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"""Utilities to get the response values of a classifier or a regressor.
It allows to make uniform checks and validation.
"""
# Authors: The scikit-learn developers
# SPDX-License-Identifier: BSD-3-Clause
import numpy as np
from ..base import is_classifier
from .multiclass import type_of_target
from .validation import _check_response_method, check_is_fitted
def _process_predict_proba(*, y_pred, target_type, classes, pos_label):
"""Get the response values when the response method is `predict_proba`.
This function process the `y_pred` array in the binary and multi-label cases.
In the binary case, it selects the column corresponding to the positive
class. In the multi-label case, it stacks the predictions if they are not
in the "compressed" format `(n_samples, n_outputs)`.
Parameters
----------
y_pred : ndarray
Output of `estimator.predict_proba`. The shape depends on the target type:
- for binary classification, it is a 2d array of shape `(n_samples, 2)`;
- for multiclass classification, it is a 2d array of shape
`(n_samples, n_classes)`;
- for multilabel classification, it is either a list of 2d arrays of shape
`(n_samples, 2)` (e.g. `RandomForestClassifier` or `KNeighborsClassifier`) or
an array of shape `(n_samples, n_outputs)` (e.g. `MLPClassifier` or
`RidgeClassifier`).
target_type : {"binary", "multiclass", "multilabel-indicator"}
Type of the target.
classes : ndarray of shape (n_classes,) or list of such arrays
Class labels as reported by `estimator.classes_`.
pos_label : int, float, bool or str
Only used with binary and multiclass targets.
Returns
-------
y_pred : ndarray of shape (n_samples,), (n_samples, n_classes) or \
(n_samples, n_output)
Compressed predictions format as requested by the metrics.
"""
if target_type == "binary" and y_pred.shape[1] < 2:
# We don't handle classifiers trained on a single class.
raise ValueError(
f"Got predict_proba of shape {y_pred.shape}, but need "
"classifier with two classes."
)
if target_type == "binary":
col_idx = np.flatnonzero(classes == pos_label)[0]
return y_pred[:, col_idx]
elif target_type == "multilabel-indicator":
# Use a compress format of shape `(n_samples, n_output)`.
# Only `MLPClassifier` and `RidgeClassifier` return an array of shape
# `(n_samples, n_outputs)`.
if isinstance(y_pred, list):
# list of arrays of shape `(n_samples, 2)`
return np.vstack([p[:, -1] for p in y_pred]).T
else:
# array of shape `(n_samples, n_outputs)`
return y_pred
return y_pred
def _process_decision_function(*, y_pred, target_type, classes, pos_label):
"""Get the response values when the response method is `decision_function`.
This function process the `y_pred` array in the binary and multi-label cases.
In the binary case, it inverts the sign of the score if the positive label
is not `classes[1]`. In the multi-label case, it stacks the predictions if
they are not in the "compressed" format `(n_samples, n_outputs)`.
Parameters
----------
y_pred : ndarray
Output of `estimator.decision_function`. The shape depends on the target type:
- for binary classification, it is a 1d array of shape `(n_samples,)` where the
sign is assuming that `classes[1]` is the positive class;
- for multiclass classification, it is a 2d array of shape
`(n_samples, n_classes)`;
- for multilabel classification, it is a 2d array of shape `(n_samples,
n_outputs)`.
target_type : {"binary", "multiclass", "multilabel-indicator"}
Type of the target.
classes : ndarray of shape (n_classes,) or list of such arrays
Class labels as reported by `estimator.classes_`.
pos_label : int, float, bool or str
Only used with binary and multiclass targets.
Returns
-------
y_pred : ndarray of shape (n_samples,), (n_samples, n_classes) or \
(n_samples, n_output)
Compressed predictions format as requested by the metrics.
"""
if target_type == "binary" and pos_label == classes[0]:
return -1 * y_pred
return y_pred
def _get_response_values(
estimator,
X,
response_method,
pos_label=None,
return_response_method_used=False,
):
"""Compute the response values of a classifier, an outlier detector, or a regressor.
The response values are predictions such that it follows the following shape:
- for binary classification, it is a 1d array of shape `(n_samples,)`;
- for multiclass classification, it is a 2d array of shape `(n_samples, n_classes)`;
- for multilabel classification, it is a 2d array of shape `(n_samples, n_outputs)`;
- for outlier detection, it is a 1d array of shape `(n_samples,)`;
- for regression, it is a 1d array of shape `(n_samples,)`.
If `estimator` is a binary classifier, also return the label for the
effective positive class.
This utility is used primarily in the displays and the scikit-learn scorers.
.. versionadded:: 1.3
Parameters
----------
estimator : estimator instance
Fitted classifier, outlier detector, or regressor or a
fitted :class:`~sklearn.pipeline.Pipeline` in which the last estimator is a
classifier, an outlier detector, or a regressor.
X : {array-like, sparse matrix} of shape (n_samples, n_features)
Input values.
response_method : {"predict_proba", "predict_log_proba", "decision_function", \
"predict"} or list of such str
Specifies the response method to use get prediction from an estimator
(i.e. :term:`predict_proba`, :term:`predict_log_proba`,
:term:`decision_function` or :term:`predict`). Possible choices are:
- if `str`, it corresponds to the name to the method to return;
- if a list of `str`, it provides the method names in order of
preference. The method returned corresponds to the first method in
the list and which is implemented by `estimator`.
pos_label : int, float, bool or str, default=None
The class considered as the positive class when computing
the metrics. If `None` and target is 'binary', `estimators.classes_[1]` is
considered as the positive class.
return_response_method_used : bool, default=False
Whether to return the response method used to compute the response
values.
.. versionadded:: 1.4
Returns
-------
y_pred : ndarray of shape (n_samples,), (n_samples, n_classes) or \
(n_samples, n_outputs)
Target scores calculated from the provided `response_method`
and `pos_label`.
pos_label : int, float, bool, str or None
The class considered as the positive class when computing
the metrics. Returns `None` if `estimator` is a regressor or an outlier
detector.
response_method_used : str
The response method used to compute the response values. Only returned
if `return_response_method_used` is `True`.
.. versionadded:: 1.4
Raises
------
ValueError
If `pos_label` is not a valid label.
If the shape of `y_pred` is not consistent for binary classifier.
If the response method can be applied to a classifier only and
`estimator` is a regressor.
"""
from sklearn.base import is_classifier, is_outlier_detector # noqa
if is_classifier(estimator):
prediction_method = _check_response_method(estimator, response_method)
classes = estimator.classes_
target_type = type_of_target(classes)
if target_type in ("binary", "multiclass"):
if pos_label is not None and pos_label not in classes.tolist():
raise ValueError(
f"pos_label={pos_label} is not a valid label: It should be "
f"one of {classes}"
)
elif pos_label is None and target_type == "binary":
pos_label = classes[-1]
y_pred = prediction_method(X)
if prediction_method.__name__ in ("predict_proba", "predict_log_proba"):
y_pred = _process_predict_proba(
y_pred=y_pred,
target_type=target_type,
classes=classes,
pos_label=pos_label,
)
elif prediction_method.__name__ == "decision_function":
y_pred = _process_decision_function(
y_pred=y_pred,
target_type=target_type,
classes=classes,
pos_label=pos_label,
)
elif is_outlier_detector(estimator):
prediction_method = _check_response_method(estimator, response_method)
y_pred, pos_label = prediction_method(X), None
else: # estimator is a regressor
if response_method != "predict":
raise ValueError(
f"{estimator.__class__.__name__} should either be a classifier to be "
f"used with response_method={response_method} or the response_method "
"should be 'predict'. Got a regressor with response_method="
f"{response_method} instead."
)
prediction_method = estimator.predict
y_pred, pos_label = prediction_method(X), None
if return_response_method_used:
return y_pred, pos_label, prediction_method.__name__
return y_pred, pos_label
def _get_response_values_binary(
estimator, X, response_method, pos_label=None, return_response_method_used=False
):
"""Compute the response values of a binary classifier.
Parameters
----------
estimator : estimator instance
Fitted classifier or a fitted :class:`~sklearn.pipeline.Pipeline`
in which the last estimator is a binary classifier.
X : {array-like, sparse matrix} of shape (n_samples, n_features)
Input values.
response_method : {'auto', 'predict_proba', 'decision_function'}
Specifies whether to use :term:`predict_proba` or
:term:`decision_function` as the target response. If set to 'auto',
:term:`predict_proba` is tried first and if it does not exist
:term:`decision_function` is tried next.
pos_label : int, float, bool or str, default=None
The class considered as the positive class when computing
the metrics. By default, `estimators.classes_[1]` is
considered as the positive class.
return_response_method_used : bool, default=False
Whether to return the response method used to compute the response
values.
.. versionadded:: 1.5
Returns
-------
y_pred : ndarray of shape (n_samples,)
Target scores calculated from the provided response_method
and pos_label.
pos_label : int, float, bool or str
The class considered as the positive class when computing
the metrics.
response_method_used : str
The response method used to compute the response values. Only returned
if `return_response_method_used` is `True`.
.. versionadded:: 1.5
"""
classification_error = "Expected 'estimator' to be a binary classifier."
check_is_fitted(estimator)
if not is_classifier(estimator):
raise ValueError(
classification_error + f" Got {estimator.__class__.__name__} instead."
)
elif len(estimator.classes_) != 2:
raise ValueError(
classification_error + f" Got {len(estimator.classes_)} classes instead."
)
if response_method == "auto":
response_method = ["predict_proba", "decision_function"]
return _get_response_values(
estimator,
X,
response_method,
pos_label=pos_label,
return_response_method_used=return_response_method_used,
)
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