Understanding the `ngram_range` argument in a CountVectorizer in sklearn

Setting the vocabulary explicitly means no vocabulary is learned from data. If you don’t set it, you get:

>>> v = CountVectorizer(ngram_range=(1, 2))
>>> pprint(v.fit(["an apple a day keeps the doctor away"]).vocabulary_)
{u'an': 0,
 u'an apple': 1,
 u'apple': 2,
 u'apple day': 3,
 u'away': 4,
 u'day': 5,
 u'day keeps': 6,
 u'doctor': 7,
 u'doctor away': 8,
 u'keeps': 9,
 u'keeps the': 10,
 u'the': 11,
 u'the doctor': 12}

An explicit vocabulary restricts the terms that will be extracted from text; the vocabulary is not changed:

>>> v = CountVectorizer(ngram_range=(1, 2), vocabulary={"keeps", "keeps the"})
>>> v.fit_transform(["an apple a day keeps the doctor away"]).toarray()
array([[1, 1]])  # unigram and bigram found

(Note that stopword filtering is applied before n-gram extraction, hence "apple day".)