Use scikit-learn to classify into multiple categories

I'm trying to use one of scikit-learn's supervised learning methods to classify pieces of text into one or more categories. The predict function of all the algorithms I tried just returns one match.

For example I have a piece of text:

"Theaters in New York compared to those in London"

And I have trained the algorithm to pick a place for every text snippet I feed it.

In the above example I would want it to return New York and London, but it only returns New York.

Is it possible to use scikit-learn to return multiple results? Or even return the label with the next highest probability?

Thanks for your help.

---Update

I tried using OneVsRestClassifier but I still only get one option back per piece of text. Below is the sample code I am using

y_train = ('New York','London')


train_set = ("new york nyc big apple", "london uk great britain")
vocab = {'new york' :0,'nyc':1,'big apple':2,'london' : 3, 'uk': 4, 'great britain' : 5}
count = CountVectorizer(analyzer=WordNGramAnalyzer(min_n=1, max_n=2),vocabulary=vocab)
test_set = ('nice day in nyc','london town','hello welcome to the big apple. enjoy it here and london too')

X_vectorized = count.transform(train_set).todense()
smatrix2  = count.transform(test_set).todense()


base_clf = MultinomialNB(alpha=1)

clf = OneVsRestClassifier(base_clf).fit(X_vectorized, y_train)
Y_pred = clf.predict(smatrix2)
print Y_pred

Result: ['New York' 'London' 'London']

What you want is called multi-label classification. Scikits-learn can do that. See here: http://scikit-learn.org/dev/modules/multiclass.html.

I'm not sure what's going wrong in your example, my version of sklearn apparently doesn't have WordNGramAnalyzer. Perhaps it's a question of using more training examples or trying a different classifier? Though note that the multi-label classifier expects the target to be a list of tuples/lists of labels.

The following works for me:

import numpy as np
from sklearn.pipeline import Pipeline
from sklearn.feature_extraction.text import CountVectorizer
from sklearn.svm import LinearSVC
from sklearn.feature_extraction.text import TfidfTransformer
from sklearn.multiclass import OneVsRestClassifier

X_train = np.array(["new york is a hell of a town",                    "new york was originally dutch",                    "the big apple is great",                    "new york is also called the big apple",                    "nyc is nice",                    "people abbreviate new york city as nyc",                    "the capital of great britain is london",                    "london is in the uk",                    "london is in england",                    "london is in great britain",                    "it rains a lot in london",                    "london hosts the british museum",                    "new york is great and so is london",                    "i like london better than new york"])
y_train = [[0],[0],[0],[0],[0],[0],[1],[1],[1],[1],[1],[1],[0,1],[0,1]]
X_test = np.array(['nice day in nyc',                   'welcome to london',                   'hello welcome to new york. enjoy it here and london too'])   
target_names = ['New York', 'London']

classifier = Pipeline([    ('vectorizer', CountVectorizer(min_n=1,max_n=2)),    ('tfidf', TfidfTransformer()),    ('clf', OneVsRestClassifier(LinearSVC()))])
classifier.fit(X_train, y_train)
predicted = classifier.predict(X_test)
for item, labels in zip(X_test, predicted):
    print '%s => %s' % (item, ', '.join(target_names[x] for x in labels))

For me, this produces the output:

nice day in nyc => New York
welcome to london => London
hello welcome to new york. enjoy it here and london too => New York, London

Hope this helps.

EDIT: Updated for Python 3, scikit-learn 0.18.1 using MultiLabelBinarizer as suggested.

I've been working on this as well, and made a slight enhancement to mwv's excellent answer that may be useful. It takes text labels as the input rather than binary labels and encodes them using MultiLabelBinarizer.

import numpy as np
from sklearn.pipeline import Pipeline
from sklearn.feature_extraction.text import CountVectorizer
from sklearn.svm import LinearSVC
from sklearn.feature_extraction.text import TfidfTransformer
from sklearn.multiclass import OneVsRestClassifier
from sklearn.preprocessing import MultiLabelBinarizer

X_train = np.array(["new york is a hell of a town",                    "new york was originally dutch",                    "the big apple is great",                    "new york is also called the big apple",                    "nyc is nice",                    "people abbreviate new york city as nyc",                    "the capital of great britain is london",                    "london is in the uk",                    "london is in england",                    "london is in great britain",                    "it rains a lot in london",                    "london hosts the british museum",                    "new york is great and so is london",                    "i like london better than new york"])
y_train_text = [["new york"],["new york"],["new york"],["new york"],["new york"],
                ["new york"],["london"],["london"],["london"],["london"],
                ["london"],["london"],["new york","london"],["new york","london"]]

X_test = np.array(['nice day in nyc',                   'welcome to london',                   'london is rainy',                   'it is raining in britian',                   'it is raining in britian and the big apple',                   'it is raining in britian and nyc',                   'hello welcome to new york. enjoy it here and london too'])
target_names = ['New York', 'London']

mlb = MultiLabelBinarizer()
Y = mlb.fit_transform(y_train_text)

classifier = Pipeline([    ('vectorizer', CountVectorizer()),    ('tfidf', TfidfTransformer()),    ('clf', OneVsRestClassifier(LinearSVC()))])

classifier.fit(X_train, Y)
predicted = classifier.predict(X_test)
all_labels = mlb.inverse_transform(predicted)

for item, labels in zip(X_test, all_labels):
    print('{0} => {1}'.format(item, ', '.join(labels)))

This gives me the following output:

nice day in nyc => new york
welcome to london => london
london is rainy => london
it is raining in britian => london
it is raining in britian and the big apple => new york
it is raining in britian and nyc => london, new york
hello welcome to new york. enjoy it here and london too => london, new york

I just ran into this as well, and the problem for me was that my y_Train was a sequence of Strings, rather than a sequence of sequences of String. Apparently, OneVsRestClassifier will decide based on the input label format whether to use multi-class vs. multi-label. So change:

y_train = ('New York','London')

to

y_train = (['New York'],['London'])

Apparently this will disappear in the future, since it breaks of all the labels are the same: https://github.com/scikit-learn/scikit-learn/pull/1987

Change this line to make it work in new versions of python

# lb = preprocessing.LabelBinarizer()
lb = preprocessing.MultiLabelBinarizer()

Few Multi classification Examples are as under :-

Example 1:-

import numpy as np
from sklearn.preprocessing import LabelBinarizer
encoder = LabelBinarizer()

arr2d = np.array([1, 2, 3,4,5,6,7,8,9,10,11,12,13,14,1])
transfomed_label = encoder.fit_transform(arr2d)
print(transfomed_label)

Output is

[[1 0 0 0 0 0 0 0 0 0 0 0 0 0]
 [0 1 0 0 0 0 0 0 0 0 0 0 0 0]
 [0 0 1 0 0 0 0 0 0 0 0 0 0 0]
 [0 0 0 1 0 0 0 0 0 0 0 0 0 0]
 [0 0 0 0 1 0 0 0 0 0 0 0 0 0]
 [0 0 0 0 0 1 0 0 0 0 0 0 0 0]
 [0 0 0 0 0 0 1 0 0 0 0 0 0 0]
 [0 0 0 0 0 0 0 1 0 0 0 0 0 0]
 [0 0 0 0 0 0 0 0 1 0 0 0 0 0]
 [0 0 0 0 0 0 0 0 0 1 0 0 0 0]
 [0 0 0 0 0 0 0 0 0 0 1 0 0 0]
 [0 0 0 0 0 0 0 0 0 0 0 1 0 0]
 [0 0 0 0 0 0 0 0 0 0 0 0 1 0]
 [0 0 0 0 0 0 0 0 0 0 0 0 0 1]
 [1 0 0 0 0 0 0 0 0 0 0 0 0 0]]

Example 2:-

import numpy as np
from sklearn.preprocessing import LabelBinarizer
encoder = LabelBinarizer()

arr2d = np.array(['Leopard','Lion','Tiger', 'Lion'])
transfomed_label = encoder.fit_transform(arr2d)
print(transfomed_label)

Output is

[[1 0 0]
 [0 1 0]
 [0 0 1]
 [0 1 0]]