I am using a simple text processing pipeline to perform sentiment
classification:

steps = [('vect', CountVectorizer()),
('tfidf', TfidfTransformer()),
('clf', LogisticRegression())]
pipe = Pipeline(steps)

With v0.15 the cross validation scores peak around 0.67 and with v0.15 they
peak at 0.55.  This seems like a significant difference to me.  My
hyperparameter gridsearch is as follows:

params = {'vect__ngram_range': [(1,1), (1,2)],
    'vect__stop_words':['english',None],
    'tfidf__use_idf': [True, False],
    'clf__C': np.logspace(-1,2,3*3+1)}

I have repeated the experiment with other classifiers (linear SVM,
naive_bayes) and seen a similar drop between v0.15 and v0.14.  Is this a
bug or am I missing some hyperparameters that need to be tuned differently
in v0.15?

- Matt Coursen
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