Partial dependence plots
  (:func:`inspection.plot_partial_dependence`) are now supported for
  any regressor or classifier (provided that they have a `predict_proba`
  method). :pr:`12599` by :user:`Trevor Stephens <trevorstephens>` and
  :user:`Nicolas Hug <NicolasHug>`.

:mod:`sklearn.isotonic`
.......................

- |Feature| Allow different dtypes (such as float32) in
  :class:`isotonic.IsotonicRegression`.
  :pr:`8769` by :user:`Vlad Niculae <vene>`

:mod:`sklearn.linear_model`
...........................

- |Enhancement| :class:`linear_model.Ridge` now preserves ``float32`` and
  ``float64`` dtypes. :issue:`8769` and :issue:`11000` by
  :user:`Guillaume Lemaitre <glemaitre>`, and :user:`Joan Massich <massich>`

- |Feature| :class:`linear_model.LogisticRegression` and
  :class:`linear_model.LogisticRegressionCV` now support Elastic-Net penalty,
  with the 'saga' solver. :pr:`11646` by :user:`Nicolas Hug <NicolasHug>`.

- |Feature| Added :class:`linear_model.lars_path_gram`, which is
  :class:`linear_model.lars_path` in the sufficient stats mode, allowing
  users to compute :class:`linear_model.lars_path` without providing
  ``X`` and ``y``. :pr:`11699` by :user:`Kuai Yu <yukuairoy>`.

- |Efficiency| :func:`linear_model.make_dataset` now preserves
  ``float32`` and ``float64`` dtypes, reducing memory consumption in stochastic
  gradient, SAG and SAGA solvers.
  :pr:`8769` and :pr:`11000` by
  :user:`Nelle Varoquaux <NelleV>`, :user:`Arthur Imbert <Henley13>`,
  :user:`Guillaume Lemaitre <glemaitre>`, and :user:`Joan Massich <massich>`

- |Enhancement| :class:`linear_model.LogisticRegression` now supports an
  unregularized objective when ``penalty='none'`` is passed. This is
  equivalent to setting ``C=np.inf`` with l2 regularization. Not supported
  by the liblinear solver. :pr:`12860` by :user:`Nicolas Hug
  <NicolasHug>`.

- |Enhancement| `sparse_cg` solver in :class:`linear_model.Ridge`
  now supports fitting the intercept (i.e. ``fit_intercept=True``) when
  inputs are sparse. :pr:`13336` by :user:`Bartosz Telenczuk <btel>`.

- |Enhancement| The coordinate descent solver used in `Lasso`, `ElasticNet`,
  etc. now issues a `ConvergenceWarning` when it completes without meeting the
  desired toleranbce.
  :pr:`11754` and :pr:`13397` by :user:`Brent Fagan <brentfagan>` and
  :user:`Adrin Jalali <adrinjalali>`.

- |Fix| Fixed a bug in :class:`linear_model.LogisticRegression` and
  :class:`linear_model.LogisticRegressionCV` with 'saga' solver, where the
  weights would not be correctly updated in some cases.
  :pr:`11646` by `Tom Dupre la Tour`_.

- |Fix| Fixed the posterior mean, posterior covariance and returned
  regularization parameters in :class:`linear_model.BayesianRidge`. The
  posterior mean and the posterior covariance were not the ones computed
  with the last update of the regularization parameters and the returned
  regularization parameters were not the final ones. Also fixed the formula of
  the log marginal likelihood used to compute the score when
  `compute_score=True`. :pr:`12174` by
  :user:`Albert Thomas <albertcthomas>`.

- |Fix| Fixed a bug in :class:`linear_model.LassoLarsIC`, where user input
  ``copy_X=False`` at instance creation would be overridden by default
  parameter value ``copy_X=True`` in ``fit``.
  :pr:`12972` by :user:`Lucio Fernandez-Arjona <luk-f-a>`

- |Fix| Fixed a bug in :class:`linear_model.LinearRegression` that
  was not returning the same coeffecients and intercepts with
  ``fit_intercept=True`` in sparse and dense case.
  :pr:`13279` by `Alexandre Gramfort`_

- |Fix| Fixed a bug in :class:`linear_model.HuberRegressor` that was
  broken when ``X`` was of dtype bool. :pr:`13328` by `Alexandre Gramfort`_.

- |Fix| Fixed a performance issue of ``saga`` and ``sag`` solvers when called
  in a :class:`joblib.Parallel` setting with ``n_jobs > 1`` and
  ``backend="threading"``, causing them to perform worse than in the sequential
  case. :pr:`13389` by :user:`Pierre Glaser <pierreglaser>`.

- |Fix| Fixed a bug in
  :class:`linear_model.stochastic_gradient.BaseSGDClassifier` that was not
  deterministic when trained in a multi-class setting on several threads.
  :pr:`13422` by :user:`Clément Doumouro <ClemDoum>`.

- |Fix| Fixed bug in :func:`linear_model.ridge_regression`,
  :class:`linear_model.Ridge` and
  :class:`linear_model.RidgeClassifier` that
  caused unhandled exception for arguments ``return_intercept=True`` and
  ``solver=auto`` (default) or any other solver different from ``sag``.
  :pr:`13363` by :user:`Bartosz Telenczuk <btel>`

- |Fix| :func:`linear_model.ridge_regression` will now raise an exception
  if ``return_intercept=True`` and solver is different from ``sag``. Previously,
  only warning was issued. :pr:`13363` by :user:`Bartosz Telenczuk <btel>`

- |Fix| :func:`linear_model.ridge_regression` will choose ``sparse_cg``
  solver for sparse inputs when ``solver=auto`` and ``sample_weight``
  is provided (previously `cholesky` solver was selected).
  :pr:`13363` by :user:`Bartosz Telenczuk <btel>`

- |API|  The use of :class:`linear_model.lars_path` with ``X=None``
  while passing ``Gram`` is deprecated in version 0.21 and will be removed
  in version 0.23. Use :class:`linear_model.lars_path_gram` instead.
  :pr:`11699` by :user:`Kuai Yu <yukuairoy>`.

- |API| :func:`linear_model.logistic_regression_path` is deprecated
  in version 0.21 and will be removed in version 0.23.
  :pr:`12821` by :user:`Nicolas Hug <NicolasHug>`.

- |Fix| :class:`linear_model.RidgeCV` with leave-one-out cross-validation
  now correctly fits an intercept when ``fit_intercept=True`` and the design
  matrix is sparse. :issue:`13350` by :user:`Jérôme Dockès <jeromedockes>`

:mod:`sklearn.manifold`
.......................

- |Efficiency| Make :func:`manifold.tsne.trustworthiness` use an inverted index
  instead of an `np.where` lookup to find the rank of neighbors in the input
  space. This improves efficiency in particular when computed with
  lots of neighbors and/or small datasets.
  :pr:`9907` by :user:`William de Vazelhes <wdevazelhes>`.

:mod:`sklearn.metrics`
......................

- |Feature| Added the :func:`metrics.max_error` metric and a corresponding
  ``'max_error'`` scorer for single output regression.
  :pr:`12232` by :user:`Krishna Sangeeth <whiletruelearn>`.

- |Feature| Add :func:`metrics.multilabel_confusion_matrix`, which calculates a
  confusion matrix with true positive, false positive, false negative and true
  negative counts for each class. This facilitates the calculation of set-wise
  metrics such as recall, specificity, fall out and miss rate.
  :pr:`11179` by :user:`Shangwu Yao <ShangwuYao>` and `Joel Nothman`_.

- |Feature| :func:`metrics.jaccard_score` has been added to calculate the
  Jaccard coefficient as an evaluation metric for binary, multilabel and
  multiclass tasks, with an interface analogous to :func:`metrics.f1_score`.
  :pr:`13151` by :user:`Gaurav Dhingra <gxyd>` and `Joel Nothman`_.

- |Feature| Added :func:`metrics.pairwise.haversine_distances` which can be
  accessed with `metric='pairwise'` through :func:`metrics.pairwise_distances`
  and estimators. (Haversine distance was previously available for nearest
  neighbors calculation.) :pr:`12568` by :user:`Wei Xue <xuewei4d>`,
  :user:`Emmanuel Arias <eamanu>` and `Joel Nothman`_.

- |Efficiency| Faster :func:`metrics.pairwise_distances` with `n_jobs`
  > 1 by using a thread-based backend, instead of process-based backends.
  :pr:`8216` by :user:`Pierre Glaser <pierreglaser>` and
  :user:`Romuald Menuet <zanospi>`

- |Efficiency| The pairwise manhattan distances with sparse input now uses the
  BLAS shipped with scipy instead of the bundled BLAS. :pr:`12732` by
  :user:`Jérémie du Boisberranger <jeremiedbb>`

- |Enhancement| Use label `accuracy` instead of `micro-average` on
  :func:`metrics.classification_report` to avoid confusion. `micro-average` is
  only shown for multi-label or multi-class with a subset of classes because
  it is otherwise identical to accuracy.
  :pr:`12334` by :user:`Emmanuel Arias <eamanu@eamanu.com>`,
  `Joel Nothman`_ and `Andreas Müller`_

- |Enhancement| Added `beta` parameter to
  :func:`metrics.homogeneity_completeness_v_measure` and
  :func:`metrics.v_measure_score` to configure the
  tradeoff between homogeneity and completeness.
  :pr:`13607` by :user:`Stephane Couvreur <scouvreur>` and
  and :user:`Ivan Sanchez <ivsanro1>`.

- |Fix| The metric :func:`metrics.r2_score` is degenerate with a single sample
  and now it returns NaN and raises :class:`exceptions.UndefinedMetricWarning`.
  :pr:`12855` by :user:`Pawel Sendyk <psendyk>`.

- |Fix| Fixed a bug where :func:`metrics.brier_score_loss` will sometimes
  return incorrect result when there's only one class in ``y_true``.
  :pr:`13628` by :user:`Hanmin Qin <qinhanmin2014>`.

- |Fix| Fixed a bug in :func:`metrics.label_ranking_average_precision_score`
  where sample_weight wasn't taken into account for samples with degenerate
  labels.
  :pr:`13447` by :user:`Dan Ellis <dpwe>`.

- |API| The parameter ``labels`` in :func:`metrics.hamming_loss` is deprecated
  in version 0.21 and will be removed in version 0.23. :pr:`10580` by
  :user:`Reshama Shaikh <reshamas>` and :user:`Sandra Mitrovic <SandraMNE>`.

- |Fix| The function :func:`metrics.pairwise.euclidean_distances`, and 
  therefore several estimators with ``metric='euclidean'``, suffered from 
  numerical precision issues with ``float32`` features. Precision has been 
  increased at the cost of a small drop of performance. :pr:`13554` by 
  :user:`Celelibi` and :user:`Jérémie du Boisberranger <jeremiedbb>`.

- |API| :func:`metrics.jaccard_similarity_score` is deprecated in favour of
  the more consistent :func:`metrics.jaccard_score`. The former behavior for
  binary and multiclass targets is broken.
  :pr:`13151` by `Joel Nothman`_.

:mod:`sklearn.mixture`
......................

- |Fix| Fixed a bug in :class:`mixture.BaseMixture` and therefore on estimators
  based on it, i.e. :class:`mixture.GaussianMixture` and
  :class:`mixture.BayesianGaussianMixture`, where ``fit_predict`` and
  ``fit.predict`` were not equivalent. :pr:`13142` by
  :user:`Jérémie du Boisberranger <jeremiedbb>`.


:mod:`sklearn.model_selection`
..............................

- |Feature| Classes :class:`~model_selection.GridSearchCV` and
  :class:`~model_selection.RandomizedSearchCV` now allow for refit=callable
  to add flexibility in identifying the best estimator.
  See :ref:`sphx_glr_auto_examples_model_selection_plot_grid_search_refit_callable.py`.
  :pr:`11354` by :user:`Wenhao Zhang <wenhaoz@ucla.edu>`,
  `Joel Nothman`_ and :user:`Adrin Jalali <adrinjalali>`.

- |Enhancement| Classes :class:`~model_selection.GridSearchCV`,
  :class:`~model_selection.RandomizedSearchCV`, and methods
  :func:`~model_selection.cross_val_score`,
  :func:`~model_selection.cross_val_predict`,
  :func:`~model_selection.cross_validate`, now print train scores when
  `return_train_scores` is True and `verbose` > 2. For
  :func:`~model_selection.learning_curve`, and
  :func:`~model_selection.validation_curve` only the latter is required.
  :pr:`12613` and :pr:`12669` by :user:`Marc Torrellas <marctorrellas>`.

- |Enhancement| Some :term:`CV splitter` classes and
  `model_selection.train_test_split` now raise ``ValueError`` when the
  resulting training set is empty.
  :pr:`12861` by :user:`Nicolas Hug <NicolasHug>`.

- |Fix| Fixed a bug where :class:`model_selection.StratifiedKFold`
  shuffles each class's samples with the same ``random_state``,
  making ``shuffle=True`` ineffective.
  :pr:`13124` by :user:`Hanmin Qin <qinhanmin2014>`.

- |Fix| Added ability for :func:`model_selection.cross_val_predict` to handle
  multi-label (and multioutput-multiclass) targets with ``predict_proba``-type
  methods. :pr:`8773` by :user:`Stephen Hoover <stephen-hoover>`.

- |Fix| Fixed an issue in :func:`~model_selection.cross_val_predict` where
  `method="predict_proba"` returned always `0.0` when one of the classes was
  excluded in a cross-validation fold.
  :pr:`13366` by :user:`Guillaume Fournier <gfournier>`

:mod:`sklearn.multiclass`
.........................

- |Fix| Fixed an issue in :func:`multiclass.OneVsOneClassifier.decision_function`
  where the decision_function value of a given sample was different depending on
  whether the decision_function was evaluated on the sample alone or on a batch
  containing this same sample due to the scaling used in decision_function.
  :pr:`10440` by :user:`Jonathan Ohayon <Johayon>`.

:mod:`sklearn.multioutput`
..........................

- |Fix| Fixed a bug in :class:`multioutput.MultiOutputClassifier` where the
  `predict_proba` method incorrectly checked for `predict_proba` attribute in
  the estimator object.
  :pr:`12222` by :user:`Rebekah Kim <rebekahkim>`
  
:mod:`sklearn.neighbors`
........................

- |MajorFeature| Added :class:`neighbors.NeighborhoodComponentsAnalysis` for
  metric learning, which implements the Neighborhood Components Analysis
  algorithm.  :pr:`10058` by :user:`William de Vazelhes <wdevazelhes>` and
  :user:`John Chiotellis <johny-c>`.

- |API| Methods in :class:`neighbors.NearestNeighbors` :
  :func:`~neighbors.NearestNeighbors.kneighbors`,
  :func:`~neighbors.NearestNeighbors.radius_neighbors`,
  :func:`~neighbors.NearestNeighbors.kneighbors_graph`,
  :func:`~neighbors.NearestNeighbors.radius_neighbors_graph`
  now raise ``NotFittedError``, rather than ``AttributeError``,
  when called before ``fit`` :pr:`12279` by :user:`Krishna Sangeeth
  <whiletruelearn>`.

:mod:`sklearn.neural_network`
.............................

- |Fix| Fixed a bug in :class:`neural_network.MLPClassifier` and
  :class:`neural_network.MLPRegressor` where the option :code:`shuffle=False`
  was being ignored. :pr:`12582` by :user:`Sam Waterbury <samwaterbury>`.

- |Fix| Fixed a bug in :class:`neural_network.MLPClassifier` where
  validation sets for early stopping were not sampled with stratification. In
  the multilabel case however, splits are still not stratified.
  :pr:`13164` by :user:`Nicolas Hug<NicolasHug>`.

:mod:`sklearn.pipeline`
.......................

- |Feature| :class:`pipeline.Pipeline` can now use indexing notation (e.g.
  ``my_pipeline[0:-1]``) to extract a subsequence of steps as another Pipeline
  instance.  A Pipeline can also be indexed directly to extract a particular
  step (e.g. ``my_pipeline['svc']``), rather than accessing ``named_steps``.
  :pr:`2568` by `Joel Nothman`_.

- |Feature| Added optional parameter ``verbose`` in :class:`pipeline.Pipeline`,
  :class:`compose.ColumnTransformer` and :class:`pipeline.FeatureUnion`
  and corresponding ``make_`` helpers for showing progress and timing of
  each step. :pr:`11364` by :user:`Baze Petrushev <petrushev>`,
  :user:`Karan Desai <karandesai-96>`, `Joel Nothman`_, and
  :user:`Thomas Fan <thomasjpfan>`.

- |Enhancement| :class:`pipeline.Pipeline` now supports using ``'passthrough'``
  as a transformer, with the same effect as ``None``.
  :pr:`11144` by :user:`Thomas Fan <thomasjpfan>`.

- |Enhancement| :class:`pipeline.Pipeline`  implements ``__len__`` and
  therefore ``len(pipeline)`` returns the number of steps in the pipeline.
  :pr:`13439` by :user:`Lakshya KD <LakshKD>`.

:mod:`sklearn.preprocessing`
............................

- |Feature| :class:`preprocessing.OneHotEncoder` now supports dropping one
  feature per category with a new drop parameter. :pr:`12908` by
  :user:`Drew Johnston <drewmjohnston>`.

- |Efficiency| :class:`preprocessing.OneHotEncoder` and
  :class:`preprocessing.OrdinalEncoder` now handle pandas DataFrames more
  efficiently. :pr:`13253` by :user:`maikia`.

- |Efficiency| Make :class:`preprocessing.MultiLabelBinarizer` cache class
  mappings instead of calculating it every time on the fly.
  :pr:`12116` by :user:`Ekaterina Krivich <kiote>` and `Joel Nothman`_.

- |Efficiency| :class:`preprocessing.PolynomialFeatures` now supports
  compressed sparse row (CSR) matrices as input for degrees 2 and 3. This is
  typically much faster than the dense case as it scales with matrix density
  and expansion degree (on the order of density^degree), and is much, much
  faster than the compressed sparse column (CSC) case.
  :pr:`12197` by :user:`Andrew Nystrom <awnystrom>`.

- |Efficiency| Speed improvement in :class:`preprocessing.PolynomialFeatures`,
  in the dense case. Also added a new parameter ``order`` which controls output
  order for further speed performances. :pr:`12251` by `Tom Dupre la Tour`_.

- |Fix| Fixed the calculation overflow when using a float16 dtype with
  :class:`preprocessing.StandardScaler`.
  :pr:`13007` by :user:`Raffaello Baluyot <baluyotraf>`

- |Fix| Fixed a bug in :class:`preprocessing.QuantileTransformer` and
  :func:`preprocessing.quantile_transform` to force n_quantiles to be at most
  equal to n_samples. Values of n_quantiles larger than n_samples were either
  useless or resulting in a wrong approximation of the cumulative distribution
  function estimator. :pr:`13333` by :user:`Albert Thomas <albertcthomas>`.

- |API| The default value of `copy` in :func:`preprocessing.quantile_transform`
  will change from False to True in 0.23 in order to make it more consistent
  with the default `copy` values of other functions in
  :mod:`preprocessing` and prevent unexpected side effects by modifying
  the value of `X` inplace.
  :pr:`13459` by :user:`Hunter McGushion <HunterMcGushion>`.

:mod:`sklearn.svm`
..................

- |Fix| Fixed an issue in :func:`svm.SVC.decision_function` when
  ``decision_function_shape='ovr'``. The decision_function value of a given
  sample was different depending on whether the decision_function was evaluated
  on the sample alone or on a batch containing this same sample due to the
  scaling used in decision_function.
  :pr:`10440` by :user:`Jonathan Ohayon <Johayon>`.

:mod:`sklearn.tree`
...................

- |Feature| Decision Trees can now be plotted with matplotlib using
  :func:`tree.plot_tree` without relying on the ``dot`` library,
  removing a hard-to-install dependency. :pr:`8508` by `Andreas Müller`_.

- |Feature| Decision Trees can now be exported in a human readable
  textual format using :func:`tree.export_text`.
  :pr:`6261` by `Giuseppe Vettigli <JustGlowing>`.

- |Feature| ``get_n_leaves()`` and ``get_depth()`` have been added to
  :class:`tree.BaseDecisionTree` and consequently all estimators based
  on it, including :class:`tree.DecisionTreeClassifier`,
  :class:`tree.DecisionTreeRegressor`, :class:`tree.ExtraTreeClassifier`,
  and :class:`tree.ExtraTreeRegressor`.
  :pr:`12300` by :user:`Adrin Jalali <adrinjalali>`.

- |Fix| Trees and forests did not previously `predict` multi-output
  classification targets with string labels, despite accepting them in `fit`.
  :pr:`11458` by :user:`Mitar Milutinovic <mitar>`.

- |Fix| Fixed an issue with :class:`tree.BaseDecisionTree`
  and consequently all estimators based
  on it, including :class:`tree.DecisionTreeClassifier`,
  :class:`tree.DecisionTreeRegressor`, :class:`tree.ExtraTreeClassifier`,
  and :class:`tree.ExtraTreeRegressor`, where they used to exceed the given
  ``max_depth`` by 1 while expanding the tree if ``max_leaf_nodes`` and
  ``max_depth`` were both specified by the user. Please note that this also
  affects all ensemble methods using decision trees.
  :pr:`12344` by :user:`Adrin Jalali <adrinjalali>`.

:mod:`sklearn.utils`
....................

- |Feature| :func:`utils.resample` now accepts a ``stratify`` parameter for
  sampling according to class distributions. :pr:`13549` by :user:`Nicolas
  Hug <NicolasHug>`.

- |API| Deprecated ``warn_on_dtype`` parameter from :func:`utils.check_array`
  and :func:`utils.check_X_y`. Added explicit warning for dtype conversion
  in :func:`check_pairwise_arrays` if the ``metric`` being passed is a
  pairwise boolean metric.
  :pr:`13382` by :user:`Prathmesh Savale <praths007>`.

Multiple modules
................

- |MajorFeature| The `__repr__()` method of all estimators (used when calling
  `print(estimator)`) has been entirely re-written, building on Python's
  pretty printing standard library. All parameters are printed by default,
  but this can be altered with the ``print_changed_only`` option in
  :func:`sklearn.set_config`. :pr:`11705` by :user:`Nicolas Hug
  <NicolasHug>`.

- |MajorFeature| Add estimators tags: these are annotations of estimators
  that allow programmatic inspection of their capabilities, such as sparse
  matrix support, supported output types and supported methods. Estimator
  tags also determine the tests that are run on an estimator when
  `check_estimator` is called. Read more in the :ref:`User Guide
  <estimator_tags>`. :pr:`8022` by :user:`Andreas Müller <amueller>`.

- |Efficiency| Memory copies are avoided when casting arrays to a different
  dtype in multiple estimators. :pr:`11973` by :user:`Roman Yurchak
  <rth>`.

- |Fix| Fixed a bug in the implementation of the :func:`our_rand_r`
  helper function that was not behaving consistently across platforms.
  :pr:`13422` by :user:`Madhura Parikh <jdnc>` and
  :user:`Clément Doumouro <ClemDoum>`.


Miscellaneous
.............

- |Enhancement| Joblib is no longer vendored in scikit-learn, and becomes a
  dependency. Minimal supported version is joblib 0.11, however using
  version >= 0.13 is strongly recommended.
  :pr:`13531` by :user:`Roman Yurchak <rth>`.


Changes to estimator checks
---------------------------

These changes mostly affect library developers.

- Add ``check_fit_idempotent`` to
  :func:`~utils.estimator_checks.check_estimator`, which checks that
  when `fit` is called twice with the same data, the output of
  `predict`, `predict_proba`, `transform`, and `decision_function` does not
  change. :pr:`12328` by :user:`Nicolas Hug <NicolasHug>`

- Many checks can now be disabled or configured with :ref:`estimator_tags`.
  :pr:`8022` by :user:`Andreas Müller <amueller>`.

Code and Documentation Contributors
-----------------------------------

Thanks to everyone who has contributed to the maintenance and improvement of the
project since version 0.20, including:

adanhawth, Aditya Vyas, Adrin Jalali, Agamemnon Krasoulis, Albert Thomas,
Alberto Torres, Alexandre Gramfort, amourav, Andrea Navarrete, Andreas Mueller,
Andrew Nystrom, assiaben, Aurélien Bellet, Bartosz Michałowski, Bartosz
Telenczuk, bauks, BenjaStudio, bertrandhaut, Bharat Raghunathan, brentfagan,
Bryan Woods, Cat Chenal, Cheuk Ting Ho, Chris Choe, Christos Aridas, Clément
Doumouro, Cole Smith, Connossor, Corey Levinson, Dan Ellis, Dan Stine, Danylo
Baibak, daten-kieker, Denis Kataev, Didi Bar-Zev, Dillon Gardner, Dmitry Mottl,
Dmitry Vukolov, Dougal J. Sutherland, Dowon, drewmjohnston, Dror Atariah,
Edward J Brown, Ekaterina Krivich, Elizabeth Sander, Emmanuel Arias, Eric
Chang, Eric Larson, Erich Schubert, esvhd, Falak, Feda Curic, Federico Caselli,
Frank Hoang, Fibinse Xavier`, Finn O'Shea, Gabriel Marzinotto, Gabriel Vacaliuc, 
Gabriele Calvo, Gael Varoquaux, GauravAhlawat, Giuseppe Vettigli, Greg Gandenberger,
Guillaume Fournier, Guillaume Lemaitre, Gustavo De Mari Pereira, Hanmin Qin,
haroldfox, hhu-luqi, Hunter McGushion, Ian Sanders, JackLangerman, Jacopo
Notarstefano, jakirkham, James Bourbeau, Jan Koch, Jan S, janvanrijn, Jarrod
Millman, jdethurens, jeremiedbb, JF, joaak, Joan Massich, Joel Nothman,
Jonathan Ohayon, Joris Van den Bossche, josephsalmon, Jérémie Méhault, Katrin
Leinweber, ken, kms15, Koen, Kossori Aruku, Krishna Sangeeth, Kuai Yu, Kulbear,
Kushal Chauhan, Kyle Jackson, Lakshya KD, Leandro Hermida, Lee Yi Jie Joel,
Lily Xiong, Lisa Sarah Thomas, Loic Esteve, louib, luk-f-a, maikia, mail-liam,
Manimaran, Manuel López-Ibáñez, Marc Torrellas, Marco Gaido, Marco Gorelli,
MarcoGorelli, marineLM, Mark Hannel, Martin Gubri, Masstran, mathurinm, Matthew
Roeschke, Max Copeland, melsyt, mferrari3, Mickaël Schoentgen, Ming Li, Mitar,
Mohammad Aftab, Mohammed AbdelAal, Mohammed Ibraheem, Muhammad Hassaan Rafique,
mwestt, Naoya Iijima, Nicholas Smith, Nicolas Goix, Nicolas Hug, Nikolay
Shebanov, Oleksandr Pavlyk, Oliver Rausch, Olivier Grisel, Orestis, Osman, Owen
Flanagan, Paul Paczuski, Pavel Soriano, pavlos kallis, Pawel Sendyk, peay,
Peter, Peter Cock, Peter Hausamann, Peter Marko, Pierre Glaser, pierretallotte,
Pim de Haan, Piotr Szymański, Prabakaran Kumaresshan, Pradeep Reddy Raamana,
Prathmesh Savale, Pulkit Maloo, Quentin Batista, Radostin Stoyanov, Raf
Baluyot, Rajdeep Dua, Ramil Nugmanov, Raúl García Calvo, Rebekah Kim, Reshama
Shaikh, Rohan Lekhwani, Rohan Singh, Rohan Varma, Rohit Kapoor, Roman
Feldbauer, Roman Yurchak, Romuald M, Roopam Sharma, Ryan, Rüdiger Busche, Sam
Waterbury, Samuel O. Ronsin, SandroCasagrande, Scott Cole, Scott Lowe,
Sebastian Raschka, Shangwu Yao, Shivam Kotwalia, Shiyu Duan, smarie, Sriharsha
Hatwar, Stephen Hoover, Stephen Tierney, Stéphane Couvreur, surgan12,
SylvainLan, TakingItCasual, Tashay Green, thibsej, Thomas Fan, Thomas J Fan,
Thomas Moreau, Tom Dupré la Tour, Tommy, Tulio Casagrande, Umar Farouk Umar,
Utkarsh Upadhyay, Vinayak Mehta, Vishaal Kapoor, Vivek Kumar, Vlad Niculae,
vqean3, Wenhao Zhang, William de Vazelhes, xhan, Xing Han Lu, xinyuliu12,
Yaroslav Halchenko, Zach Griffith, Zach Miller, Zayd Hammoudeh, Zhuyi Xue,
Zijie (ZJ) Poh, ^__^
