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<li><a class="reference internal" href="#">Glossary of Common Terms and API Elements</a><ul>
<li><a class="reference internal" href="#general-concepts">General Concepts</a></li>
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<section id="glossary-of-common-terms-and-api-elements">
<span id="glossary"></span><h1>Glossary of Common Terms and API Elements<a class="headerlink" href="#glossary-of-common-terms-and-api-elements" title="Link to this heading">¶</a></h1>
<p>This glossary hopes to definitively represent the tacit and explicit
conventions applied in Scikit-learn and its API, while providing a reference
for users and contributors. It aims to describe the concepts and either detail
their corresponding API or link to other relevant parts of the documentation
which do so. By linking to glossary entries from the API Reference and User
Guide, we may minimize redundancy and inconsistency.</p>
<p>We begin by listing general concepts (and any that didn’t fit elsewhere), but
more specific sets of related terms are listed below:
<a class="reference internal" href="#glossary-estimator-types"><span class="std std-ref">Class APIs and Estimator Types</span></a>, <a class="reference internal" href="#glossary-target-types"><span class="std std-ref">Target Types</span></a>,
<a class="reference internal" href="#glossary-methods"><span class="std std-ref">Methods</span></a>, <a class="reference internal" href="#glossary-parameters"><span class="std std-ref">Parameters</span></a>,
<a class="reference internal" href="#glossary-attributes"><span class="std std-ref">Attributes</span></a>, <a class="reference internal" href="#glossary-sample-props"><span class="std std-ref">Data and sample properties</span></a>.</p>
<section id="general-concepts">
<h2>General Concepts<a class="headerlink" href="#general-concepts" title="Link to this heading">¶</a></h2>
<dl class="glossary">
<dt id="term-1d">1d<a class="headerlink" href="#term-1d" title="Link to this term">¶</a></dt><dt id="term-1d-array">1d array<a class="headerlink" href="#term-1d-array" title="Link to this term">¶</a></dt><dd><p>One-dimensional array. A NumPy array whose <code class="docutils literal notranslate"><span class="pre">.shape</span></code> has length 1.
A vector.</p>
</dd>
<dt id="term-2d">2d<a class="headerlink" href="#term-2d" title="Link to this term">¶</a></dt><dt id="term-2d-array">2d array<a class="headerlink" href="#term-2d-array" title="Link to this term">¶</a></dt><dd><p>Two-dimensional array. A NumPy array whose <code class="docutils literal notranslate"><span class="pre">.shape</span></code> has length 2.
Often represents a matrix.</p>
</dd>
<dt id="term-API">API<a class="headerlink" href="#term-API" title="Link to this term">¶</a></dt><dd><p>Refers to both the <em>specific</em> interfaces for estimators implemented in
Scikit-learn and the <em>generalized</em> conventions across types of
estimators as described in this glossary and <a class="reference internal" href="developers/develop.html#api-overview"><span class="std std-ref">overviewed in the
contributor documentation</span></a>.</p>
<p>The specific interfaces that constitute Scikit-learn’s public API are
largely documented in <a class="reference internal" href="modules/classes.html#api-ref"><span class="std std-ref">API Reference</span></a>. However, we less formally consider
anything as public API if none of the identifiers required to access it
begins with <code class="docutils literal notranslate"><span class="pre">_</span></code>. We generally try to maintain <a class="reference internal" href="#term-backwards-compatibility"><span class="xref std std-term">backwards
compatibility</span></a> for all objects in the public API.</p>
<p>Private API, including functions, modules and methods beginning <code class="docutils literal notranslate"><span class="pre">_</span></code>
are not assured to be stable.</p>
</dd>
<dt id="term-array-like">array-like<a class="headerlink" href="#term-array-like" title="Link to this term">¶</a></dt><dd><p>The most common data format for <em>input</em> to Scikit-learn estimators and
functions, array-like is any type object for which
<a class="reference external" href="https://numpy.org/doc/stable/reference/generated/numpy.asarray.html#numpy.asarray" title="(in NumPy v1.26)"><code class="xref py py-func docutils literal notranslate"><span class="pre">numpy.asarray</span></code></a> will produce an array of appropriate shape
(usually 1 or 2-dimensional) of appropriate dtype (usually numeric).</p>
<p>This includes:</p>
<ul class="simple">
<li><p>a numpy array</p></li>
<li><p>a list of numbers</p></li>
<li><p>a list of length-k lists of numbers for some fixed length k</p></li>
<li><p>a <a class="reference external" href="https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.html#pandas.DataFrame" title="(in pandas v2.2.2)"><code class="xref py py-class docutils literal notranslate"><span class="pre">pandas.DataFrame</span></code></a> with all columns numeric</p></li>
<li><p>a numeric <a class="reference external" href="https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.html#pandas.Series" title="(in pandas v2.2.2)"><code class="xref py py-class docutils literal notranslate"><span class="pre">pandas.Series</span></code></a></p></li>
</ul>
<p>It excludes:</p>
<ul class="simple">
<li><p>a <a class="reference internal" href="#term-sparse-matrix"><span class="xref std std-term">sparse matrix</span></a></p></li>
<li><p>a sparse array</p></li>
<li><p>an iterator</p></li>
<li><p>a generator</p></li>
</ul>
<p>Note that <em>output</em> from scikit-learn estimators and functions (e.g.
predictions) should generally be arrays or sparse matrices, or lists
thereof (as in multi-output <a class="reference internal" href="modules/generated/sklearn.tree.DecisionTreeClassifier.html#sklearn.tree.DecisionTreeClassifier" title="sklearn.tree.DecisionTreeClassifier"><code class="xref py py-class docutils literal notranslate"><span class="pre">tree.DecisionTreeClassifier</span></code></a>’s
<code class="docutils literal notranslate"><span class="pre">predict_proba</span></code>). An estimator where <code class="docutils literal notranslate"><span class="pre">predict()</span></code> returns a list or
a <code class="docutils literal notranslate"><span class="pre">pandas.Series</span></code> is not valid.</p>
</dd>
<dt id="term-attribute">attribute<a class="headerlink" href="#term-attribute" title="Link to this term">¶</a></dt><dt id="term-attributes">attributes<a class="headerlink" href="#term-attributes" title="Link to this term">¶</a></dt><dd><p>We mostly use attribute to refer to how model information is stored on
an estimator during fitting. Any public attribute stored on an
estimator instance is required to begin with an alphabetic character
and end in a single underscore if it is set in <a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a> or
<a class="reference internal" href="#term-partial_fit"><span class="xref std std-term">partial_fit</span></a>. These are what is documented under an estimator’s
<em>Attributes</em> documentation. The information stored in attributes is
usually either: sufficient statistics used for prediction or
transformation; <a class="reference internal" href="#term-transductive"><span class="xref std std-term">transductive</span></a> outputs such as <a class="reference internal" href="#term-labels_"><span class="xref std std-term">labels_</span></a> or
<a class="reference internal" href="#term-embedding_"><span class="xref std std-term">embedding_</span></a>; or diagnostic data, such as
<a class="reference internal" href="#term-feature_importances_"><span class="xref std std-term">feature_importances_</span></a>.
Common attributes are listed <a class="reference internal" href="#glossary-attributes"><span class="std std-ref">below</span></a>.</p>
<p>A public attribute may have the same name as a constructor
<a class="reference internal" href="#term-parameter"><span class="xref std std-term">parameter</span></a>, with a <code class="docutils literal notranslate"><span class="pre">_</span></code> appended. This is used to store a
validated or estimated version of the user’s input. For example,
<a class="reference internal" href="modules/generated/sklearn.decomposition.PCA.html#sklearn.decomposition.PCA" title="sklearn.decomposition.PCA"><code class="xref py py-class docutils literal notranslate"><span class="pre">decomposition.PCA</span></code></a> is constructed with an <code class="docutils literal notranslate"><span class="pre">n_components</span></code>
parameter. From this, together with other parameters and the data,
PCA estimates the attribute <code class="docutils literal notranslate"><span class="pre">n_components_</span></code>.</p>
<p>Further private attributes used in prediction/transformation/etc. may
also be set when fitting. These begin with a single underscore and are
not assured to be stable for public access.</p>
<p>A public attribute on an estimator instance that does not end in an
underscore should be the stored, unmodified value of an <code class="docutils literal notranslate"><span class="pre">__init__</span></code>
<a class="reference internal" href="#term-parameter"><span class="xref std std-term">parameter</span></a> of the same name. Because of this equivalence, these
are documented under an estimator’s <em>Parameters</em> documentation.</p>
</dd>
<dt id="term-backwards-compatibility">backwards compatibility<a class="headerlink" href="#term-backwards-compatibility" title="Link to this term">¶</a></dt><dd><p>We generally try to maintain backward compatibility (i.e. interfaces
and behaviors may be extended but not changed or removed) from release
to release but this comes with some exceptions:</p>
<dl class="simple">
<dt>Public API only</dt><dd><p>The behavior of objects accessed through private identifiers
(those beginning <code class="docutils literal notranslate"><span class="pre">_</span></code>) may be changed arbitrarily between
versions.</p>
</dd>
<dt>As documented</dt><dd><p>We will generally assume that the users have adhered to the
documented parameter types and ranges. If the documentation asks
for a list and the user gives a tuple, we do not assure consistent
behavior from version to version.</p>
</dd>
<dt>Deprecation</dt><dd><p>Behaviors may change following a <a class="reference internal" href="#term-deprecation"><span class="xref std std-term">deprecation</span></a> period
(usually two releases long). Warnings are issued using Python’s
<a class="reference external" href="https://docs.python.org/3/library/warnings.html#module-warnings" title="(in Python v3.12)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">warnings</span></code></a> module.</p>
</dd>
<dt>Keyword arguments</dt><dd><p>We may sometimes assume that all optional parameters (other than X
and y to <a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a> and similar methods) are passed as keyword
arguments only and may be positionally reordered.</p>
</dd>
<dt>Bug fixes and enhancements</dt><dd><p>Bug fixes and – less often – enhancements may change the behavior
of estimators, including the predictions of an estimator trained on
the same data and <a class="reference internal" href="#term-random_state"><span class="xref std std-term">random_state</span></a>. When this happens, we
attempt to note it clearly in the changelog.</p>
</dd>
<dt>Serialization</dt><dd><p>We make no assurances that pickling an estimator in one version
will allow it to be unpickled to an equivalent model in the
subsequent version. (For estimators in the sklearn package, we
issue a warning when this unpickling is attempted, even if it may
happen to work.) See <a class="reference internal" href="model_persistence.html#persistence-limitations"><span class="std std-ref">Security & maintainability limitations for pickle and joblib</span></a>.</p>
</dd>
<dt><a class="reference internal" href="modules/generated/sklearn.utils.estimator_checks.check_estimator.html#sklearn.utils.estimator_checks.check_estimator" title="sklearn.utils.estimator_checks.check_estimator"><code class="xref py py-func docutils literal notranslate"><span class="pre">utils.estimator_checks.check_estimator</span></code></a></dt><dd><p>We provide limited backwards compatibility assurances for the
estimator checks: we may add extra requirements on estimators
tested with this function, usually when these were informally
assumed but not formally tested.</p>
</dd>
</dl>
<p>Despite this informal contract with our users, the software is provided
as is, as stated in the license. When a release inadvertently
introduces changes that are not backward compatible, these are known
as software regressions.</p>
</dd>
<dt id="term-callable">callable<a class="headerlink" href="#term-callable" title="Link to this term">¶</a></dt><dd><p>A function, class or an object which implements the <code class="docutils literal notranslate"><span class="pre">__call__</span></code>
method; anything that returns True when the argument of <a class="reference external" href="https://docs.python.org/3/library/functions.html#callable">callable()</a>.</p>
</dd>
<dt id="term-categorical-feature">categorical feature<a class="headerlink" href="#term-categorical-feature" title="Link to this term">¶</a></dt><dd><p>A categorical or nominal <a class="reference internal" href="#term-feature"><span class="xref std std-term">feature</span></a> is one that has a
finite set of discrete values across the population of data.
These are commonly represented as columns of integers or
strings. Strings will be rejected by most scikit-learn
estimators, and integers will be treated as ordinal or
count-valued. For the use with most estimators, categorical
variables should be one-hot encoded. Notable exceptions include
tree-based models such as random forests and gradient boosting
models that often work better and faster with integer-coded
categorical variables.
<a class="reference internal" href="modules/generated/sklearn.preprocessing.OrdinalEncoder.html#sklearn.preprocessing.OrdinalEncoder" title="sklearn.preprocessing.OrdinalEncoder"><code class="xref py py-class docutils literal notranslate"><span class="pre">OrdinalEncoder</span></code></a> helps encoding
string-valued categorical features as ordinal integers, and
<a class="reference internal" href="modules/generated/sklearn.preprocessing.OneHotEncoder.html#sklearn.preprocessing.OneHotEncoder" title="sklearn.preprocessing.OneHotEncoder"><code class="xref py py-class docutils literal notranslate"><span class="pre">OneHotEncoder</span></code></a> can be used to
one-hot encode categorical features.
See also <a class="reference internal" href="modules/preprocessing.html#preprocessing-categorical-features"><span class="std std-ref">Encoding categorical features</span></a> and the
<a class="reference external" href="https://github.com/scikit-learn-contrib/category_encoders">categorical-encoding</a>
package for tools related to encoding categorical features.</p>
</dd>
<dt id="term-clone">clone<a class="headerlink" href="#term-clone" title="Link to this term">¶</a></dt><dt id="term-cloned">cloned<a class="headerlink" href="#term-cloned" title="Link to this term">¶</a></dt><dd><p>To copy an <a class="reference internal" href="#term-estimator-instance"><span class="xref std std-term">estimator instance</span></a> and create a new one with
identical <a class="reference internal" href="#term-parameters"><span class="xref std std-term">parameters</span></a>, but without any fitted
<a class="reference internal" href="#term-attributes"><span class="xref std std-term">attributes</span></a>, using <a class="reference internal" href="modules/generated/sklearn.base.clone.html#sklearn.base.clone" title="sklearn.base.clone"><code class="xref py py-func docutils literal notranslate"><span class="pre">clone</span></code></a>.</p>
<p>When <code class="docutils literal notranslate"><span class="pre">fit</span></code> is called, a <a class="reference internal" href="#term-meta-estimator"><span class="xref std std-term">meta-estimator</span></a> usually clones
a wrapped estimator instance before fitting the cloned instance.
(Exceptions, for legacy reasons, include
<a class="reference internal" href="modules/generated/sklearn.pipeline.Pipeline.html#sklearn.pipeline.Pipeline" title="sklearn.pipeline.Pipeline"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pipeline</span></code></a> and
<a class="reference internal" href="modules/generated/sklearn.pipeline.FeatureUnion.html#sklearn.pipeline.FeatureUnion" title="sklearn.pipeline.FeatureUnion"><code class="xref py py-class docutils literal notranslate"><span class="pre">FeatureUnion</span></code></a>.)</p>
<p>If the estimator’s <code class="docutils literal notranslate"><span class="pre">random_state</span></code> parameter is an integer (or if the
estimator doesn’t have a <code class="docutils literal notranslate"><span class="pre">random_state</span></code> parameter), an <em>exact clone</em>
is returned: the clone and the original estimator will give the exact
same results. Otherwise, <em>statistical clone</em> is returned: the clone
might yield different results from the original estimator. More
details can be found in <a class="reference internal" href="common_pitfalls.html#randomness"><span class="std std-ref">Controlling randomness</span></a>.</p>
</dd>
<dt id="term-common-tests">common tests<a class="headerlink" href="#term-common-tests" title="Link to this term">¶</a></dt><dd><p>This refers to the tests run on almost every estimator class in
Scikit-learn to check they comply with basic API conventions. They are
available for external use through
<a class="reference internal" href="modules/generated/sklearn.utils.estimator_checks.check_estimator.html#sklearn.utils.estimator_checks.check_estimator" title="sklearn.utils.estimator_checks.check_estimator"><code class="xref py py-func docutils literal notranslate"><span class="pre">utils.estimator_checks.check_estimator</span></code></a>, with most of the
implementation in <code class="docutils literal notranslate"><span class="pre">sklearn/utils/estimator_checks.py</span></code>.</p>
<p>Note: Some exceptions to the common testing regime are currently
hard-coded into the library, but we hope to replace this by marking
exceptional behaviours on the estimator using semantic <a class="reference internal" href="#term-estimator-tags"><span class="xref std std-term">estimator
tags</span></a>.</p>
</dd>
<dt id="term-cross-fitting">cross-fitting<a class="headerlink" href="#term-cross-fitting" title="Link to this term">¶</a></dt><dt id="term-0">cross fitting<a class="headerlink" href="#term-0" title="Link to this term">¶</a></dt><dd><p>A resampling method that iteratively partitions data into mutually
exclusive subsets to fit two stages. During the first stage, the
mutually exclusive subsets enable predictions or transformations to be
computed on data not seen during training. The computed data is then
used in the second stage. The objective is to avoid having any
overfitting in the first stage introduce bias into the input data
distribution of the second stage.
For examples of its use, see: <a class="reference internal" href="modules/generated/sklearn.preprocessing.TargetEncoder.html#sklearn.preprocessing.TargetEncoder" title="sklearn.preprocessing.TargetEncoder"><code class="xref py py-class docutils literal notranslate"><span class="pre">TargetEncoder</span></code></a>,
<a class="reference internal" href="modules/generated/sklearn.ensemble.StackingClassifier.html#sklearn.ensemble.StackingClassifier" title="sklearn.ensemble.StackingClassifier"><code class="xref py py-class docutils literal notranslate"><span class="pre">StackingClassifier</span></code></a>,
<a class="reference internal" href="modules/generated/sklearn.ensemble.StackingRegressor.html#sklearn.ensemble.StackingRegressor" title="sklearn.ensemble.StackingRegressor"><code class="xref py py-class docutils literal notranslate"><span class="pre">StackingRegressor</span></code></a> and
<a class="reference internal" href="modules/generated/sklearn.calibration.CalibratedClassifierCV.html#sklearn.calibration.CalibratedClassifierCV" title="sklearn.calibration.CalibratedClassifierCV"><code class="xref py py-class docutils literal notranslate"><span class="pre">CalibratedClassifierCV</span></code></a>.</p>
</dd>
<dt id="term-cross-validation">cross-validation<a class="headerlink" href="#term-cross-validation" title="Link to this term">¶</a></dt><dt id="term-1">cross validation<a class="headerlink" href="#term-1" title="Link to this term">¶</a></dt><dd><p>A resampling method that iteratively partitions data into mutually
exclusive ‘train’ and ‘test’ subsets so model performance can be
evaluated on unseen data. This conserves data as avoids the need to hold
out a ‘validation’ dataset and accounts for variability as multiple
rounds of cross validation are generally performed.
See <a class="reference internal" href="modules/cross_validation.html#cross-validation"><span class="std std-ref">User Guide</span></a> for more details.</p>
</dd>
<dt id="term-deprecation">deprecation<a class="headerlink" href="#term-deprecation" title="Link to this term">¶</a></dt><dd><p>We use deprecation to slowly violate our <a class="reference internal" href="#term-backwards-compatibility"><span class="xref std std-term">backwards
compatibility</span></a> assurances, usually to:</p>
<ul class="simple">
<li><p>change the default value of a parameter; or</p></li>
<li><p>remove a parameter, attribute, method, class, etc.</p></li>
</ul>
<p>We will ordinarily issue a warning when a deprecated element is used,
although there may be limitations to this. For instance, we will raise
a warning when someone sets a parameter that has been deprecated, but
may not when they access that parameter’s attribute on the estimator
instance.</p>
<p>See the <a class="reference internal" href="developers/contributing.html#contributing-deprecation"><span class="std std-ref">Contributors’ Guide</span></a>.</p>
</dd>
<dt id="term-dimensionality">dimensionality<a class="headerlink" href="#term-dimensionality" title="Link to this term">¶</a></dt><dd><p>May be used to refer to the number of <a class="reference internal" href="#term-features"><span class="xref std std-term">features</span></a> (i.e.
<a class="reference internal" href="#term-n_features"><span class="xref std std-term">n_features</span></a>), or columns in a 2d feature matrix.
Dimensions are, however, also used to refer to the length of a NumPy
array’s shape, distinguishing a 1d array from a 2d matrix.</p>
</dd>
<dt id="term-docstring">docstring<a class="headerlink" href="#term-docstring" title="Link to this term">¶</a></dt><dd><p>The embedded documentation for a module, class, function, etc., usually
in code as a string at the beginning of the object’s definition, and
accessible as the object’s <code class="docutils literal notranslate"><span class="pre">__doc__</span></code> attribute.</p>
<p>We try to adhere to <a class="reference external" href="https://www.python.org/dev/peps/pep-0257/">PEP257</a>, and follow <a class="reference external" href="https://numpydoc.readthedocs.io/en/latest/format.html">NumpyDoc
conventions</a>.</p>
</dd>
<dt id="term-double-underscore">double underscore<a class="headerlink" href="#term-double-underscore" title="Link to this term">¶</a></dt><dt id="term-double-underscore-notation">double underscore notation<a class="headerlink" href="#term-double-underscore-notation" title="Link to this term">¶</a></dt><dd><p>When specifying parameter names for nested estimators, <code class="docutils literal notranslate"><span class="pre">__</span></code> may be
used to separate between parent and child in some contexts. The most
common use is when setting parameters through a meta-estimator with
<a class="reference internal" href="#term-set_params"><span class="xref std std-term">set_params</span></a> and hence in specifying a search grid in
<a class="reference internal" href="modules/grid_search.html#grid-search"><span class="std std-ref">parameter search</span></a>. See <a class="reference internal" href="#term-parameter"><span class="xref std std-term">parameter</span></a>.
It is also used in <a class="reference internal" href="modules/generated/sklearn.pipeline.Pipeline.html#sklearn.pipeline.Pipeline.fit" title="sklearn.pipeline.Pipeline.fit"><code class="xref py py-meth docutils literal notranslate"><span class="pre">pipeline.Pipeline.fit</span></code></a> for passing
<a class="reference internal" href="#term-sample-properties"><span class="xref std std-term">sample properties</span></a> to the <code class="docutils literal notranslate"><span class="pre">fit</span></code> methods of estimators in
the pipeline.</p>
</dd>
<dt id="term-dtype">dtype<a class="headerlink" href="#term-dtype" title="Link to this term">¶</a></dt><dt id="term-data-type">data type<a class="headerlink" href="#term-data-type" title="Link to this term">¶</a></dt><dd><p>NumPy arrays assume a homogeneous data type throughout, available in
the <code class="docutils literal notranslate"><span class="pre">.dtype</span></code> attribute of an array (or sparse matrix). We generally
assume simple data types for scikit-learn data: float or integer.
We may support object or string data types for arrays before encoding
or vectorizing. Our estimators do not work with struct arrays, for
instance.</p>
<p>Our documentation can sometimes give information about the dtype
precision, e.g. <code class="docutils literal notranslate"><span class="pre">np.int32</span></code>, <code class="docutils literal notranslate"><span class="pre">np.int64</span></code>, etc. When the precision is
provided, it refers to the NumPy dtype. If an arbitrary precision is
used, the documentation will refer to dtype <code class="docutils literal notranslate"><span class="pre">integer</span></code> or <code class="docutils literal notranslate"><span class="pre">floating</span></code>.
Note that in this case, the precision can be platform dependent.
The <code class="docutils literal notranslate"><span class="pre">numeric</span></code> dtype refers to accepting both <code class="docutils literal notranslate"><span class="pre">integer</span></code> and <code class="docutils literal notranslate"><span class="pre">floating</span></code>.</p>
<p>When it comes to choosing between 64-bit dtype (i.e. <code class="docutils literal notranslate"><span class="pre">np.float64</span></code> and
<code class="docutils literal notranslate"><span class="pre">np.int64</span></code>) and 32-bit dtype (i.e. <code class="docutils literal notranslate"><span class="pre">np.float32</span></code> and <code class="docutils literal notranslate"><span class="pre">np.int32</span></code>), it
boils down to a trade-off between efficiency and precision. The 64-bit
types offer more accurate results due to their lower floating-point
error, but demand more computational resources, resulting in slower
operations and increased memory usage. In contrast, 32-bit types
promise enhanced operation speed and reduced memory consumption, but
introduce a larger floating-point error. The efficiency improvement are
dependent on lower level optimization such as like vectorization,
single instruction multiple dispatch (SIMD), or cache optimization but
crucially on the compatibility of the algorithm in use.</p>
<p>Specifically, the choice of precision should account for whether the
employed algorithm can effectively leverage <code class="docutils literal notranslate"><span class="pre">np.float32</span></code>. Some
algorithms, especially certain minimization methods, are exclusively
coded for <code class="docutils literal notranslate"><span class="pre">np.float64</span></code>, meaning that even if <code class="docutils literal notranslate"><span class="pre">np.float32</span></code> is passed, it
triggers an automatic conversion back to <code class="docutils literal notranslate"><span class="pre">np.float64</span></code>. This not only
negates the intended computational savings but also introduces
additional overhead, making operations with <code class="docutils literal notranslate"><span class="pre">np.float32</span></code> unexpectedly
slower and more memory-intensive due to this extra conversion step.</p>
</dd>
<dt id="term-duck-typing">duck typing<a class="headerlink" href="#term-duck-typing" title="Link to this term">¶</a></dt><dd><p>We try to apply <a class="reference external" href="https://en.wikipedia.org/wiki/Duck_typing">duck typing</a> to determine how to
handle some input values (e.g. checking whether a given estimator is
a classifier). That is, we avoid using <code class="docutils literal notranslate"><span class="pre">isinstance</span></code> where possible,
and rely on the presence or absence of attributes to determine an
object’s behaviour. Some nuance is required when following this
approach:</p>
<ul>
<li><p>For some estimators, an attribute may only be available once it is
<a class="reference internal" href="#term-fitted"><span class="xref std std-term">fitted</span></a>. For instance, we cannot a priori determine if
<a class="reference internal" href="#term-predict_proba"><span class="xref std std-term">predict_proba</span></a> is available in a grid search where the grid
includes alternating between a probabilistic and a non-probabilistic
predictor in the final step of the pipeline. In the following, we
can only determine if <code class="docutils literal notranslate"><span class="pre">clf</span></code> is probabilistic after fitting it on
some data:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">sklearn.model_selection</span> <span class="kn">import</span> <span class="n">GridSearchCV</span>
<span class="gp">>>> </span><span class="kn">from</span> <span class="nn">sklearn.linear_model</span> <span class="kn">import</span> <span class="n">SGDClassifier</span>
<span class="gp">>>> </span><span class="n">clf</span> <span class="o">=</span> <span class="n">GridSearchCV</span><span class="p">(</span><span class="n">SGDClassifier</span><span class="p">(),</span>
<span class="gp">... </span> <span class="n">param_grid</span><span class="o">=</span><span class="p">{</span><span class="s1">'loss'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'log_loss'</span><span class="p">,</span> <span class="s1">'hinge'</span><span class="p">]})</span>
</pre></div>
</div>
<p>This means that we can only check for duck-typed attributes after
fitting, and that we must be careful to make <a class="reference internal" href="#term-meta-estimators"><span class="xref std std-term">meta-estimators</span></a>
only present attributes according to the state of the underlying
estimator after fitting.</p>
</li>
<li><p>Checking if an attribute is present (using <code class="docutils literal notranslate"><span class="pre">hasattr</span></code>) is in general
just as expensive as getting the attribute (<code class="docutils literal notranslate"><span class="pre">getattr</span></code> or dot
notation). In some cases, getting the attribute may indeed be
expensive (e.g. for some implementations of
<a class="reference internal" href="#term-feature_importances_"><span class="xref std std-term">feature_importances_</span></a>, which may suggest this is an API design
flaw). So code which does <code class="docutils literal notranslate"><span class="pre">hasattr</span></code> followed by <code class="docutils literal notranslate"><span class="pre">getattr</span></code> should
be avoided; <code class="docutils literal notranslate"><span class="pre">getattr</span></code> within a try-except block is preferred.</p></li>
<li><p>For determining some aspects of an estimator’s expectations or
support for some feature, we use <a class="reference internal" href="#term-estimator-tags"><span class="xref std std-term">estimator tags</span></a> instead of
duck typing.</p></li>
</ul>
</dd>
<dt id="term-early-stopping">early stopping<a class="headerlink" href="#term-early-stopping" title="Link to this term">¶</a></dt><dd><p>This consists in stopping an iterative optimization method before the
convergence of the training loss, to avoid over-fitting. This is
generally done by monitoring the generalization score on a validation
set. When available, it is activated through the parameter
<code class="docutils literal notranslate"><span class="pre">early_stopping</span></code> or by setting a positive <a class="reference internal" href="#term-n_iter_no_change"><span class="xref std std-term">n_iter_no_change</span></a>.</p>
</dd>
<dt id="term-estimator-instance">estimator instance<a class="headerlink" href="#term-estimator-instance" title="Link to this term">¶</a></dt><dd><p>We sometimes use this terminology to distinguish an <a class="reference internal" href="#term-estimator"><span class="xref std std-term">estimator</span></a>
class from a constructed instance. For example, in the following,
<code class="docutils literal notranslate"><span class="pre">cls</span></code> is an estimator class, while <code class="docutils literal notranslate"><span class="pre">est1</span></code> and <code class="docutils literal notranslate"><span class="pre">est2</span></code> are
instances:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="bp">cls</span> <span class="o">=</span> <span class="n">RandomForestClassifier</span>
<span class="n">est1</span> <span class="o">=</span> <span class="bp">cls</span><span class="p">()</span>
<span class="n">est2</span> <span class="o">=</span> <span class="n">RandomForestClassifier</span><span class="p">()</span>
</pre></div>
</div>
</dd>
<dt id="term-examples">examples<a class="headerlink" href="#term-examples" title="Link to this term">¶</a></dt><dd><p>We try to give examples of basic usage for most functions and
classes in the API:</p>
<ul class="simple">
<li><p>as doctests in their docstrings (i.e. within the <code class="docutils literal notranslate"><span class="pre">sklearn/</span></code> library
code itself).</p></li>
<li><p>as examples in the <a class="reference internal" href="auto_examples/index.html#general-examples"><span class="std std-ref">example gallery</span></a>
rendered (using <a class="reference external" href="https://sphinx-gallery.readthedocs.io/">sphinx-gallery</a>) from scripts in the
<code class="docutils literal notranslate"><span class="pre">examples/</span></code> directory, exemplifying key features or parameters
of the estimator/function. These should also be referenced from the
User Guide.</p></li>
<li><p>sometimes in the <a class="reference internal" href="user_guide.html#user-guide"><span class="std std-ref">User Guide</span></a> (built from <code class="docutils literal notranslate"><span class="pre">doc/</span></code>)
alongside a technical description of the estimator.</p></li>
</ul>
</dd>
<dt id="term-experimental">experimental<a class="headerlink" href="#term-experimental" title="Link to this term">¶</a></dt><dd><p>An experimental tool is already usable but its public API, such as
default parameter values or fitted attributes, is still subject to
change in future versions without the usual <a class="reference internal" href="#term-deprecation"><span class="xref std std-term">deprecation</span></a>
warning policy.</p>
</dd>
<dt id="term-evaluation-metric">evaluation metric<a class="headerlink" href="#term-evaluation-metric" title="Link to this term">¶</a></dt><dt id="term-evaluation-metrics">evaluation metrics<a class="headerlink" href="#term-evaluation-metrics" title="Link to this term">¶</a></dt><dd><p>Evaluation metrics give a measure of how well a model performs. We may
use this term specifically to refer to the functions in <a class="reference internal" href="modules/classes.html#module-sklearn.metrics" title="sklearn.metrics"><code class="xref py py-mod docutils literal notranslate"><span class="pre">metrics</span></code></a>
(disregarding <a class="reference internal" href="modules/classes.html#module-sklearn.metrics.pairwise" title="sklearn.metrics.pairwise"><code class="xref py py-mod docutils literal notranslate"><span class="pre">pairwise</span></code></a>), as distinct from the
<a class="reference internal" href="#term-score"><span class="xref std std-term">score</span></a> method and the <a class="reference internal" href="#term-scoring"><span class="xref std std-term">scoring</span></a> API used in cross
validation. See <a class="reference internal" href="modules/model_evaluation.html#model-evaluation"><span class="std std-ref">Metrics and scoring: quantifying the quality of predictions</span></a>.</p>
<p>These functions usually accept a ground truth (or the raw data
where the metric evaluates clustering without a ground truth) and a
prediction, be it the output of <a class="reference internal" href="#term-predict"><span class="xref std std-term">predict</span></a> (<code class="docutils literal notranslate"><span class="pre">y_pred</span></code>),
of <a class="reference internal" href="#term-predict_proba"><span class="xref std std-term">predict_proba</span></a> (<code class="docutils literal notranslate"><span class="pre">y_proba</span></code>), or of an arbitrary score
function including <a class="reference internal" href="#term-decision_function"><span class="xref std std-term">decision_function</span></a> (<code class="docutils literal notranslate"><span class="pre">y_score</span></code>).
Functions are usually named to end with <code class="docutils literal notranslate"><span class="pre">_score</span></code> if a greater
score indicates a better model, and <code class="docutils literal notranslate"><span class="pre">_loss</span></code> if a lesser score
indicates a better model. This diversity of interface motivates
the scoring API.</p>
<p>Note that some estimators can calculate metrics that are not included
in <a class="reference internal" href="modules/classes.html#module-sklearn.metrics" title="sklearn.metrics"><code class="xref py py-mod docutils literal notranslate"><span class="pre">metrics</span></code></a> and are estimator-specific, notably model
likelihoods.</p>
</dd>
<dt id="term-estimator-tags">estimator tags<a class="headerlink" href="#term-estimator-tags" title="Link to this term">¶</a></dt><dd><p>A proposed feature (e.g. <a class="reference external" href="https://github.com/scikit-learn/scikit-learn/issues/8022">#8022</a>) by which the capabilities of an
estimator are described through a set of semantic tags. This would
enable some runtime behaviors based on estimator inspection, but it
also allows each estimator to be tested for appropriate invariances
while being excepted from other <a class="reference internal" href="#term-common-tests"><span class="xref std std-term">common tests</span></a>.</p>
<p>Some aspects of estimator tags are currently determined through
the <a class="reference internal" href="#term-duck-typing"><span class="xref std std-term">duck typing</span></a> of methods like <code class="docutils literal notranslate"><span class="pre">predict_proba</span></code> and through
some special attributes on estimator objects:</p>
<dl class="simple glossary">
<dt id="term-_estimator_type"><code class="docutils literal notranslate"><span class="pre">_estimator_type</span></code><a class="headerlink" href="#term-_estimator_type" title="Link to this term">¶</a></dt><dd><p>This string-valued attribute identifies an estimator as being a
classifier, regressor, etc. It is set by mixins such as
<a class="reference internal" href="modules/generated/sklearn.base.ClassifierMixin.html#sklearn.base.ClassifierMixin" title="sklearn.base.ClassifierMixin"><code class="xref py py-class docutils literal notranslate"><span class="pre">base.ClassifierMixin</span></code></a>, but needs to be more explicitly
adopted on a <a class="reference internal" href="#term-meta-estimator"><span class="xref std std-term">meta-estimator</span></a>. Its value should usually be
checked by way of a helper such as <a class="reference internal" href="modules/generated/sklearn.base.is_classifier.html#sklearn.base.is_classifier" title="sklearn.base.is_classifier"><code class="xref py py-func docutils literal notranslate"><span class="pre">base.is_classifier</span></code></a>.</p>
</dd>
</dl>
<p>For more detailed info, see <a class="reference internal" href="developers/develop.html#estimator-tags"><span class="std std-ref">Estimator Tags</span></a>.</p>
</dd>
<dt id="term-feature">feature<a class="headerlink" href="#term-feature" title="Link to this term">¶</a></dt><dt id="term-features">features<a class="headerlink" href="#term-features" title="Link to this term">¶</a></dt><dt id="term-feature-vector">feature vector<a class="headerlink" href="#term-feature-vector" title="Link to this term">¶</a></dt><dd><p>In the abstract, a feature is a function (in its mathematical sense)
mapping a sampled object to a numeric or categorical quantity.
“Feature” is also commonly used to refer to these quantities, being the
individual elements of a vector representing a sample. In a data
matrix, features are represented as columns: each column contains the
result of applying a feature function to a set of samples.</p>
<p>Elsewhere features are known as attributes, predictors, regressors, or
independent variables.</p>
<p>Nearly all estimators in scikit-learn assume that features are numeric,
finite and not missing, even when they have semantically distinct
domains and distributions (categorical, ordinal, count-valued,
real-valued, interval). See also <a class="reference internal" href="#term-categorical-feature"><span class="xref std std-term">categorical feature</span></a> and
<a class="reference internal" href="#term-missing-values"><span class="xref std std-term">missing values</span></a>.</p>
<p><code class="docutils literal notranslate"><span class="pre">n_features</span></code> indicates the number of features in a dataset.</p>
</dd>
<dt id="term-fitting">fitting<a class="headerlink" href="#term-fitting" title="Link to this term">¶</a></dt><dd><p>Calling <a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a> (or <a class="reference internal" href="#term-fit_transform"><span class="xref std std-term">fit_transform</span></a>, <a class="reference internal" href="#term-fit_predict"><span class="xref std std-term">fit_predict</span></a>,
etc.) on an estimator.</p>
</dd>
<dt id="term-fitted">fitted<a class="headerlink" href="#term-fitted" title="Link to this term">¶</a></dt><dd><p>The state of an estimator after <a class="reference internal" href="#term-fitting"><span class="xref std std-term">fitting</span></a>.</p>
<p>There is no conventional procedure for checking if an estimator
is fitted. However, an estimator that is not fitted:</p>
<ul class="simple">
<li><p>should raise <a class="reference internal" href="modules/generated/sklearn.exceptions.NotFittedError.html#sklearn.exceptions.NotFittedError" title="sklearn.exceptions.NotFittedError"><code class="xref py py-class docutils literal notranslate"><span class="pre">exceptions.NotFittedError</span></code></a> when a prediction
method (<a class="reference internal" href="#term-predict"><span class="xref std std-term">predict</span></a>, <a class="reference internal" href="#term-transform"><span class="xref std std-term">transform</span></a>, etc.) is called.
(<a class="reference internal" href="modules/generated/sklearn.utils.validation.check_is_fitted.html#sklearn.utils.validation.check_is_fitted" title="sklearn.utils.validation.check_is_fitted"><code class="xref py py-func docutils literal notranslate"><span class="pre">utils.validation.check_is_fitted</span></code></a> is used internally
for this purpose.)</p></li>
<li><p>should not have any <a class="reference internal" href="#term-attributes"><span class="xref std std-term">attributes</span></a> beginning with an alphabetic
character and ending with an underscore. (Note that a descriptor for
the attribute may still be present on the class, but hasattr should
return False)</p></li>
</ul>
</dd>
<dt id="term-function">function<a class="headerlink" href="#term-function" title="Link to this term">¶</a></dt><dd><p>We provide ad hoc function interfaces for many algorithms, while
<a class="reference internal" href="#term-estimator"><span class="xref std std-term">estimator</span></a> classes provide a more consistent interface.</p>
<p>In particular, Scikit-learn may provide a function interface that fits
a model to some data and returns the learnt model parameters, as in
<a class="reference internal" href="modules/generated/sklearn.linear_model.enet_path.html#sklearn.linear_model.enet_path" title="sklearn.linear_model.enet_path"><code class="xref py py-func docutils literal notranslate"><span class="pre">linear_model.enet_path</span></code></a>. For transductive models, this also
returns the embedding or cluster labels, as in
<a class="reference internal" href="modules/generated/sklearn.manifold.spectral_embedding.html#sklearn.manifold.spectral_embedding" title="sklearn.manifold.spectral_embedding"><code class="xref py py-func docutils literal notranslate"><span class="pre">manifold.spectral_embedding</span></code></a> or <a class="reference internal" href="modules/generated/dbscan-function.html#sklearn.cluster.dbscan" title="sklearn.cluster.dbscan"><code class="xref py py-func docutils literal notranslate"><span class="pre">cluster.dbscan</span></code></a>. Many
preprocessing transformers also provide a function interface, akin to
calling <a class="reference internal" href="#term-fit_transform"><span class="xref std std-term">fit_transform</span></a>, as in
<a class="reference internal" href="modules/generated/sklearn.preprocessing.maxabs_scale.html#sklearn.preprocessing.maxabs_scale" title="sklearn.preprocessing.maxabs_scale"><code class="xref py py-func docutils literal notranslate"><span class="pre">preprocessing.maxabs_scale</span></code></a>. Users should be careful to avoid
<a class="reference internal" href="#term-data-leakage"><span class="xref std std-term">data leakage</span></a> when making use of these
<code class="docutils literal notranslate"><span class="pre">fit_transform</span></code>-equivalent functions.</p>
<p>We do not have a strict policy about when to or when not to provide
function forms of estimators, but maintainers should consider
consistency with existing interfaces, and whether providing a function
would lead users astray from best practices (as regards data leakage,
etc.)</p>
</dd>
<dt id="term-gallery">gallery<a class="headerlink" href="#term-gallery" title="Link to this term">¶</a></dt><dd><p>See <a class="reference internal" href="#term-examples"><span class="xref std std-term">examples</span></a>.</p>
</dd>
<dt id="term-hyperparameter">hyperparameter<a class="headerlink" href="#term-hyperparameter" title="Link to this term">¶</a></dt><dt id="term-hyper-parameter">hyper-parameter<a class="headerlink" href="#term-hyper-parameter" title="Link to this term">¶</a></dt><dd><p>See <a class="reference internal" href="#term-parameter"><span class="xref std std-term">parameter</span></a>.</p>
</dd>
<dt id="term-impute">impute<a class="headerlink" href="#term-impute" title="Link to this term">¶</a></dt><dt id="term-imputation">imputation<a class="headerlink" href="#term-imputation" title="Link to this term">¶</a></dt><dd><p>Most machine learning algorithms require that their inputs have no
<a class="reference internal" href="#term-missing-values"><span class="xref std std-term">missing values</span></a>, and will not work if this requirement is
violated. Algorithms that attempt to fill in (or impute) missing values
are referred to as imputation algorithms.</p>
</dd>
<dt id="term-indexable">indexable<a class="headerlink" href="#term-indexable" title="Link to this term">¶</a></dt><dd><p>An <a class="reference internal" href="#term-array-like"><span class="xref std std-term">array-like</span></a>, <a class="reference internal" href="#term-sparse-matrix"><span class="xref std std-term">sparse matrix</span></a>, pandas DataFrame or
sequence (usually a list).</p>
</dd>
<dt id="term-induction">induction<a class="headerlink" href="#term-induction" title="Link to this term">¶</a></dt><dt id="term-inductive">inductive<a class="headerlink" href="#term-inductive" title="Link to this term">¶</a></dt><dd><p>Inductive (contrasted with <a class="reference internal" href="#term-transductive"><span class="xref std std-term">transductive</span></a>) machine learning
builds a model of some data that can then be applied to new instances.
Most estimators in Scikit-learn are inductive, having <a class="reference internal" href="#term-predict"><span class="xref std std-term">predict</span></a>
and/or <a class="reference internal" href="#term-transform"><span class="xref std std-term">transform</span></a> methods.</p>
</dd>
<dt id="term-joblib">joblib<a class="headerlink" href="#term-joblib" title="Link to this term">¶</a></dt><dd><p>A Python library (<a class="reference external" href="https://joblib.readthedocs.io">https://joblib.readthedocs.io</a>) used in Scikit-learn to
facilite simple parallelism and caching. Joblib is oriented towards
efficiently working with numpy arrays, such as through use of
<a class="reference internal" href="#term-memory-mapping"><span class="xref std std-term">memory mapping</span></a>. See <a class="reference internal" href="computing/parallelism.html#parallelism"><span class="std std-ref">Parallelism</span></a> for more
information.</p>
</dd>
<dt id="term-label-indicator-matrix">label indicator matrix<a class="headerlink" href="#term-label-indicator-matrix" title="Link to this term">¶</a></dt><dt id="term-multilabel-indicator-matrix">multilabel indicator matrix<a class="headerlink" href="#term-multilabel-indicator-matrix" title="Link to this term">¶</a></dt><dt id="term-multilabel-indicator-matrices">multilabel indicator matrices<a class="headerlink" href="#term-multilabel-indicator-matrices" title="Link to this term">¶</a></dt><dd><p>The format used to represent multilabel data, where each row of a 2d
array or sparse matrix corresponds to a sample, each column
corresponds to a class, and each element is 1 if the sample is labeled
with the class and 0 if not.</p>
</dd>
<dt id="term-leakage">leakage<a class="headerlink" href="#term-leakage" title="Link to this term">¶</a></dt><dt id="term-data-leakage">data leakage<a class="headerlink" href="#term-data-leakage" title="Link to this term">¶</a></dt><dd><p>A problem in cross validation where generalization performance can be
over-estimated since knowledge of the test data was inadvertently
included in training a model. This is a risk, for instance, when
applying a <a class="reference internal" href="#term-transformer"><span class="xref std std-term">transformer</span></a> to the entirety of a dataset rather
than each training portion in a cross validation split.</p>
<p>We aim to provide interfaces (such as <a class="reference internal" href="modules/classes.html#module-sklearn.pipeline" title="sklearn.pipeline"><code class="xref py py-mod docutils literal notranslate"><span class="pre">pipeline</span></code></a> and
<a class="reference internal" href="modules/classes.html#module-sklearn.model_selection" title="sklearn.model_selection"><code class="xref py py-mod docutils literal notranslate"><span class="pre">model_selection</span></code></a>) that shield the user from data leakage.</p>
</dd>
<dt id="term-memmapping">memmapping<a class="headerlink" href="#term-memmapping" title="Link to this term">¶</a></dt><dt id="term-memory-map">memory map<a class="headerlink" href="#term-memory-map" title="Link to this term">¶</a></dt><dt id="term-memory-mapping">memory mapping<a class="headerlink" href="#term-memory-mapping" title="Link to this term">¶</a></dt><dd><p>A memory efficiency strategy that keeps data on disk rather than
copying it into main memory. Memory maps can be created for arrays
that can be read, written, or both, using <a class="reference external" href="https://numpy.org/doc/stable/reference/generated/numpy.memmap.html#numpy.memmap" title="(in NumPy v1.26)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">numpy.memmap</span></code></a>. When
using <a class="reference internal" href="#term-joblib"><span class="xref std std-term">joblib</span></a> to parallelize operations in Scikit-learn, it
may automatically memmap large arrays to reduce memory duplication
overhead in multiprocessing.</p>
</dd>
<dt id="term-missing-values">missing values<a class="headerlink" href="#term-missing-values" title="Link to this term">¶</a></dt><dd><p>Most Scikit-learn estimators do not work with missing values. When they
do (e.g. in <a class="reference internal" href="modules/generated/sklearn.impute.SimpleImputer.html#sklearn.impute.SimpleImputer" title="sklearn.impute.SimpleImputer"><code class="xref py py-class docutils literal notranslate"><span class="pre">impute.SimpleImputer</span></code></a>), NaN is the preferred
representation of missing values in float arrays. If the array has
integer dtype, NaN cannot be represented. For this reason, we support
specifying another <code class="docutils literal notranslate"><span class="pre">missing_values</span></code> value when <a class="reference internal" href="#term-imputation"><span class="xref std std-term">imputation</span></a> or
learning can be performed in integer space.
<a class="reference internal" href="#term-unlabeled-data"><span class="xref std std-term">Unlabeled data</span></a> is a special case of missing
values in the <a class="reference internal" href="#term-target"><span class="xref std std-term">target</span></a>.</p>
</dd>
<dt id="term-n_features"><code class="docutils literal notranslate"><span class="pre">n_features</span></code><a class="headerlink" href="#term-n_features" title="Link to this term">¶</a></dt><dd><p>The number of <a class="reference internal" href="#term-features"><span class="xref std std-term">features</span></a>.</p>
</dd>
<dt id="term-n_outputs"><code class="docutils literal notranslate"><span class="pre">n_outputs</span></code><a class="headerlink" href="#term-n_outputs" title="Link to this term">¶</a></dt><dd><p>The number of <a class="reference internal" href="#term-outputs"><span class="xref std std-term">outputs</span></a> in the <a class="reference internal" href="#term-target"><span class="xref std std-term">target</span></a>.</p>
</dd>
<dt id="term-n_samples"><code class="docutils literal notranslate"><span class="pre">n_samples</span></code><a class="headerlink" href="#term-n_samples" title="Link to this term">¶</a></dt><dd><p>The number of <a class="reference internal" href="#term-samples"><span class="xref std std-term">samples</span></a>.</p>
</dd>
<dt id="term-n_targets"><code class="docutils literal notranslate"><span class="pre">n_targets</span></code><a class="headerlink" href="#term-n_targets" title="Link to this term">¶</a></dt><dd><p>Synonym for <a class="reference internal" href="#term-n_outputs"><span class="xref std std-term">n_outputs</span></a>.</p>
</dd>
<dt id="term-narrative-docs">narrative docs<a class="headerlink" href="#term-narrative-docs" title="Link to this term">¶</a></dt><dt id="term-narrative-documentation">narrative documentation<a class="headerlink" href="#term-narrative-documentation" title="Link to this term">¶</a></dt><dd><p>An alias for <a class="reference internal" href="user_guide.html#user-guide"><span class="std std-ref">User Guide</span></a>, i.e. documentation written
in <code class="docutils literal notranslate"><span class="pre">doc/modules/</span></code>. Unlike the <a class="reference internal" href="modules/classes.html#api-ref"><span class="std std-ref">API reference</span></a> provided
through docstrings, the User Guide aims to:</p>
<ul class="simple">
<li><p>group tools provided by Scikit-learn together thematically or in
terms of usage;</p></li>
<li><p>motivate why someone would use each particular tool, often through
comparison;</p></li>
<li><p>provide both intuitive and technical descriptions of tools;</p></li>
<li><p>provide or link to <a class="reference internal" href="#term-examples"><span class="xref std std-term">examples</span></a> of using key features of a
tool.</p></li>
</ul>
</dd>
<dt id="term-np">np<a class="headerlink" href="#term-np" title="Link to this term">¶</a></dt><dd><p>A shorthand for Numpy due to the conventional import statement:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
</pre></div>
</div>
</dd>
<dt id="term-online-learning">online learning<a class="headerlink" href="#term-online-learning" title="Link to this term">¶</a></dt><dd><p>Where a model is iteratively updated by receiving each batch of ground
truth <a class="reference internal" href="#term-targets"><span class="xref std std-term">targets</span></a> soon after making predictions on corresponding
batch of data. Intrinsically, the model must be usable for prediction
after each batch. See <a class="reference internal" href="#term-partial_fit"><span class="xref std std-term">partial_fit</span></a>.</p>
</dd>
<dt id="term-out-of-core">out-of-core<a class="headerlink" href="#term-out-of-core" title="Link to this term">¶</a></dt><dd><p>An efficiency strategy where not all the data is stored in main memory
at once, usually by performing learning on batches of data. See
<a class="reference internal" href="#term-partial_fit"><span class="xref std std-term">partial_fit</span></a>.</p>
</dd>
<dt id="term-outputs">outputs<a class="headerlink" href="#term-outputs" title="Link to this term">¶</a></dt><dd><p>Individual scalar/categorical variables per sample in the
<a class="reference internal" href="#term-target"><span class="xref std std-term">target</span></a>. For example, in multilabel classification each
possible label corresponds to a binary output. Also called <em>responses</em>,
<em>tasks</em> or <em>targets</em>.
See <a class="reference internal" href="#term-multiclass-multioutput"><span class="xref std std-term">multiclass multioutput</span></a> and <a class="reference internal" href="#term-continuous-multioutput"><span class="xref std std-term">continuous multioutput</span></a>.</p>
</dd>
<dt id="term-pair">pair<a class="headerlink" href="#term-pair" title="Link to this term">¶</a></dt><dd><p>A tuple of length two.</p>
</dd>
<dt id="term-parameter">parameter<a class="headerlink" href="#term-parameter" title="Link to this term">¶</a></dt><dt id="term-parameters">parameters<a class="headerlink" href="#term-parameters" title="Link to this term">¶</a></dt><dt id="term-param">param<a class="headerlink" href="#term-param" title="Link to this term">¶</a></dt><dt id="term-params">params<a class="headerlink" href="#term-params" title="Link to this term">¶</a></dt><dd><p>We mostly use <em>parameter</em> to refer to the aspects of an estimator that
can be specified in its construction. For example, <code class="docutils literal notranslate"><span class="pre">max_depth</span></code> and
<code class="docutils literal notranslate"><span class="pre">random_state</span></code> are parameters of <a class="reference internal" href="modules/generated/sklearn.ensemble.RandomForestClassifier.html#sklearn.ensemble.RandomForestClassifier" title="sklearn.ensemble.RandomForestClassifier"><code class="xref py py-class docutils literal notranslate"><span class="pre">RandomForestClassifier</span></code></a>.
Parameters to an estimator’s constructor are stored unmodified as
attributes on the estimator instance, and conventionally start with an
alphabetic character and end with an alphanumeric character. Each
estimator’s constructor parameters are described in the estimator’s
docstring.</p>
<p>We do not use parameters in the statistical sense, where parameters are
values that specify a model and can be estimated from data. What we
call parameters might be what statisticians call hyperparameters to the
model: aspects for configuring model structure that are often not
directly learnt from data. However, our parameters are also used to
prescribe modeling operations that do not affect the learnt model, such
as <a class="reference internal" href="#term-n_jobs"><span class="xref std std-term">n_jobs</span></a> for controlling parallelism.</p>
<p>When talking about the parameters of a <a class="reference internal" href="#term-meta-estimator"><span class="xref std std-term">meta-estimator</span></a>, we may
also be including the parameters of the estimators wrapped by the
meta-estimator. Ordinarily, these nested parameters are denoted by
using a <a class="reference internal" href="#term-double-underscore"><span class="xref std std-term">double underscore</span></a> (<code class="docutils literal notranslate"><span class="pre">__</span></code>) to separate between the
estimator-as-parameter and its parameter. Thus <code class="docutils literal notranslate"><span class="pre">clf</span> <span class="pre">=</span>
<span class="pre">BaggingClassifier(estimator=DecisionTreeClassifier(max_depth=3))</span></code>
has a deep parameter <code class="docutils literal notranslate"><span class="pre">estimator__max_depth</span></code> with value <code class="docutils literal notranslate"><span class="pre">3</span></code>,
which is accessible with <code class="docutils literal notranslate"><span class="pre">clf.estimator.max_depth</span></code> or
<code class="docutils literal notranslate"><span class="pre">clf.get_params()['estimator__max_depth']</span></code>.</p>
<p>The list of parameters and their current values can be retrieved from
an <a class="reference internal" href="#term-estimator-instance"><span class="xref std std-term">estimator instance</span></a> using its <a class="reference internal" href="#term-get_params"><span class="xref std std-term">get_params</span></a> method.</p>
<p>Between construction and fitting, parameters may be modified using
<a class="reference internal" href="#term-set_params"><span class="xref std std-term">set_params</span></a>. To enable this, parameters are not ordinarily
validated or altered when the estimator is constructed, or when each
parameter is set. Parameter validation is performed when <a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a> is
called.</p>
<p>Common parameters are listed <a class="reference internal" href="#glossary-parameters"><span class="std std-ref">below</span></a>.</p>
</dd>
<dt id="term-pairwise-metric">pairwise metric<a class="headerlink" href="#term-pairwise-metric" title="Link to this term">¶</a></dt><dt id="term-pairwise-metrics">pairwise metrics<a class="headerlink" href="#term-pairwise-metrics" title="Link to this term">¶</a></dt><dd><p>In its broad sense, a pairwise metric defines a function for measuring
similarity or dissimilarity between two samples (with each ordinarily
represented as a <a class="reference internal" href="#term-feature-vector"><span class="xref std std-term">feature vector</span></a>). We particularly provide
implementations of distance metrics (as well as improper metrics like
Cosine Distance) through <a class="reference internal" href="modules/generated/sklearn.metrics.pairwise_distances.html#sklearn.metrics.pairwise_distances" title="sklearn.metrics.pairwise_distances"><code class="xref py py-func docutils literal notranslate"><span class="pre">metrics.pairwise_distances</span></code></a>, and of
kernel functions (a constrained class of similarity functions) in
<a class="reference internal" href="modules/generated/sklearn.metrics.pairwise.pairwise_kernels.html#sklearn.metrics.pairwise.pairwise_kernels" title="sklearn.metrics.pairwise.pairwise_kernels"><code class="xref py py-func docutils literal notranslate"><span class="pre">metrics.pairwise.pairwise_kernels</span></code></a>. These can compute pairwise distance
matrices that are symmetric and hence store data redundantly.</p>
<p>See also <a class="reference internal" href="#term-precomputed"><span class="xref std std-term">precomputed</span></a> and <a class="reference internal" href="#term-metric"><span class="xref std std-term">metric</span></a>.</p>
<p>Note that for most distance metrics, we rely on implementations from
<a class="reference external" href="https://docs.scipy.org/doc/scipy/reference/spatial.distance.html#module-scipy.spatial.distance" title="(in SciPy v1.13.0)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">scipy.spatial.distance</span></code></a>, but may reimplement for efficiency in
our context. The <a class="reference internal" href="modules/generated/sklearn.metrics.DistanceMetric.html#sklearn.metrics.DistanceMetric" title="sklearn.metrics.DistanceMetric"><code class="xref py py-class docutils literal notranslate"><span class="pre">metrics.DistanceMetric</span></code></a> interface is used to implement
distance metrics for integration with efficient neighbors search.</p>
</dd>
<dt id="term-pd">pd<a class="headerlink" href="#term-pd" title="Link to this term">¶</a></dt><dd><p>A shorthand for <a class="reference external" href="https://pandas.pydata.org">Pandas</a> due to the
conventional import statement:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">pandas</span> <span class="k">as</span> <span class="nn">pd</span>
</pre></div>
</div>
</dd>
<dt id="term-precomputed">precomputed<a class="headerlink" href="#term-precomputed" title="Link to this term">¶</a></dt><dd><p>Where algorithms rely on <a class="reference internal" href="#term-pairwise-metrics"><span class="xref std std-term">pairwise metrics</span></a>, and can be computed
from pairwise metrics alone, we often allow the user to specify that
the <a class="reference internal" href="#term-X"><span class="xref std std-term">X</span></a> provided is already in the pairwise (dis)similarity
space, rather than in a feature space. That is, when passed to
<a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a>, it is a square, symmetric matrix, with each vector
indicating (dis)similarity to every sample, and when passed to
prediction/transformation methods, each row corresponds to a testing
sample and each column to a training sample.</p>
<p>Use of precomputed X is usually indicated by setting a <code class="docutils literal notranslate"><span class="pre">metric</span></code>,
<code class="docutils literal notranslate"><span class="pre">affinity</span></code> or <code class="docutils literal notranslate"><span class="pre">kernel</span></code> parameter to the string ‘precomputed’. If
this is the case, then the estimator should set the <code class="docutils literal notranslate"><span class="pre">pairwise</span></code>
estimator tag as True.</p>
</dd>
<dt id="term-rectangular">rectangular<a class="headerlink" href="#term-rectangular" title="Link to this term">¶</a></dt><dd><p>Data that can be represented as a matrix with <a class="reference internal" href="#term-samples"><span class="xref std std-term">samples</span></a> on the
first axis and a fixed, finite set of <a class="reference internal" href="#term-features"><span class="xref std std-term">features</span></a> on the second
is called rectangular.</p>
<p>This term excludes samples with non-vectorial structures, such as text,
an image of arbitrary size, a time series of arbitrary length, a set of
vectors, etc. The purpose of a <a class="reference internal" href="#term-vectorizer"><span class="xref std std-term">vectorizer</span></a> is to produce
rectangular forms of such data.</p>
</dd>
<dt id="term-sample">sample<a class="headerlink" href="#term-sample" title="Link to this term">¶</a></dt><dt id="term-samples">samples<a class="headerlink" href="#term-samples" title="Link to this term">¶</a></dt><dd><p>We usually use this term as a noun to indicate a single feature vector.
Elsewhere a sample is called an instance, data point, or observation.
<code class="docutils literal notranslate"><span class="pre">n_samples</span></code> indicates the number of samples in a dataset, being the
number of rows in a data array <a class="reference internal" href="#term-X"><span class="xref std std-term">X</span></a>.</p>
</dd>
<dt id="term-sample-property">sample property<a class="headerlink" href="#term-sample-property" title="Link to this term">¶</a></dt><dt id="term-sample-properties">sample properties<a class="headerlink" href="#term-sample-properties" title="Link to this term">¶</a></dt><dd><p>A sample property is data for each sample (e.g. an array of length
n_samples) passed to an estimator method or a similar function,
alongside but distinct from the <a class="reference internal" href="#term-features"><span class="xref std std-term">features</span></a> (<code class="docutils literal notranslate"><span class="pre">X</span></code>) and
<a class="reference internal" href="#term-target"><span class="xref std std-term">target</span></a> (<code class="docutils literal notranslate"><span class="pre">y</span></code>). The most prominent example is
<a class="reference internal" href="#term-sample_weight"><span class="xref std std-term">sample_weight</span></a>; see others at <a class="reference internal" href="#glossary-sample-props"><span class="std std-ref">Data and sample properties</span></a>.</p>
<p>As of version 0.19 we do not have a consistent approach to handling
sample properties and their routing in <a class="reference internal" href="#term-meta-estimators"><span class="xref std std-term">meta-estimators</span></a>, though
a <code class="docutils literal notranslate"><span class="pre">fit_params</span></code> parameter is often used.</p>
</dd>
<dt id="term-scikit-learn-contrib">scikit-learn-contrib<a class="headerlink" href="#term-scikit-learn-contrib" title="Link to this term">¶</a></dt><dd><p>A venue for publishing Scikit-learn-compatible libraries that are
broadly authorized by the core developers and the contrib community,
but not maintained by the core developer team.
See <a class="reference external" href="https://scikit-learn-contrib.github.io">https://scikit-learn-contrib.github.io</a>.</p>
</dd>
<dt id="term-scikit-learn-enhancement-proposals">scikit-learn enhancement proposals<a class="headerlink" href="#term-scikit-learn-enhancement-proposals" title="Link to this term">¶</a></dt><dt id="term-SLEP">SLEP<a class="headerlink" href="#term-SLEP" title="Link to this term">¶</a></dt><dt id="term-SLEPs">SLEPs<a class="headerlink" href="#term-SLEPs" title="Link to this term">¶</a></dt><dd><p>Changes to the API principles and changes to dependencies or supported
versions happen via a <a class="reference internal" href="governance.html#slep"><span class="std std-ref">SLEP</span></a> and follows the
decision-making process outlined in <a class="reference internal" href="governance.html#governance"><span class="std std-ref">Scikit-learn governance and decision-making</span></a>.
For all votes, a proposal must have been made public and discussed before the
vote. Such a proposal must be a consolidated document, in the form of a
“Scikit-Learn Enhancement Proposal” (SLEP), rather than a long discussion on an
issue. A SLEP must be submitted as a pull-request to
<a class="reference external" href="https://scikit-learn-enhancement-proposals.readthedocs.io">enhancement proposals</a> using the
<a class="reference external" href="https://scikit-learn-enhancement-proposals.readthedocs.io/en/latest/slep_template.html">SLEP template</a>.</p>
</dd>
<dt id="term-semi-supervised">semi-supervised<a class="headerlink" href="#term-semi-supervised" title="Link to this term">¶</a></dt><dt id="term-semi-supervised-learning">semi-supervised learning<a class="headerlink" href="#term-semi-supervised-learning" title="Link to this term">¶</a></dt><dt id="term-semisupervised">semisupervised<a class="headerlink" href="#term-semisupervised" title="Link to this term">¶</a></dt><dd><p>Learning where the expected prediction (label or ground truth) is only
available for some samples provided as training data when
<a class="reference internal" href="#term-fitting"><span class="xref std std-term">fitting</span></a> the model. We conventionally apply the label <code class="docutils literal notranslate"><span class="pre">-1</span></code>
to <a class="reference internal" href="#term-unlabeled"><span class="xref std std-term">unlabeled</span></a> samples in semi-supervised classification.</p>
</dd>
<dt id="term-sparse-matrix">sparse matrix<a class="headerlink" href="#term-sparse-matrix" title="Link to this term">¶</a></dt><dt id="term-sparse-graph">sparse graph<a class="headerlink" href="#term-sparse-graph" title="Link to this term">¶</a></dt><dd><p>A representation of two-dimensional numeric data that is more memory
efficient the corresponding dense numpy array where almost all elements
are zero. We use the <a class="reference external" href="https://docs.scipy.org/doc/scipy/reference/sparse.html#module-scipy.sparse" title="(in SciPy v1.13.0)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">scipy.sparse</span></code></a> framework, which provides
several underlying sparse data representations, or <em>formats</em>.
Some formats are more efficient than others for particular tasks, and
when a particular format provides especial benefit, we try to document
this fact in Scikit-learn parameter descriptions.</p>
<p>Some sparse matrix formats (notably CSR, CSC, COO and LIL) distinguish
between <em>implicit</em> and <em>explicit</em> zeros. Explicit zeros are stored
(i.e. they consume memory in a <code class="docutils literal notranslate"><span class="pre">data</span></code> array) in the data structure,
while implicit zeros correspond to every element not otherwise defined
in explicit storage.</p>
<p>Two semantics for sparse matrices are used in Scikit-learn:</p>
<dl class="simple">
<dt>matrix semantics</dt><dd><p>The sparse matrix is interpreted as an array with implicit and
explicit zeros being interpreted as the number 0. This is the
interpretation most often adopted, e.g. when sparse matrices
are used for feature matrices or <a class="reference internal" href="#term-multilabel-indicator-matrices"><span class="xref std std-term">multilabel indicator
matrices</span></a>.</p>
</dd>
<dt>graph semantics</dt><dd><p>As with <a class="reference external" href="https://docs.scipy.org/doc/scipy/reference/sparse.csgraph.html#module-scipy.sparse.csgraph" title="(in SciPy v1.13.0)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">scipy.sparse.csgraph</span></code></a>, explicit zeros are
interpreted as the number 0, but implicit zeros indicate a masked
or absent value, such as the absence of an edge between two
vertices of a graph, where an explicit value indicates an edge’s
weight. This interpretation is adopted to represent connectivity
in clustering, in representations of nearest neighborhoods
(e.g. <a class="reference internal" href="modules/generated/sklearn.neighbors.kneighbors_graph.html#sklearn.neighbors.kneighbors_graph" title="sklearn.neighbors.kneighbors_graph"><code class="xref py py-func docutils literal notranslate"><span class="pre">neighbors.kneighbors_graph</span></code></a>), and for precomputed
distance representation where only distances in the neighborhood
of each point are required.</p>
</dd>
</dl>
<p>When working with sparse matrices, we assume that it is sparse for a
good reason, and avoid writing code that densifies a user-provided
sparse matrix, instead maintaining sparsity or raising an error if not
possible (i.e. if an estimator does not / cannot support sparse
matrices).</p>
</dd>
<dt id="term-stateless">stateless<a class="headerlink" href="#term-stateless" title="Link to this term">¶</a></dt><dd><p>An estimator is stateless if it does not store any information that is
obtained during <a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a>. This information can be either parameters
learned during <a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a> or statistics computed from the
training data. An estimator is stateless if it has no <a class="reference internal" href="#term-attributes"><span class="xref std std-term">attributes</span></a>
apart from ones set in <code class="docutils literal notranslate"><span class="pre">__init__</span></code>. Calling <a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a> for these
estimators will only validate the public <a class="reference internal" href="#term-attributes"><span class="xref std std-term">attributes</span></a> passed
in <code class="docutils literal notranslate"><span class="pre">__init__</span></code>.</p>
</dd>
<dt id="term-supervised">supervised<a class="headerlink" href="#term-supervised" title="Link to this term">¶</a></dt><dt id="term-supervised-learning">supervised learning<a class="headerlink" href="#term-supervised-learning" title="Link to this term">¶</a></dt><dd><p>Learning where the expected prediction (label or ground truth) is
available for each sample when <a class="reference internal" href="#term-fitting"><span class="xref std std-term">fitting</span></a> the model, provided as
<a class="reference internal" href="#term-y"><span class="xref std std-term">y</span></a>. This is the approach taken in a <a class="reference internal" href="#term-classifier"><span class="xref std std-term">classifier</span></a> or
<a class="reference internal" href="#term-regressor"><span class="xref std std-term">regressor</span></a> among other estimators.</p>
</dd>
<dt id="term-target">target<a class="headerlink" href="#term-target" title="Link to this term">¶</a></dt><dt id="term-targets">targets<a class="headerlink" href="#term-targets" title="Link to this term">¶</a></dt><dd><p>The <em>dependent variable</em> in <a class="reference internal" href="#term-supervised"><span class="xref std std-term">supervised</span></a> (and
<a class="reference internal" href="#term-semisupervised"><span class="xref std std-term">semisupervised</span></a>) learning, passed as <a class="reference internal" href="#term-y"><span class="xref std std-term">y</span></a> to an estimator’s
<a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a> method. Also known as <em>dependent variable</em>, <em>outcome
variable</em>, <em>response variable</em>, <em>ground truth</em> or <em>label</em>. Scikit-learn
works with targets that have minimal structure: a class from a finite
set, a finite real-valued number, multiple classes, or multiple
numbers. See <a class="reference internal" href="#glossary-target-types"><span class="std std-ref">Target Types</span></a>.</p>
</dd>
<dt id="term-transduction">transduction<a class="headerlink" href="#term-transduction" title="Link to this term">¶</a></dt><dt id="term-transductive">transductive<a class="headerlink" href="#term-transductive" title="Link to this term">¶</a></dt><dd><p>A transductive (contrasted with <a class="reference internal" href="#term-inductive"><span class="xref std std-term">inductive</span></a>) machine learning
method is designed to model a specific dataset, but not to apply that
model to unseen data. Examples include <a class="reference internal" href="modules/generated/sklearn.manifold.TSNE.html#sklearn.manifold.TSNE" title="sklearn.manifold.TSNE"><code class="xref py py-class docutils literal notranslate"><span class="pre">manifold.TSNE</span></code></a>,
<a class="reference internal" href="modules/generated/sklearn.cluster.AgglomerativeClustering.html#sklearn.cluster.AgglomerativeClustering" title="sklearn.cluster.AgglomerativeClustering"><code class="xref py py-class docutils literal notranslate"><span class="pre">cluster.AgglomerativeClustering</span></code></a> and
<a class="reference internal" href="modules/generated/sklearn.neighbors.LocalOutlierFactor.html#sklearn.neighbors.LocalOutlierFactor" title="sklearn.neighbors.LocalOutlierFactor"><code class="xref py py-class docutils literal notranslate"><span class="pre">neighbors.LocalOutlierFactor</span></code></a>.</p>
</dd>
<dt id="term-unlabeled">unlabeled<a class="headerlink" href="#term-unlabeled" title="Link to this term">¶</a></dt><dt id="term-unlabeled-data">unlabeled data<a class="headerlink" href="#term-unlabeled-data" title="Link to this term">¶</a></dt><dd><p>Samples with an unknown ground truth when fitting; equivalently,
<a class="reference internal" href="#term-missing-values"><span class="xref std std-term">missing values</span></a> in the <a class="reference internal" href="#term-target"><span class="xref std std-term">target</span></a>. See also
<a class="reference internal" href="#term-semisupervised"><span class="xref std std-term">semisupervised</span></a> and <a class="reference internal" href="#term-unsupervised"><span class="xref std std-term">unsupervised</span></a> learning.</p>
</dd>
<dt id="term-unsupervised">unsupervised<a class="headerlink" href="#term-unsupervised" title="Link to this term">¶</a></dt><dt id="term-unsupervised-learning">unsupervised learning<a class="headerlink" href="#term-unsupervised-learning" title="Link to this term">¶</a></dt><dd><p>Learning where the expected prediction (label or ground truth) is not
available for each sample when <a class="reference internal" href="#term-fitting"><span class="xref std std-term">fitting</span></a> the model, as in
<a class="reference internal" href="#term-clusterers"><span class="xref std std-term">clusterers</span></a> and <a class="reference internal" href="#term-outlier-detectors"><span class="xref std std-term">outlier detectors</span></a>. Unsupervised
estimators ignore any <a class="reference internal" href="#term-y"><span class="xref std std-term">y</span></a> passed to <a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a>.</p>
</dd>
</dl>
</section>
<section id="class-apis-and-estimator-types">
<span id="glossary-estimator-types"></span><h2>Class APIs and Estimator Types<a class="headerlink" href="#class-apis-and-estimator-types" title="Link to this heading">¶</a></h2>
<dl class="glossary">
<dt id="term-classifier">classifier<a class="headerlink" href="#term-classifier" title="Link to this term">¶</a></dt><dt id="term-classifiers">classifiers<a class="headerlink" href="#term-classifiers" title="Link to this term">¶</a></dt><dd><p>A <a class="reference internal" href="#term-supervised"><span class="xref std std-term">supervised</span></a> (or <a class="reference internal" href="#term-semi-supervised"><span class="xref std std-term">semi-supervised</span></a>) <a class="reference internal" href="#term-predictor"><span class="xref std std-term">predictor</span></a>
with a finite set of discrete possible output values.</p>
<p>A classifier supports modeling some of <a class="reference internal" href="#term-binary"><span class="xref std std-term">binary</span></a>,
<a class="reference internal" href="#term-multiclass"><span class="xref std std-term">multiclass</span></a>, <a class="reference internal" href="#term-multilabel"><span class="xref std std-term">multilabel</span></a>, or <a class="reference internal" href="#term-multiclass-multioutput"><span class="xref std std-term">multiclass
multioutput</span></a> targets. Within scikit-learn, all classifiers support
multi-class classification, defaulting to using a one-vs-rest
strategy over the binary classification problem.</p>
<p>Classifiers must store a <a class="reference internal" href="#term-classes_"><span class="xref std std-term">classes_</span></a> attribute after fitting,
and usually inherit from <a class="reference internal" href="modules/generated/sklearn.base.ClassifierMixin.html#sklearn.base.ClassifierMixin" title="sklearn.base.ClassifierMixin"><code class="xref py py-class docutils literal notranslate"><span class="pre">base.ClassifierMixin</span></code></a>, which sets
their <a class="reference internal" href="#term-_estimator_type"><span class="xref std std-term">_estimator_type</span></a> attribute.</p>
<p>A classifier can be distinguished from other estimators with
<a class="reference internal" href="modules/generated/sklearn.base.is_classifier.html#sklearn.base.is_classifier" title="sklearn.base.is_classifier"><code class="xref py py-func docutils literal notranslate"><span class="pre">is_classifier</span></code></a>.</p>
<p>A classifier must implement:</p>
<ul class="simple">
<li><p><a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a></p></li>
<li><p><a class="reference internal" href="#term-predict"><span class="xref std std-term">predict</span></a></p></li>
<li><p><a class="reference internal" href="#term-score"><span class="xref std std-term">score</span></a></p></li>
</ul>
<p>It may also be appropriate to implement <a class="reference internal" href="#term-decision_function"><span class="xref std std-term">decision_function</span></a>,
<a class="reference internal" href="#term-predict_proba"><span class="xref std std-term">predict_proba</span></a> and <a class="reference internal" href="#term-predict_log_proba"><span class="xref std std-term">predict_log_proba</span></a>.</p>
</dd>
<dt id="term-clusterer">clusterer<a class="headerlink" href="#term-clusterer" title="Link to this term">¶</a></dt><dt id="term-clusterers">clusterers<a class="headerlink" href="#term-clusterers" title="Link to this term">¶</a></dt><dd><p>A <a class="reference internal" href="#term-unsupervised"><span class="xref std std-term">unsupervised</span></a> <a class="reference internal" href="#term-predictor"><span class="xref std std-term">predictor</span></a> with a finite set of discrete
output values.</p>
<p>A clusterer usually stores <a class="reference internal" href="#term-labels_"><span class="xref std std-term">labels_</span></a> after fitting, and must do
so if it is <a class="reference internal" href="#term-transductive"><span class="xref std std-term">transductive</span></a>.</p>
<p>A clusterer must implement:</p>
<ul class="simple">
<li><p><a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a></p></li>
<li><p><a class="reference internal" href="#term-fit_predict"><span class="xref std std-term">fit_predict</span></a> if <a class="reference internal" href="#term-transductive"><span class="xref std std-term">transductive</span></a></p></li>
<li><p><a class="reference internal" href="#term-predict"><span class="xref std std-term">predict</span></a> if <a class="reference internal" href="#term-inductive"><span class="xref std std-term">inductive</span></a></p></li>
</ul>
</dd>
<dt id="term-density-estimator">density estimator<a class="headerlink" href="#term-density-estimator" title="Link to this term">¶</a></dt><dd><p>An <a class="reference internal" href="#term-unsupervised"><span class="xref std std-term">unsupervised</span></a> estimation of input probability density
function. Commonly used techniques are:</p>
<ul class="simple">
<li><p><a class="reference internal" href="modules/density.html#kernel-density"><span class="std std-ref">Kernel Density Estimation</span></a> - uses a kernel function, controlled by the
bandwidth parameter to represent density;</p></li>
<li><p><a class="reference internal" href="modules/mixture.html#mixture"><span class="std std-ref">Gaussian mixture</span></a> - uses mixture of Gaussian models
to represent density.</p></li>
</ul>
</dd>
<dt id="term-estimator">estimator<a class="headerlink" href="#term-estimator" title="Link to this term">¶</a></dt><dt id="term-estimators">estimators<a class="headerlink" href="#term-estimators" title="Link to this term">¶</a></dt><dd><p>An object which manages the estimation and decoding of a model. The
model is estimated as a deterministic function of:</p>
<ul class="simple">
<li><p><a class="reference internal" href="#term-parameters"><span class="xref std std-term">parameters</span></a> provided in object construction or with
<a class="reference internal" href="#term-set_params"><span class="xref std std-term">set_params</span></a>;</p></li>
<li><p>the global <a class="reference external" href="https://numpy.org/doc/stable/reference/random/index.html#module-numpy.random" title="(in NumPy v1.26)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">numpy.random</span></code></a> random state if the estimator’s
<a class="reference internal" href="#term-random_state"><span class="xref std std-term">random_state</span></a> parameter is set to None; and</p></li>
<li><p>any data or <a class="reference internal" href="#term-sample-properties"><span class="xref std std-term">sample properties</span></a> passed to the most recent
call to <a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a>, <a class="reference internal" href="#term-fit_transform"><span class="xref std std-term">fit_transform</span></a> or <a class="reference internal" href="#term-fit_predict"><span class="xref std std-term">fit_predict</span></a>,
or data similarly passed in a sequence of calls to
<a class="reference internal" href="#term-partial_fit"><span class="xref std std-term">partial_fit</span></a>.</p></li>
</ul>
<p>The estimated model is stored in public and private <a class="reference internal" href="#term-attributes"><span class="xref std std-term">attributes</span></a>
on the estimator instance, facilitating decoding through prediction
and transformation methods.</p>
<p>Estimators must provide a <a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a> method, and should provide
<a class="reference internal" href="#term-set_params"><span class="xref std std-term">set_params</span></a> and <a class="reference internal" href="#term-get_params"><span class="xref std std-term">get_params</span></a>, although these are usually
provided by inheritance from <a class="reference internal" href="modules/generated/sklearn.base.BaseEstimator.html#sklearn.base.BaseEstimator" title="sklearn.base.BaseEstimator"><code class="xref py py-class docutils literal notranslate"><span class="pre">base.BaseEstimator</span></code></a>.</p>
<p>The core functionality of some estimators may also be available as a
<a class="reference internal" href="#term-function"><span class="xref std std-term">function</span></a>.</p>
</dd>
<dt id="term-feature-extractor">feature extractor<a class="headerlink" href="#term-feature-extractor" title="Link to this term">¶</a></dt><dt id="term-feature-extractors">feature extractors<a class="headerlink" href="#term-feature-extractors" title="Link to this term">¶</a></dt><dd><p>A <a class="reference internal" href="#term-transformer"><span class="xref std std-term">transformer</span></a> which takes input where each sample is not
represented as an <a class="reference internal" href="#term-array-like"><span class="xref std std-term">array-like</span></a> object of fixed length, and
produces an <a class="reference internal" href="#term-array-like"><span class="xref std std-term">array-like</span></a> object of <a class="reference internal" href="#term-features"><span class="xref std std-term">features</span></a> for each
sample (and thus a 2-dimensional array-like for a set of samples). In
other words, it (lossily) maps a non-rectangular data representation
into <a class="reference internal" href="#term-rectangular"><span class="xref std std-term">rectangular</span></a> data.</p>
<p>Feature extractors must implement at least:</p>
<ul class="simple">
<li><p><a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a></p></li>
<li><p><a class="reference internal" href="#term-transform"><span class="xref std std-term">transform</span></a></p></li>
<li><p><a class="reference internal" href="#term-get_feature_names_out"><span class="xref std std-term">get_feature_names_out</span></a></p></li>
</ul>
</dd>
<dt id="term-meta-estimator">meta-estimator<a class="headerlink" href="#term-meta-estimator" title="Link to this term">¶</a></dt><dt id="term-meta-estimators">meta-estimators<a class="headerlink" href="#term-meta-estimators" title="Link to this term">¶</a></dt><dt id="term-metaestimator">metaestimator<a class="headerlink" href="#term-metaestimator" title="Link to this term">¶</a></dt><dt id="term-metaestimators">metaestimators<a class="headerlink" href="#term-metaestimators" title="Link to this term">¶</a></dt><dd><p>An <a class="reference internal" href="#term-estimator"><span class="xref std std-term">estimator</span></a> which takes another estimator as a parameter.
Examples include <a class="reference internal" href="modules/generated/sklearn.pipeline.Pipeline.html#sklearn.pipeline.Pipeline" title="sklearn.pipeline.Pipeline"><code class="xref py py-class docutils literal notranslate"><span class="pre">pipeline.Pipeline</span></code></a>,
<a class="reference internal" href="modules/generated/sklearn.model_selection.GridSearchCV.html#sklearn.model_selection.GridSearchCV" title="sklearn.model_selection.GridSearchCV"><code class="xref py py-class docutils literal notranslate"><span class="pre">model_selection.GridSearchCV</span></code></a>,
<a class="reference internal" href="modules/generated/sklearn.feature_selection.SelectFromModel.html#sklearn.feature_selection.SelectFromModel" title="sklearn.feature_selection.SelectFromModel"><code class="xref py py-class docutils literal notranslate"><span class="pre">feature_selection.SelectFromModel</span></code></a> and
<a class="reference internal" href="modules/generated/sklearn.ensemble.BaggingClassifier.html#sklearn.ensemble.BaggingClassifier" title="sklearn.ensemble.BaggingClassifier"><code class="xref py py-class docutils literal notranslate"><span class="pre">ensemble.BaggingClassifier</span></code></a>.</p>
<p>In a meta-estimator’s <a class="reference internal" href="#term-fit"><span class="xref std std-term">fit</span></a> method, any contained estimators
should be <a class="reference internal" href="#term-cloned"><span class="xref std std-term">cloned</span></a> before they are fit (although FIXME: Pipeline
and FeatureUnion do not do this currently). An exception to this is
that an estimator may explicitly document that it accepts a pre-fitted
estimator (e.g. using <code class="docutils literal notranslate"><span class="pre">prefit=True</span></code> in
<a class="reference internal" href="modules/generated/sklearn.feature_selection.SelectFromModel.html#sklearn.feature_selection.SelectFromModel" title="sklearn.feature_selection.SelectFromModel"><code class="xref py py-class docutils literal notranslate"><span class="pre">feature_selection.SelectFromModel</span></code></a>). One known issue with this
is that the pre-fitted estimator will lose its model if the
meta-estimator is cloned. A meta-estimator should have <code class="docutils literal notranslate"><span class="pre">fit</span></code> called
before prediction, even if all contained estimators are pre-fitted.</p>
<p>In cases where a meta-estimator’s primary behaviors (e.g.
<a class="reference internal" href="#term-predict"><span class="xref std std-term">predict</span></a> or <a class="reference internal" href="#term-transform"><span class="xref std std-term">transform</span></a> implementation) are functions of
prediction/transformation methods of the provided <em>base estimator</em> (or
multiple base estimators), a meta-estimator should provide at least the
standard methods provided by the base estimator. It may not be
possible to identify which methods are provided by the underlying
estimator until the meta-estimator has been <a class="reference internal" href="#term-fitted"><span class="xref std std-term">fitted</span></a> (see also
<a class="reference internal" href="#term-duck-typing"><span class="xref std std-term">duck typing</span></a>), for which
<a class="reference internal" href="modules/generated/sklearn.utils.metaestimators.available_if.html#sklearn.utils.metaestimators.available_if" title="sklearn.utils.metaestimators.available_if"><code class="xref py py-func docutils literal notranslate"><span class="pre">utils.metaestimators.available_if</span></code></a> may help. It
should also provide (or modify) the <a class="reference internal" href="#term-estimator-tags"><span class="xref std std-term">estimator tags</span></a> and
<a class="reference internal" href="#term-classes_"><span class="xref std std-term">classes_</span></a> attribute provided by the base estimator.</p>
<p>Meta-estimators should be careful to validate data as minimally as
possible before passing it to an underlying estimator. This saves
computation time, and may, for instance, allow the underlying
estimator to easily work with data that is not <a class="reference internal" href="#term-rectangular"><span class="xref std std-term">rectangular</span></a>.</p>
</dd>