scikit-learn-intro

Credits: Forked from PyCon 2015 Scikit-learn Tutorial by Jake VanderPlas

• Machine Learning Models Cheat Sheet
• Estimators
• Introduction: Iris Dataset
• K-Nearest Neighbors Classifier

%matplotlib inline
import numpy as np
import matplotlib.pyplot as plt
import seaborn; 
from sklearn.linear_model import LinearRegression
from scipy import stats
import pylab as pl

seaborn.set()

Machine Learning Models Cheat Sheet

from IPython.display import Image
Image("http://scikit-learn.org/dev/_static/ml_map.png", width=800)

Estimators

Given a scikit-learn estimator object named model, the following methods are available:

• Available in all Estimators
  • model.fit() : fit training data. For supervised learning applications, this accepts two arguments: the data X and the labels y (e.g. model.fit(X, y)). For unsupervised learning applications, this accepts only a single argument, the data X (e.g. model.fit(X)).
• Available in supervised estimators
  • model.predict() : given a trained model, predict the label of a new set of data. This method accepts one argument, the new data X_new (e.g. model.predict(X_new)), and returns the learned label for each object in the array.
  • model.predict_proba() : For classification problems, some estimators also provide this method, which returns the probability that a new observation has each categorical label. In this case, the label with the highest probability is returned by model.predict().
  • model.score() : for classification or regression problems, most (all?) estimators implement a score method. Scores are between 0 and 1, with a larger score indicating a better fit.
• Available in unsupervised estimators
  • model.predict() : predict labels in clustering algorithms.
  • model.transform() : given an unsupervised model, transform new data into the new basis. This also accepts one argument X_new, and returns the new representation of the data based on the unsupervised model.
  • model.fit_transform() : some estimators implement this method, which more efficiently performs a fit and a transform on the same input data.

Introduction: Iris Dataset

from sklearn.datasets import load_iris
iris = load_iris()

n_samples, n_features = iris.data.shape
print(iris.keys())
print((n_samples, n_features))
print(iris.data.shape)
print(iris.target.shape)
print(iris.target_names)
print(iris.feature_names)

import numpy as np
import matplotlib.pyplot as plt

# 'sepal width (cm)'
x_index = 1
# 'petal length (cm)'
y_index = 2

# this formatter will label the colorbar with the correct target names
formatter = plt.FuncFormatter(lambda i, *args: iris.target_names[int(i)])

plt.scatter(iris.data[:, x_index], iris.data[:, y_index],
            c=iris.target, cmap=plt.cm.get_cmap('RdYlBu', 3))
plt.colorbar(ticks=[0, 1, 2], format=formatter)
plt.clim(-0.5, 2.5)
plt.xlabel(iris.feature_names[x_index])
plt.ylabel(iris.feature_names[y_index]);

K-Nearest Neighbors Classifier

The K-Nearest Neighbors (KNN) algorithm is a method used for algorithm used for classification or for regression. In both cases, the input consists of the k closest training examples in the feature space. Given a new, unknown observation, look up which points have the closest features and assign the predominant class.

from sklearn import neighbors, datasets

iris = datasets.load_iris()
X, y = iris.data, iris.target

# create the model
knn = neighbors.KNeighborsClassifier(n_neighbors=5, weights='uniform')

# fit the model
knn.fit(X, y)

# What kind of iris has 3cm x 5cm sepal and 4cm x 2cm petal?
X_pred = [3, 5, 4, 2]
result = knn.predict([X_pred, ])

print(iris.target_names[result])
print(iris.target_names)
print(knn.predict_proba([X_pred, ]))

from fig_code import plot_iris_knn
plot_iris_knn()

Note we see overfitting in the K-Nearest Neighbors model above. We'll be addressing overfitting and model validation in a later notebook.