'Train the classifier with a labeled feature set and return
the classifier. Takes the same arguments as the wrapped NLTK class.
This method is implicitly called when calling ``classify`` or
``accuracy`` methods and is included only to allow passing in arguments
to the ``train`` method of the wrapped NLTK class.
.. versionadded:: 0.6.2
:rtype: A classifier'

def train(self, *args, **kwargs):

    try:
        self.classifier = self.nltk_class.train(self.train_features, *args, **kwargs)
        return self.classifier
    except AttributeError:
        raise ValueError('NLTKClassifier    must    have    a    nltk_class    variable    that    is    not    None.')
