Guide To PyOD: A Python Toolkit For Outlier Detection – Analytics India Magazine


PyOD is a versatile and scalable toolkit designed for detecting outliers or anomalies in multivariate information; therefore the identify PyOD (Python Outlier Detection). It was launched by Yue Zhao, Zain Nasrullah and Zeng Li in May 2019 (JMLR (Journal of Machine studying) paper).

Before going into the small print of PyOD, allow us to perceive briefly what outlier detection means.

What is outlier detection?

Outliers in information evaluation check with these information factors which differ considerably from nearly all of observations or don’t conform to the pattern/sample adopted by them. The strategy of figuring out such suspicious information factors is called outlier detection. Detecting fraudulent transactions within the banking sector is an instance of outlier detection. Following are a few of our helpful articles for detailed data on outlier detection:

Overview of PyOD

PyOD is an open-source Python toolbox that gives over 20 outlier detection algorithms until date – starting from conventional strategies like native outlier issue to novel neural community architectures resembling adversarial fashions or autoencoders. The full listing of supported algorithms is obtainable here



  • It is suitable with each Python 2 and Python Three throughout Linux, MacOS and Windows working techniques. The compatibility is achieved utilizing six library.
  • PyOD may give cumulative outcomes by combining numerous outlier detection strategies, detectors and ensembles.
  • It contains an easy-to-use API and interactive examples for the supported algorithms.
  • Optimization strategies resembling parallelization and Just-In-Time (JIT) compilation might be employed for chosen fashions at any time when required.
  • Practices resembling unit testing, code protection, steady integration and code maintainability checks are thought of by the supported fashions.

Essential dependencies

Practical implementation

Here’s an illustration of making use of eight totally different outlier detection algorithms utilizing PyOD library and evaluating their visualization outcomes. The code demonstrated right here is examined with Google Colab having Python 3.7.10 and PyOD 0.8.7 variations. 

Following fashions have been included within the demonstration:

  • Angle-Based Outlier Detector (ABOD)
  • Cluster-based Local Outlier Factor (CBLOF)
  • Isolation Forest
  • k-Nearest Neighbors (KNN)
  • Average KNN 
  • Local Outlier Factor (LOF)
  • One-Class SVM (OCSVM)
  • Principal Component Analysis (PCA)

Step-wise clarification of the code is as follows:

  1. Install PyOD and combo toolbox
 !pip set up --upgarde pod
 !pip set up combo 
  1. Import required libraries 
 from __future__ import division
 from __future__ import print_function
 import os
 import sys
 from time import time
 import numpy as np
 from numpy import percentile
 import matplotlib.pyplot as plt
 import matplotlib.font_manager 
  1. Import all of the fashions for use from pyod.fashions module
 from pyod.fashions.abod import ABOD
 from pyod.fashions.cblof import CBLOF
 from pyod.fashions.iforest import IForest
 from pyod.fashions.knn import KNN
 from pyod.fashions.lof import LOF
 from pyod.fashions.ocsvm import OCSVM
 from pyod.fashions.pca import PCA 
  1. Define variety of inliers and outliers.
 num_samples = 500
 out_frac = 0.30 
  1. Initialize inliers and outliers information
 clusters_separation = [0]
 x, y = np.meshgrid(np.linspace(-7, 7, 100), np.linspace(-7, 7, 100))
"""
(1 - fraction of outliers) will give the fraction of inliers; multiplying  it with the overall quantity #of samples will give variety of inliers
"""
 num_inliers = int((1. - outl_frac) * num_samples)
"""
Multiply fraction of outliers with whole variety of samples to compute variety of outliers
"""
 num_outliers = int(outl_frac * num_samples)
"""
Create floor reality array with Zero and 1 representing outliers and inliers respectively
"""
 ground_truth = np.zeros(num_samples, dtype=int)
 ground_truth[-num_outliers:] = 1 
  1. Display the variety of inliers and outliers and the bottom reality array.
 print('No. of inliers: %i' % num_inliers)
 print('No. of outliers: %i' % num_outliers)
 print('Ground reality arrayy form is form. Outlier are 1 and inlier are  
 0.n'.format(form=ground_truth.form))
 print(ground_truth) 

Output:

See Also


  1. Define a dictionary of outlier detection strategies to be in contrast
 rs = np.random.RandomState(42)  #random state
 #dictionary of classifiers
 clf =     
     'Angle-based Outlier Detector (ABOD)':
         ABOD(contamination=out_frac),
     'Cluster-based Local Outlier Factor (CBLOF)':
         CBLOF(contamination=out_frac,
          check_estimator=False, random_state=rs),
     'Isolation Forest': IForest(contamination=out_frac,
                                 random_state=rs),
     'Ok Nearest Neighbors (KNN)': KNN(
         contamination=out_frac),
     'Average KNN': KNN(methodology='imply',
                        contamination=out_frac),
     'Local Outlier Factor (LOF)':
         LOF(n_neighbors=35, contamination=out_frac),
     'One-class SVM (OCSVM)': OCSVM(contamination=out_frac),
     'Principal Component Analysis (PCA)': PCA(
         contamination=out_frac, random_state=rs),
  
  1. Display the names of classifiers used
 for i, classifier in enumerate(clf.keys()):
     print('Model', i + 1, classifier) 

Output:

 Model 1 Angle-based Outlier Detector (ABOD)
 Model 2 Cluster-based Local Outlier Factor (CBLOF)
 Model 3 Isolation Forest
 Model Four Ok Nearest Neighbors (KNN)
 Model 5 Average KNN
 Model 6 Local Outlier Factor (LOF)
 Model 7 One-class SVM (OCSVM)
 Model 8 Principal Component Analysis (PCA) 
  1. Fit the fashions to the info and visualize their outcomes
 for i, offset in enumerate(clusters_separation):
     np.random.seed(42)
     # Data technology
     X1 = 0.3 * np.random.randn(num_inliers // 2, 2) - offset  #inliers information
     X2 = 0.3 * np.random.randn(num_inliers // 2, 2) + offset  #outlier information
 #Build an array having X1 and X2 utilizing numpy.r_
     X = np.r_[X1, X2]
     # Add outliers to X array
     X = np.r_[X, np.random.uniform(low=-6, high=6, size=(num_outliers, 2))]
"""
numpy.random.uniform() attracts samples from the uniform distribution of inliers and outliers
"""
    # Fit the fashions one-by-one
     plt.determine(figsize=(15, 12))
 #For every classifier to be examined
     for i, (classifier_name, classifier) in enumerate(clf.gadgets()):
 #match the classifier to information X
         classifier.match(X)
       #compute confidence rating
         scores_pred = classifier.decision_function(X) * -1
      #make prediction utilizing the classifier
         y_pred = classifier.predict(X)
 #compute percentile rank of the arrogance rating
         threshold = percentile(scores_pred, 100 * out_frac)
"""
compute variety of errors from distinction between predicted and floor 
reality values
"""
         num_errors = (y_pred != ground_truth).sum()
         # plot the degrees traces and the factors
         Z = classifier.decision_function(np.c_[x.ravel(), y.ravel()]) * -1
         Z = Z.reshape(x.form)
       #2 rows having Four subplots every
         subplot = plt.subplot(2, 4, i + 1)
"""
plot stuffed and unfilled contours utilizing contourf() and contour() respectively
"""
       subplot.contourf(x, y, Z, ranges=np.linspace(Z.min(), threshold, 7),
                        cmap=plt.cm.Blues_r)
         a = subplot.contour(x, y, Z, ranges=[threshold],
                             linewidths=2, colours="red")
       #for discovered resolution operate
         subplot.contourf(x, y, Z, ranges=[threshold, Z.max()],
                          colours="orange")
        #for true inliers
        b = subplot.scatter(X[:-num_outliers, 0], X[:-num_outliers, 1], 
        c="white",s=20, edgecolor="k")   
       #for true outliers
       c = subplot.scatter(X[-num_outliers:, 0], X[-num_outliers:, 1], 
       c="black",s=20, edgecolor="k")
         subplot.axis('tight')
        #legend of the subplots
         subplot.legend(
             [a.collections[0], b, c],
             ['learned decision function', 'true inliers', 'true outliers'],
             prop=matplotlib.font_manager.FontProperties(measurement=10),
             loc="lower right")
        # X-axis label
         subplot.set_xlabel("%d. %s (errors: %d)" % (i + 1, classifier_name, 
         num_errors))
      #marking limits of each the axes
         subplot.set_xlim((-7, 7))
         subplot.set_ylim((-7, 7))
    #format parameters
     plt.subplots_adjust(0.04, 0.1, 0.96, 0.94, 0.1, 0.26)
 #centered title to be given to the determine
     plt.suptitle("Outlier detection by 8 models")
 plt.present()     #show the plots 

Output:

References

For in-depth understanding of the PyOD toolkit and its tutorials, check with the next sources:


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