from sklearn.feature_extraction.text import CountVectorizer

vec = CountVectorizer(

    analyzer='word',            # tokenise by character ngrams

    max_features=4000,     # keep the most common 4000 ngrams，表示抽取最常見的4000個單詞

#在x_train上提取詞袋模型特徵

vec.fit(x_train)  

classifier = MultinomialNB()

# vec.transform(x_train)轉化訓練集樣本，轉變之後矩陣維度是[n_samples, 4000]

classifier.fit(vec.transform(x_train), y_train)

#加入抽取2-gram和3-gram的統計特徵

vec = CountVectorizer(

    analyzer='word',   # tokenise by character ngrams

    ngram_range=(1,4),  # use ngrams of size 1 and 2

max_features=20000,)  # keep the most common 1000 ngrams

更可靠的驗證效果的方式是交叉驗證，但是交叉驗證最好保證每一份裡面的樣本類別也是相對均衡的，我們這裡使用StratifiedKFold

from sklearn.cross_validation import StratifiedKFold

#x是訓練資料，y是標籤，train_index : test_index = 4:1

stratifiedk_fold = StratifiedKFold(y, n_folds=n_folds, shuffle=shuffle)

    for train_index, test_index in stratifiedk_fold:

        X_train, X_test = x[train_index], x[test_index]

        y_train = y[train_index]