Column Transformer with Heterogeneous Data Sources

Datasets can often contain components of that require different feature extraction and processing pipelines. This scenario might occur when:

1. Your dataset consists of heterogeneous data types (e.g. raster images and text captions)
2. Your dataset is stored in a Pandas DataFrame and different columns require different processing pipelines.

This example demonstrates how to use sklearn.compose.ColumnTransformer on a dataset containing different types of features. We use the 20-newsgroups dataset and compute standard bag-of-words features for the subject line and body in separate pipelines as well as ad hoc features on the body. We combine them (with weights) using a ColumnTransformer and finally train a classifier on the combined set of features.

The choice of features is not particularly helpful, but serves to illustrate the technique.

Out:

/home/circleci/project/sklearn/svm/base.py:931: ConvergenceWarning: Liblinear failed to converge, increase the number of iterations.
  "the number of iterations.", ConvergenceWarning)
              precision    recall  f1-score   support

           0       0.96      0.62      0.76       494
           1       0.25      0.84      0.39        76

   micro avg       0.65      0.65      0.65       570
   macro avg       0.61      0.73      0.57       570
weighted avg       0.87      0.65      0.71       570

# Author: Matt Terry <matt.terry@gmail.com>
#
# License: BSD 3 clause
from __future__ import print_function

import numpy as np

from sklearn.base import BaseEstimator, TransformerMixin
from sklearn.datasets import fetch_20newsgroups
from sklearn.datasets.twenty_newsgroups import strip_newsgroup_footer
from sklearn.datasets.twenty_newsgroups import strip_newsgroup_quoting
from sklearn.decomposition import TruncatedSVD
from sklearn.feature_extraction import DictVectorizer
from sklearn.feature_extraction.text import TfidfVectorizer
from sklearn.metrics import classification_report
from sklearn.pipeline import Pipeline
from sklearn.compose import ColumnTransformer
from sklearn.svm import LinearSVC


class TextStats(BaseEstimator, TransformerMixin):
    """Extract features from each document for DictVectorizer"""

    def fit(self, x, y=None):
        return self

    def transform(self, posts):
        return [{'length': len(text),
                 'num_sentences': text.count('.')}
                for text in posts]


class SubjectBodyExtractor(BaseEstimator, TransformerMixin):
    """Extract the subject & body from a usenet post in a single pass.

    Takes a sequence of strings and produces a dict of sequences.  Keys are
    `subject` and `body`.
    """
    def fit(self, x, y=None):
        return self

    def transform(self, posts):
        # construct object dtype array with two columns
        # first column = 'subject' and second column = 'body'
        features = np.empty(shape=(len(posts), 2), dtype=object)
        for i, text in enumerate(posts):
            headers, _, bod = text.partition('\n\n')
            bod = strip_newsgroup_footer(bod)
            bod = strip_newsgroup_quoting(bod)
            features[i, 1] = bod

            prefix = 'Subject:'
            sub = ''
            for line in headers.split('\n'):
                if line.startswith(prefix):
                    sub = line[len(prefix):]
                    break
            features[i, 0] = sub

        return features


pipeline = Pipeline([
    # Extract the subject & body
    ('subjectbody', SubjectBodyExtractor()),

    # Use ColumnTransformer to combine the features from subject and body
    ('union', ColumnTransformer(
        [
            # Pulling features from the post's subject line (first column)
            ('subject', TfidfVectorizer(min_df=50), 0),

            # Pipeline for standard bag-of-words model for body (second column)
            ('body_bow', Pipeline([
                ('tfidf', TfidfVectorizer()),
                ('best', TruncatedSVD(n_components=50)),
            ]), 1),

            # Pipeline for pulling ad hoc features from post's body
            ('body_stats', Pipeline([
                ('stats', TextStats()),  # returns a list of dicts
                ('vect', DictVectorizer()),  # list of dicts -> feature matrix
            ]), 1),
        ],

        # weight components in ColumnTransformer
        transformer_weights={
            'subject': 0.8,
            'body_bow': 0.5,
            'body_stats': 1.0,
        }
    )),

    # Use a SVC classifier on the combined features
    ('svc', LinearSVC()),
])

# limit the list of categories to make running this example faster.
categories = ['alt.atheism', 'talk.religion.misc']
train = fetch_20newsgroups(random_state=1,
                           subset='train',
                           categories=categories,
                           )
test = fetch_20newsgroups(random_state=1,
                          subset='test',
                          categories=categories,
                          )

pipeline.fit(train.data, train.target)
y = pipeline.predict(test.data)
print(classification_report(y, test.target))