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# `sklearn.multiclass`.OneVsOneClassifier

class sklearn.multiclass.OneVsOneClassifier(estimator, *, n_jobs=None)

  One-vs-one multiclass strategy.

  This strategy consists in fitting one classifier per class pair. At prediction time, the class which received the most votes is selected. Since it requires to fit `n_classes * (n_classes - 1) / 2` classifiers, this method is usually slower than one-vs-the-rest, due to its O(n_classes^2) complexity. However, this method may be advantageous for algorithms such as kernel algorithms which don’t scale well with `n_samples`. This is because each individual learning problem only involves a small subset of the data whereas, with one-vs-the-rest, the complete dataset is used `n_classes` times.

  Read more in the User Guide.

  Parameters:
    estimator estimator object

      A regressor or a classifier that implements fit. When a classifier is passed, decision_function will be used in priority and it will fallback to predict_proba if it is not available. When a regressor is passed, predict is used.

    n_jobs int, default=None

      The number of jobs to use for the computation: the `n_classes * ( n_classes - 1) / 2` OVO problems are computed in parallel.

      `None` means 1 unless in a `joblib.parallel_backend` context. `-1` means using all processors. See Glossary for more details.

  Attributes:
    estimators_list of `n_classes * (n_classes - 1) / 2` estimators

      Estimators used for predictions.

    classes_numpy array of shape [n_classes]

      Array containing labels.

    `n_classes_`int

      Number of classes.

    pairwise_indices_list, length = `len(estimators_)`, or `None`

      Indices of samples used when training the estimators. `None` when `estimator`’s `pairwise` tag is False.

    n_features_in_int

      Number of features seen during fit.

      New in version 0.24.

    feature_names_in_ndarray of shape (`n_features_in_`,)

      Names of features seen during fit. Defined only when `X` has feature names that are all strings.

      New in version 1.0.

  See also

  `OneVsRestClassifier`

    One-vs-all multiclass strategy.

  `OutputCodeClassifier`

    (Error-Correcting) Output-Code multiclass strategy.

  Examples

      >>> from sklearn.datasets import load_iris
      >>> from sklearn.model_selection import train_test_split
      >>> from sklearn.multiclass import OneVsOneClassifier
      >>> from sklearn.svm import LinearSVC
      >>> X, y = load_iris(return_X_y=True)
      >>> X_train, X_test, y_train, y_test = train_test_split(
      ...     X, y, test_size=0.33, shuffle=True, random_state=0)
      >>> clf = OneVsOneClassifier(
      ...     LinearSVC(dual="auto", random_state=0)).fit(X_train, y_train)
      >>> clf.predict(X_test[:10])
      array([2, 1, 0, 2, 0, 2, 0, 1, 1, 1])

  Methods

  `decision_function`(X) | Decision function for the OneVsOneClassifier.
  `fit`(X, y) | Fit underlying estimators.
  `get_metadata_routing`() | Get metadata routing of this object.
  `get_params`([deep]) | Get parameters for this estimator.
  `partial_fit`(X, y[, classes]) | Partially fit underlying estimators.
  `predict`(X) | Estimate the best class label for each sample in X.
  `score`(X, y[, sample_weight]) | Return the mean accuracy on the given test data and labels.
  `set_params`(**params) | Set the parameters of this estimator.
  `set_partial_fit_request`(*[, classes]) | Request metadata passed to the `partial_fit` method.
  `set_score_request`(*[, sample_weight]) | Request metadata passed to the `score` method.

  decision_function(X)

    Decision function for the OneVsOneClassifier.

    The decision values for the samples are computed by adding the normalized sum of pair-wise classification confidence levels to the votes in order to disambiguate between the decision values when the votes for all the classes are equal leading to a tie.

    Parameters:
      X array-like of shape (n_samples, n_features)

        Input data.

    Returns:
      Y array-like of shape (n_samples, n_classes) or (n_samples,)

        Result of calling `decision_function` on the final estimator.

        Changed in version 0.19: output shape changed to `(n_samples,)` to conform to scikit-learn conventions for binary classification.

  fit(X, y)

    Fit underlying estimators.

    Parameters:
      X{array-like, sparse matrix} of shape (n_samples, n_features)

        Data.

      y array-like of shape (n_samples,)

        Multi-class targets.

    Returns:
      self object

        The fitted underlying estimator.

  get_metadata_routing()

    Get metadata routing of this object.

    Please check User Guide on how the routing mechanism works.

    Returns:
      routing MetadataRequest

        A `MetadataRequest` encapsulating routing information.

  get_params(deep=True)

    Get parameters for this estimator.

    Parameters:
      deep bool, default=True

        If True, will return the parameters for this estimator and contained subobjects that are estimators.

    Returns:
      params dict

        Parameter names mapped to their values.

  property n_classes_

    Number of classes.

  partial_fit(X, y, classes=None)

    Partially fit underlying estimators.

    Should be used when memory is inefficient to train all data. Chunks of data can be passed in several iteration, where the first call should have an array of all target variables.

    Parameters:
      X{array-like, sparse matrix) of shape (n_samples, n_features)

        Data.

      y array-like of shape (n_samples,)

        Multi-class targets.

      classes array, shape (n_classes, )

        Classes across all calls to partial_fit. Can be obtained via `np.unique(y_all)`, where y_all is the target vector of the entire dataset. This argument is only required in the first call of partial_fit and can be omitted in the subsequent calls.

    Returns:
      self object

        The partially fitted underlying estimator.

  predict(X)

    Estimate the best class label for each sample in X.

    This is implemented as `argmax(decision_function(X), axis=1)` which will return the label of the class with most votes by estimators predicting the outcome of a decision for each possible class pair.

    Parameters:
      X{array-like, sparse matrix} of shape (n_samples, n_features)

        Data.

    Returns:
      y numpy array of shape [n_samples]

        Predicted multi-class targets.

  score(X, y, sample_weight=None)

    Return the mean accuracy on the given test data and labels.

    In multi-label classification, this is the subset accuracy which is a harsh metric since you require for each sample that each label set be correctly predicted.

    Parameters:
      X array-like of shape (n_samples, n_features)

        Test samples.

      y array-like of shape (n_samples,) or (n_samples, n_outputs)

        True labels for `X`.

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

        Sample weights.

    Returns:
      score float

        Mean accuracy of `self.predict(X)` w.r.t. `y`.

  set_params(**params)

    Set the parameters of this estimator.

    The method works on simple estimators as well as on nested objects (such as `Pipeline`). The latter have parameters of the form `<component>__<parameter>` so that it’s possible to update each component of a nested object.

    Parameters:
      **params dict

        Estimator parameters.

    Returns:
      self estimator instance

        Estimator instance.

  set_partial_fit_request(*, classes: bool | None | str = '$UNCHANGED$') → OneVsOneClassifier

    Request metadata passed to the `partial_fit` method.

    Note that this method is only relevant if `enable_metadata_routing=True` (see `sklearn.set_config`). Please see User Guide on how the routing mechanism works.

    The options for each parameter are:

    - `True`: metadata is requested, and passed to `partial_fit` if provided. The request is ignored if metadata is not provided.

    - `False`: metadata is not requested and the meta-estimator will not pass it to `partial_fit`.

    - `None`: metadata is not requested, and the meta-estimator will raise an error if the user provides it.

    - `str`: metadata should be passed to the meta-estimator with this given alias instead of the original name.

    The default (`sklearn.utils.metadata_routing.UNCHANGED`) retains the existing request. This allows you to change the request for some parameters and not others.

    New in version 1.3.

    Note

    This method is only relevant if this estimator is used as a sub-estimator of a meta-estimator, e.g. used inside a `Pipeline`. Otherwise it has no effect.

    Parameters:
      classes str, True, False, or None, default=sklearn.utils.metadata_routing.UNCHANGED

        Metadata routing for `classes` parameter in `partial_fit`.

    Returns:
      self object

        The updated object.

  set_score_request(*, sample_weight: bool | None | str = '$UNCHANGED$') → OneVsOneClassifier

    Request metadata passed to the `score` method.

    Note that this method is only relevant if `enable_metadata_routing=True` (see `sklearn.set_config`). Please see User Guide on how the routing mechanism works.

    The options for each parameter are:

    - `True`: metadata is requested, and passed to `score` if provided. The request is ignored if metadata is not provided.

    - `False`: metadata is not requested and the meta-estimator will not pass it to `score`.

    - `None`: metadata is not requested, and the meta-estimator will raise an error if the user provides it.

    - `str`: metadata should be passed to the meta-estimator with this given alias instead of the original name.

    The default (`sklearn.utils.metadata_routing.UNCHANGED`) retains the existing request. This allows you to change the request for some parameters and not others.

    New in version 1.3.

    Note

    This method is only relevant if this estimator is used as a sub-estimator of a meta-estimator, e.g. used inside a `Pipeline`. Otherwise it has no effect.

    Parameters:
      sample_weight str, True, False, or None, default=sklearn.utils.metadata_routing.UNCHANGED

        Metadata routing for `sample_weight` parameter in `score`.

    Returns:
      self object

        The updated object.