Python – Text features input format for classification algorithms in scikit-learn

I'm starting to use the scikit-learn to do some NLP. I've already used some classifiers from NLTK and now I want to try the ones implemented in scikit-learn.

My data is basically sentences, and I extract features from some words of those sentences to do some classification task. Most of my features are nominal: part-of-speech (POS) of the word, word-to-the-left, POS word-to-the-left, word-to-the-right, POS word-to-the-right, syntactic relations path from one word to another, etc.

When I made some experiments using the NLTK classifiers (Decision Tree, Naive Bayes), the feature set was just a dictionary with the corresponding values for the features: the nominal values. Such as: [ {"postag":"noun", "wleft":"house", "path":"VPNPNP",…},…. ]. I just had to pass this to the classifiers and they did their job.

This is part of the code used:

def train_classifier(self):
        if self.reader == None:
            raise ValueError("No reader was provided for accessing training instances.")

        # Get the argument candidates
        argcands = self.get_argcands(self.reader)

        # Extract the necessary features from the argument candidates
        training_argcands = []
        for argcand in argcands:
            if argcand["info"]["label"] == "NULL":
                training_argcands.append( (self.extract_features(argcand), "NULL") )
            else:
                training_argcands.append( (self.extract_features(argcand), "ARG") )

        # Train the appropriate supervised model
        self.classifier = DecisionTreeClassifier.train(training_argcands)

        return

Here's an example of one of the feature sets extracted:

[({'phrase': u'np', 'punct_right': 'NULL', 'phrase_left-sibling': 'NULL', 'subcat': 'fcl=np np vp np pu', 'pred_lemma': u'revelar', 'phrase_right-sibling': u'np', 'partial_path': 'vp fcl', 'first_word-postag': 'Bras\xc3\xadlia PROP', 'last_word-postag': 'Bras\xc3\xadlia PROP', 'phrase_parent': u'fcl', 'pred_context_right': u'um', 'pred_form': u'revela', 'punct_left': 'NULL', 'path': 'vp\xc2\xa1fcl!np', 'position': 0, 'pred_context_left_postag': u'ADV', 'voice': 0, 'pred_context_right_postag': u'ART', 'pred_context_left': u'hoje'}, 'NULL')]

As I mentioned before, most of the features are nominal (a string value).

Now, I want try out the classifiers in the scikit-learn package. As I understand, this type of feature sets are not acceptable for the algorithms implemented in sklearn, since all feature values must be numeric, and they have to be in an array or matrix. Therefore, I transformed the "original" feature sets using the DictVectorizer class. However, when I pass this transformed vectors, I get the following errors:

# With DecisionTreeClass
Traceback (most recent call last): 
.....
self.classifier.fit(train_argcands_feats,new_train_argcands_target)
  File "/usr/local/lib/python2.7/dist-packages/sklearn/tree/tree.py", line 458, in fit
    X = np.asarray(X, dtype=DTYPE, order='F')
  File "/usr/local/lib/python2.7/dist-packages/numpy/core/numeric.py", line 235, in asarray
    return array(a, dtype, copy=False, order=order)
TypeError: float() argument must be a string or a number


# With GaussianNB

Traceback (most recent call last):
....
self.classifier.fit(train_argcands_feats,new_train_argcands_target)
  File "/usr/local/lib/python2.7/dist-packages/sklearn/naive_bayes.py", line 156, in fit
    n_samples, n_features = X.shape
ValueError: need more than 0 values to unpack

I get these errors when I just use DictVectorizer(). However, if I use DictVectorizer(sparse=False), I get the errors even before the code gets to the training part:

Traceback (most recent call last):
train_argcands_feats = self.feat_vectorizer.fit_transform(train_argcands_feats)
  File "/usr/local/lib/python2.7/dist-packages/sklearn/feature_extraction/dict_vectorizer.py", line 123, in fit_transform
    return self.transform(X)
  File "/usr/local/lib/python2.7/dist-packages/sklearn/feature_extraction/dict_vectorizer.py", line 212, in transform
    Xa = np.zeros((len(X), len(vocab)), dtype=dtype)
ValueError: array is too big.

Because of this error, it's obvious that a sparse representation has to be used.

So the question is: how do I transform my nominal features so as to use the classification algorithms provided by scikit-learn?

Thanks in advance for all the help you could give me.

UPDATE

As suggested by an answer below, I tried to use the NLTK wrapper for scikit-learn. I just changed the code line that creates the classifier:

self.classifier = SklearnClassifier(DecisionTreeClassifier())

Then, when I call the "train" method I get the following:

File "/usr/local/lib/python2.7/dist-packages/nltk/classify/scikitlearn.py", line 100, in train
    X = self._convert(featuresets)
  File "/usr/local/lib/python2.7/dist-packages/nltk/classify/scikitlearn.py", line 109, in _convert
    return self._featuresets_to_coo(featuresets)
  File "/usr/local/lib/python2.7/dist-packages/nltk/classify/scikitlearn.py", line 126, in _featuresets_to_coo
    values.append(self._dtype(v))
ValueError: could not convert string to float: np

So, apparently, the wrapper can't create the sparse matrix because the features are nominal. Then, I'm back to the original problem.