# Authors: Nicolas Goix <nicolas.goix@telecom-paristech.fr>
# Alexandre Gramfort <alexandre.gramfort@telecom-paristech.fr>
# License: BSD 3 clause
import numbers
import numpy as np
from scipy.sparse import issparse
from warnings import warn
from ..tree import ExtraTreeRegressor
from ..utils import (
check_random_state,
check_array,
gen_batches,
get_chunk_n_rows,
)
from ..utils.fixes import _joblib_parallel_args
from ..utils.validation import check_is_fitted, _num_samples
from ..utils.validation import _deprecate_positional_args
from ..base import OutlierMixin
from ._bagging import BaseBagging
__all__ = ["IsolationForest"]
class IsolationForest(OutlierMixin, BaseBagging):
"""
Isolation Forest Algorithm.
Return the anomaly score of each sample using the IsolationForest algorithm
The IsolationForest 'isolates' observations by randomly selecting a feature
and then randomly selecting a split value between the maximum and minimum
values of the selected feature.
Since recursive partitioning can be represented by a tree structure, the
number of splittings required to isolate a sample is equivalent to the path
length from the root node to the terminating node.
This path length, averaged over a forest of such random trees, is a
measure of normality and our decision function.
Random partitioning produces noticeably shorter paths for anomalies.
Hence, when a forest of random trees collectively produce shorter path
lengths for particular samples, they are highly likely to be anomalies.
Read more in the :ref:`User Guide <isolation_forest>`.
.. versionadded:: 0.18
Parameters
----------
n_estimators : int, default=100
The number of base estimators in the ensemble.
max_samples : "auto", int or float, default="auto"
The number of samples to draw from X to train each base estimator.
- If int, then draw `max_samples` samples.
- If float, then draw `max_samples * X.shape[0]` samples.
- If "auto", then `max_samples=min(256, n_samples)`.
If max_samples is larger than the number of samples provided,
all samples will be used for all trees (no sampling).
contamination : 'auto' or float, default='auto'
The amount of contamination of the data set, i.e. the proportion
of outliers in the data set. Used when fitting to define the threshold
on the scores of the samples.
- If 'auto', the threshold is determined as in the
original paper.
- If float, the contamination should be in the range [0, 0.5].
.. versionchanged:: 0.22
The default value of ``contamination`` changed from 0.1
to ``'auto'``.
max_features : int or float, default=1.0
The number of features to draw from X to train each base estimator.
- If int, then draw `max_features` features.
- If float, then draw `max_features * X.shape[1]` features.
bootstrap : bool, default=False
If True, individual trees are fit on random subsets of the training
data sampled with replacement. If False, sampling without replacement
is performed.
n_jobs : int, default=None
The number of jobs to run in parallel for both :meth:`fit` and
:meth:`predict`. ``None`` means 1 unless in a
:obj:`joblib.parallel_backend` context. ``-1`` means using all
processors. See :term:`Glossary <n_jobs>` for more details.
behaviour : str, default='deprecated'
This parameter has no effect, is deprecated, and will be removed.
.. versionadded:: 0.20
``behaviour`` is added in 0.20 for back-compatibility purpose.
.. deprecated:: 0.20
``behaviour='old'`` is deprecated in 0.20 and will not be possible
in 0.22.
.. deprecated:: 0.22
``behaviour`` parameter is deprecated in 0.22 and removed in
0.24.
random_state : int or RandomState, default=None
Controls the pseudo-randomness of the selection of the feature
and split values for each branching step and each tree in the forest.
Pass an int for reproducible results across multiple function calls.
See :term:`Glossary <random_state>`.
verbose : int, default=0
Controls the verbosity of the tree building process.
warm_start : bool, default=False
When set to ``True``, reuse the solution of the previous call to fit
and add more estimators to the ensemble, otherwise, just fit a whole
new forest. See :term:`the Glossary <warm_start>`.
.. versionadded:: 0.21
Attributes
----------
estimators_ : list of DecisionTreeClassifier
The collection of fitted sub-estimators.
estimators_samples_ : list of arrays
The subset of drawn samples (i.e., the in-bag samples) for each base
estimator.
max_samples_ : int
The actual number of samples.
offset_ : float
Offset used to define the decision function from the raw scores. We
have the relation: ``decision_function = score_samples - offset_``.
``offset_`` is defined as follows. When the contamination parameter is
set to "auto", the offset is equal to -0.5 as the scores of inliers are
close to 0 and the scores of outliers are close to -1. When a
contamination parameter different than "auto" is provided, the offset
is defined in such a way we obtain the expected number of outliers
(samples with decision function < 0) in training.
.. versionadded:: 0.20
estimators_features_ : list of arrays
The subset of drawn features for each base estimator.
Notes
-----
The implementation is based on an ensemble of ExtraTreeRegressor. The
maximum depth of each tree is set to ``ceil(log_2(n))`` where
:math:`n` is the number of samples used to build the tree
(see (Liu et al., 2008) for more details).
References
----------
.. [1] Liu, Fei Tony, Ting, Kai Ming and Zhou, Zhi-Hua. "Isolation forest."
Data Mining, 2008. ICDM'08. Eighth IEEE International Conference on.
.. [2] Liu, Fei Tony, Ting, Kai Ming and Zhou, Zhi-Hua. "Isolation-based
anomaly detection." ACM Transactions on Knowledge Discovery from
Data (TKDD) 6.1 (2012): 3.
See Also
----------
sklearn.covariance.EllipticEnvelope : An object for detecting outliers in a
Gaussian distributed dataset.
sklearn.svm.OneClassSVM : Unsupervised Outlier Detection.
Estimate the support of a high-dimensional distribution.
The implementation is based on libsvm.
sklearn.neighbors.LocalOutlierFactor : Unsupervised Outlier Detection
using Local Outlier Factor (LOF).
Examples
--------
>>> from sklearn.ensemble import IsolationForest
>>> X = [[-1.1], [0.3], [0.5], [100]]
>>> clf = IsolationForest(random_state=0).fit(X)
>>> clf.predict([[0.1], [0], [90]])
array([ 1, 1, -1])
"""
@_deprecate_positional_args
def __init__(self, *,
n_estimators=100,
max_samples="auto",
contamination="auto",
max_features=1.,
bootstrap=False,
n_jobs=None,
behaviour='deprecated',
random_state=None,
verbose=0,
warm_start=False):
super().__init__(
base_estimator=ExtraTreeRegressor(
max_features=1,
splitter='random',
random_state=random_state),
# here above max_features has no links with self.max_features
bootstrap=bootstrap,
bootstrap_features=False,
n_estimators=n_estimators,
max_samples=max_samples,
max_features=max_features,
warm_start=warm_start,
n_jobs=n_jobs,
random_state=random_state,
verbose=verbose)
self.behaviour = behaviour
self.contamination = contamination
def _set_oob_score(self, X, y):
raise NotImplementedError("OOB score not supported by iforest")
def _parallel_args(self):
# ExtraTreeRegressor releases the GIL, so it's more efficient to use
# a thread-based backend rather than a process-based backend so as
# to avoid suffering from communication overhead and extra memory
# copies.
return _joblib_parallel_args(prefer='threads')
def fit(self, X, y=None, sample_weight=None):
"""
Fit estimator.
Parameters
----------
X : {array-like, sparse matrix} of shape (n_samples, n_features)
The input samples. Use ``dtype=np.float32`` for maximum
efficiency. Sparse matrices are also supported, use sparse
``csc_matrix`` for maximum efficiency.
y : Ignored
Not used, present for API consistency by convention.
sample_weight : array-like of shape (n_samples,), default=None
Sample weights. If None, then samples are equally weighted.
Returns
-------
self : object
Fitted estimator.
"""
if self.behaviour != 'deprecated':
if self.behaviour == 'new':
warn(
"'behaviour' is deprecated in 0.22 and will be removed "
"in 0.24. You should not pass or set this parameter.",
FutureWarning
)
else:
raise NotImplementedError(
"The old behaviour of IsolationForest is not implemented "
"anymore. Remove the 'behaviour' parameter."
)
X = check_array(X, accept_sparse=['csc'])
if issparse(X):
# Pre-sort indices to avoid that each individual tree of the
# ensemble sorts the indices.
X.sort_indices()
rnd = check_random_state(self.random_state)
y = rnd.uniform(size=X.shape[0])
# ensure that max_sample is in [1, n_samples]:
n_samples = X.shape[0]
if isinstance(self.max_samples, str):
if self.max_samples == 'auto':
max_samples = min(256, n_samples)
else:
raise ValueError('max_samples (%s) is not supported.'
'Valid choices are: "auto", int or'
'float' % self.max_samples)
elif isinstance(self.max_samples, numbers.Integral):
if self.max_samples > n_samples:
warn("max_samples (%s) is greater than the "
"total number of samples (%s). max_samples "
"will be set to n_samples for estimation."
% (self.max_samples, n_samples))
max_samples = n_samples
else:
max_samples = self.max_samples
else: # float
if not 0. < self.max_samples <= 1.:
raise ValueError("max_samples must be in (0, 1], got %r"
% self.max_samples)
max_samples = int(self.max_samples * X.shape[0])
self.max_samples_ = max_samples
max_depth = int(np.ceil(np.log2(max(max_samples, 2))))
super()._fit(X, y, max_samples,
max_depth=max_depth,
sample_weight=sample_weight)
if self.contamination == "auto":
# 0.5 plays a special role as described in the original paper.
# we take the opposite as we consider the opposite of their score.
self.offset_ = -0.5
return self
# else, define offset_ wrt contamination parameter
self.offset_ = np.percentile(self.score_samples(X),
100. * self.contamination)
return self
def predict(self, X):
"""
Predict if a particular sample is an outlier or not.
Parameters
----------
X : {array-like, sparse matrix} of shape (n_samples, n_features)
The input samples. Internally, it will be converted to
``dtype=np.float32`` and if a sparse matrix is provided
to a sparse ``csr_matrix``.
Returns
-------
is_inlier : ndarray of shape (n_samples,)
For each observation, tells whether or not (+1 or -1) it should
be considered as an inlier according to the fitted model.
"""
check_is_fitted(self)
X = check_array(X, accept_sparse='csr')
is_inlier = np.ones(X.shape[0], dtype=int)
is_inlier[self.decision_function(X) < 0] = -1
return is_inlier
def decision_function(self, X):
"""
Average anomaly score of X of the base classifiers.
The anomaly score of an input sample is computed as
the mean anomaly score of the trees in the forest.
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