Introduction to Machine Learning with Python

Chapter 6. Algorithm Chains and Pipelines

For many machine learning algorithms, the particular representation of the data that you provide is very important, as we discussed in Chapter 4. This starts with scaling the data and combining features by hand and goes all the way to learning features using unsupervised machine learning, as we saw in Chapter 3. Consequently, most machine learning applications require not only the application of a single algorithm, but the chaining together of many different processing steps and machine learning models. In this chapter, we will cover how to use the Pipeline class to simplify the process of building chains of transformations and models. In particular, we will see how we can combine Pipeline and GridSearchCV to search over parameters for all processing steps at once.

As an example of the importance of chaining models, we noticed that we can greatly improve the performance of a kernel SVM on the cancer dataset by using the MinMaxScaler for preprocessing. Here’s code for splitting the data, computing the minimum and maximum, scaling the data, and training the SVM:

In[1]:

from sklearn.svm import SVC
from sklearn.datasets import load_breast_cancer
from sklearn.model_selection import train_test_split
from sklearn.preprocessing import MinMaxScaler

# load and split the data
cancer = load_breast_cancer()
X_train, X_test, y_train, y_test = train_test_split(
    cancer.data, cancer.target, random_state=0)

# compute minimum and maximum on the training ...