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"""
==================================================
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
:class:`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.
"""
# 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))
|
