Using Sklearn’s TfidfVectorizer transform

I am trying to get the tf-idf vector for a single document using Sklearn’s TfidfVectorizer object. I create a vocabulary based on some training documents and use fit_transform to train the TfidfVectorizer. Then, I want to find the tf-idf vectors for any given testing document.

from sklearn.feature_extraction.text import TfidfVectorizer

self.vocabulary = "a list of words I want to look for in the documents".split()
self.vect = TfidfVectorizer(sublinear_tf=True, max_df=0.5, analyzer='word', 
                 stop_words='english')
self.vect.fit_transform(self.vocabulary)

...

doc = "some string I want to get tf-idf vector for"
tfidf = self.vect.transform(doc)

The problem is that this returns a matrix with n rows where n is the size of my doc string. I want it to return just a single vector representing the tf-idf for the entire string. How can I make this see the string as a single document, rather than each character being a document? Also, I am very new to text mining so if I am doing something wrong conceptually, that would be great to know. Any help is appreciated.

Answer

If you want to compute tf-idf only for a given vocabulary, use vocabulary argument to TfidfVectorizer constructor,

vocabulary = "a list of words I want to look for in the documents".split()
vect = TfidfVectorizer(sublinear_tf=True, max_df=0.5, analyzer='word', 
           stop_words='english', vocabulary=vocabulary)

Then, to fit, i.e. calculate counts, with a given corpus, i.e. an iterable of documents, use fit:

vect.fit(corpus)

Method fit_transform is a shortening for

vect.fit(corpus)
corpus_tf_idf = vect.transform(corpus) 

Last, transform method accepts a corpus, so for a single document, you should pass it as list, or it is treated as iterable of symbols, each symbol being a document.

doc_tfidf = vect.transform([doc])