# Source code for pyod.models.iforest

```
# -*- coding: utf-8 -*-
"""IsolationForest Outlier Detector. Implemented on scikit-learn library.
"""
# Author: Yue Zhao <yzhao062@gmail.com>
# License: BSD 2 clause
import numpy as np
from joblib import Parallel
from joblib.parallel import delayed
from sklearn.ensemble import IsolationForest
from sklearn.utils import check_array
from sklearn.utils.validation import check_is_fitted
from .base import BaseDetector
# noinspection PyProtectedMember
from ..utils.utility import invert_order
# TODO: behavior of Isolation Forest will change in sklearn 0.22. See below.
# in 0.22, scikit learn will start adjust decision_function values by
# offset to make the values below zero as outliers. In other words, it is
# an absolute shift, which SHOULD NOT affect the result of PyOD at all as
# the order is still preserved.
# Behaviour of the decision_function which can be either ‘old’ or ‘new’.
# Passing behaviour='new' makes the decision_function change to match other
# anomaly detection algorithm API which will be the default behaviour in the
# future. As explained in details in the offset_ attribute documentation,
# the decision_function becomes dependent on the contamination parameter,
# in such a way that 0 becomes its natural threshold to detect outliers.
# offset_ : float
# Offset used to define the decision function from the raw scores.
# We have the relation: decision_function = score_samples - offset_.
# Assuming behaviour == ‘new’, 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.
# Assuming the behaviour parameter is set to ‘old’,
# we always have offset_ = -0.5, making the decision function independent from
# the contamination parameter.
# check https://scikit-learn.org/stable/modules/generated/sklearn.ensemble.IsolationForest.html for more information
[docs]
class IForest(BaseDetector):
"""Wrapper of scikit-learn Isolation Forest with more functionalities.
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.
See :cite:`liu2008isolation,liu2012isolation` for details.
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.
Parameters
----------
n_estimators : int, optional (default=100)
The number of base estimators in the ensemble.
max_samples : int or float, optional (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 : float in (0., 0.5), optional (default=0.1)
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 decision function.
max_features : int or float, optional (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, optional (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 : integer, optional (default=1)
The number of jobs to run in parallel for both `fit` and `predict`.
If -1, then the number of jobs is set to the number of cores.
behaviour : str, default='old'
Behaviour of the ``decision_function`` which can be either 'old' or
'new'. Passing ``behaviour='new'`` makes the ``decision_function``
change to match other anomaly detection algorithm API which will be
the default behaviour in the future. As explained in details in the
``offset_`` attribute documentation, the ``decision_function`` becomes
dependent on the contamination parameter, in such a way that 0 becomes
its natural threshold to detect outliers.
.. versionadded:: 0.7.0
``behaviour`` is added in 0.7.0 for back-compatibility purpose.
.. deprecated:: 0.20
``behaviour='old'`` is deprecated in sklearn 0.20 and will not be
possible in 0.22.
.. deprecated:: 0.22
``behaviour`` parameter will be deprecated in sklearn 0.22 and
removed in 0.24.
.. warning::
Only applicable for sklearn 0.20 above.
random_state : int, RandomState instance or None, optional (default=None)
If int, random_state is the seed used by the random number generator;
If RandomState instance, random_state is the random number generator;
If None, the random number generator is the RandomState instance used
by `np.random`.
verbose : int, optional (default=0)
Controls the verbosity of the tree building process.
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_ : integer
The actual number of samples
decision_scores_ : numpy array of shape (n_samples,)
The outlier scores of the training data.
The higher, the more abnormal. Outliers tend to have higher
scores. This value is available once the detector is
fitted.
threshold_ : float
The threshold is based on ``contamination``. It is the
``n_samples * contamination`` most abnormal samples in
``decision_scores_``. The threshold is calculated for generating
binary outlier labels.
labels_ : int, either 0 or 1
The binary labels of the training data. 0 stands for inliers
and 1 for outliers/anomalies. It is generated by applying
``threshold_`` on ``decision_scores_``.
"""
def __init__(self, n_estimators=100,
max_samples="auto",
contamination=0.1,
max_features=1.,
bootstrap=False,
n_jobs=1,
behaviour='old',
random_state=None,
verbose=0):
super(IForest, self).__init__(contamination=contamination)
self.n_estimators = n_estimators
self.max_samples = max_samples
self.max_features = max_features
self.bootstrap = bootstrap
self.n_jobs = n_jobs
self.behaviour = behaviour
self.random_state = random_state
self.verbose = verbose
[docs]
def fit(self, X, y=None):
"""Fit detector. y is ignored in unsupervised methods.
Parameters
----------
X : numpy array of shape (n_samples, n_features)
The input samples.
y : Ignored
Not used, present for API consistency by convention.
Returns
-------
self : object
Fitted estimator.
"""
# validate inputs X and y (optional)
X = check_array(X)
self._set_n_classes(y)
# In sklearn 0.20+ new behaviour is added (arg behaviour={'new','old'})
# to IsolationForest that shifts the location of the anomaly scores
# noinspection PyProtectedMember
self.detector_ = IsolationForest(n_estimators=self.n_estimators,
max_samples=self.max_samples,
contamination=self.contamination,
max_features=self.max_features,
bootstrap=self.bootstrap,
n_jobs=self.n_jobs,
random_state=self.random_state,
verbose=self.verbose)
self.detector_.fit(X=X, y=None, sample_weight=None)
# invert decision_scores_. Outliers comes with higher outlier scores.
self.decision_scores_ = invert_order(
self.detector_.decision_function(X))
self._process_decision_scores()
return self
[docs]
def decision_function(self, X):
"""Predict raw anomaly score of X using the fitted detector.
The anomaly score of an input sample is computed based on different
detector algorithms. For consistency, outliers are assigned with
larger anomaly scores.
Parameters
----------
X : numpy array of shape (n_samples, n_features)
The training input samples. Sparse matrices are accepted only
if they are supported by the base estimator.
Returns
-------
anomaly_scores : numpy array of shape (n_samples,)
The anomaly score of the input samples.
"""
check_is_fitted(self, ['decision_scores_', 'threshold_', 'labels_'])
# invert outlier scores. Outliers comes with higher outlier scores
return invert_order(self.detector_.decision_function(X))
@property
def estimators_(self):
"""The collection of fitted sub-estimators.
Decorator for scikit-learn Isolation Forest attributes.
"""
return self.detector_.estimators_
@property
def estimators_samples_(self):
"""The subset of drawn samples (i.e., the in-bag samples) for
each base estimator.
Decorator for scikit-learn Isolation Forest attributes.
"""
return self.detector_.estimators_samples_
@property
def max_samples_(self):
"""The actual number of samples.
Decorator for scikit-learn Isolation Forest attributes.
"""
return self.detector_.max_samples_
@property
def estimators_features_(self):
"""The indeces of the subset of features used to train the estimators.
Decorator for scikit-learn Isolation Forest attributes.
"""
return self.detector_.estimators_features_
@property
def n_features_in_(self):
"""The number of features seen during the fit.
Decorator for scikit-learn Isolation Forest attributes.
"""
return self.detector_.n_features_in_
@property
def offset_(self):
"""Offset used to define the decision function from the raw scores.
Decorator for scikit-learn Isolation Forest attributes.
"""
return self.detector_.offset_
@property
def feature_importances_(self):
"""The impurity-based feature importance. The higher, the more
important the feature. The importance of a feature is computed as the
(normalized) total reduction of the criterion brought by that feature.
It is also known as the Gini importance.
.. warning::
impurity-based feature importance can be misleading for
high cardinality features (many unique values). See
https://scikit-learn.org/stable/modules/generated/sklearn.inspection.permutation_importance.html
as an alternative.
Returns
-------
feature_importances_ : ndarray of shape (n_features,)
The values of this array sum to 1, unless all trees are single node
trees consisting of only the root node, in which case it will be an
array of zeros.
"""
check_is_fitted(self)
all_importances = Parallel(
n_jobs=self.n_jobs)(
delayed(getattr)(tree, "feature_importances_")
for tree in self.detector_.estimators_
if tree.tree_.node_count > 1
)
if not all_importances:
return np.zeros(self.n_features_in_, dtype=np.float64)
all_importances = np.mean(all_importances, axis=0, dtype=np.float64)
return all_importances / np.sum(all_importances)
```