Python TfidfVectorizer throwing : empty vocabulary; perhaps the documents only contain stop words"

Question

I'm trying to use Python's Tfidf to transform a corpus of text. However, when I try to fit_transform it, I get a value error ValueError: empty vocabulary; perhaps the documents only contain stop words.

In [69]: TfidfVectorizer().fit_transform(smallcorp)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-69-ac16344f3129> in <module>()
----> 1 TfidfVectorizer().fit_transform(smallcorp)

/Users/maxsong/anaconda/lib/python2.7/site-packages/sklearn/feature_extraction/text.pyc in fit_transform(self, raw_documents, y)
   1217         vectors : array, [n_samples, n_features]
   1218         """
-> 1219         X = super(TfidfVectorizer, self).fit_transform(raw_documents)
   1220         self._tfidf.fit(X)
   1221         # X is already a transformed view of raw_documents so

/Users/maxsong/anaconda/lib/python2.7/site-packages/sklearn/feature_extraction/text.pyc in fit_transform(self, raw_documents, y)
    778         max_features = self.max_features
    779 
--> 780         vocabulary, X = self._count_vocab(raw_documents, self.fixed_vocabulary)
    781         X = X.tocsc()
    782 

/Users/maxsong/anaconda/lib/python2.7/site-packages/sklearn/feature_extraction/text.pyc in _count_vocab(self, raw_documents, fixed_vocab)
    725             vocabulary = dict(vocabulary)
    726             if not vocabulary:
--> 727                 raise ValueError("empty vocabulary; perhaps the documents only"
    728                                  " contain stop words")
    729 

ValueError: empty vocabulary; perhaps the documents only contain stop words

I read through the SO question here: Problems using a custom vocabulary for TfidfVectorizer scikit-learn and tried ogrisel's suggestion of using TfidfVectorizer(**params).build_analyzer()(dataset2) to check the results of the text analysis step and that seems to be working as expected: snippet below:

In [68]: TfidfVectorizer().build_analyzer()(smallcorp)
Out[68]: 
[u'due',
 u'to',
 u'lack',
 u'of',
 u'personal',
 u'biggest',
 u'education',
 u'and',
 u'husband',
 u'to',

Is there something else that I am doing wrong? the corpus I am feeding it is just one giant long string punctuated by newlines.

Thanks!

Answer 1

I guess it's because you just have one string. Try splitting it into a list of strings, e.g.:

In [51]: smallcorp
Out[51]: 'Ah! Now I have done Philosophy,\nI have finished Law and Medicine,\nAnd sadly even Theology:\nTaken fierce pains, from end to end.\nNow here I am, a fool for sure!\nNo wiser than I was before:'

In [52]: tf = TfidfVectorizer()

In [53]: tf.fit_transform(smallcorp.split('\n'))
Out[53]: 
<6x28 sparse matrix of type '<type 'numpy.float64'>'
    with 31 stored elements in Compressed Sparse Row format>