def get_performance(clf_sel, train_features, test_features):
    ref_set = collections.defaultdict(set)
    test_set = collections.defaultdict(set)
    classification_error = False

    clf = SklearnClassifier(clf_sel)
    try:
        classifier = clf.train(train_features)
    except:
        classification_error = True
        # print (str(clf_sel.__class__),'NA')

    if str(clf_sel.__class__) == "<class 'sklearn.naive_bayes.MultinomialNB'>":
        pickle_cls(classifier, 'MultinomialNB')

    # print(str(clf_sel), 'accuracy:'(nltk.classify.accuracy(classifier, test_features)) * 100)

    if not classification_error:
        clf_acc = nltk.classify.accuracy(classifier, test_features)

        for i, (features, label) in enumerate(test_features):
            ref_set[label].add(i)
            predicted = classifier.classify(features)
            test_set[predicted].add(i)

        pos_precision = precision(ref_set['pos'], test_set['pos'])
        pos_recall = recall(ref_set['pos'], test_set['pos'])
        neg_precision = precision(ref_set['neg'], test_set['neg'])
        neg_recall = recall(ref_set['neg'], test_set['neg'])

        print(
            "{0},{1},{2},{3},{4},{5}".format(clf_sel.__class__, clf_acc, pos_precision, pos_recall, neg_precision,
                                             neg_recall))

	def validate(self, validation_set):
		if self.classifier is None:
			raise Exception("self.classifier is None")
		reference=defaultdict(set)
		observed=defaultdict(set)
		observed['neutral']=set()

		for i, (tweet, label) in enumerate(validation_set):
			reference[label].add(i)
			observation=self.classify(tweet)
			observed[observation].add(i)
		acc=classify.accuracy(self.classifier, observed)
		posp=precision(reference['positive'],observed['positive'])
		posr=recall(reference['positive'], observed['positive'])
		posf=f_measure(reference['positive'], observed['positive'])
		negp=precision(reference['negative'],observed['negative'])
		negr=recall(reference['negative'], observed['negative'])
		negf=f_measure(reference['negative'], observed['negative'])
		
		print "accuracy: %s" % acc
		print "pos precision: %s" % posp
		print "pos recall: %s" % posr
		print "pos f-measure: %s" % posf
		print "neg precision: %s" % negp
		print "neg recall: %s" % negr
		print "neg f-measure: %s" % negf
		return (acc, posp, posr, posf, negp, negr, negf)

def print_precision_recall(classifier, test_dict):
    refsets = defaultdict(set)
    testsets = defaultdict(set)
    for i, (feats, label) in enumerate(test_dict):
        refsets[label].add(i)
        observed = classifier.classify(feats)
        testsets[observed].add(i)
    print 'pos precision:', precision(refsets['positive'], testsets['positive'])
    print 'pos recall:', recall(refsets['positive'], testsets['positive'])
    print 'pos F-measure:', f_measure(refsets['positive'], testsets['positive'])
    print 'neg precision:', precision(refsets['negative'], testsets['negative'])
    print 'neg recall:', recall(refsets['negative'], testsets['negative'])
    print 'neg F-measure:', f_measure(refsets['negative'], testsets['negative'])

    def GetEvaluacion(self):
        '''
            Devuelve las medidas de precision, recall, y matriz de confusion del clasificador.
            Para esto usamos las funciones precision, recall y confusion matrix de nltk y el conjunto
            de testeo.

            Retorna una tupla (positivos, negativos, matriz) donde positivos y negativos es otra tupla con los valores (precision, recall)

            Precision: Fraccion de las instancias que se clasificaron correctamente / Fraccion de las instancias que se clasificaron en la clase: 
                TP / (TP + FP)
                Cuanto mayor es esto, menor es la cantidad de falsos positivos, es decir, esto me da el porcentaje de los elementos que
                fueron clasificados correctamente en esta clase.

            Recall: Fraccion de las instancias que se clasificaron correctamente / Fraccion de las instancias que realmente estaban en la clase:
                TP / (TP + FN)      
                Cuanto mayor es esto, menor es la cantidad de falsos negativos, es decir, del total de elementos que realmente existen
                en la clase, cuantos clasifique.


            Tanto precision y recall son para las clases y no en general (o sea, hay un valor de precision/recall para la clase de comentarios
            positivos y otros para la clase de comentarios negativos)

            Vale la pena leer de aca: http://streamhacker.com/2010/05/17/text-classification-sentiment-analysis-precision-recall/
        '''

        clasificador = self.GetClasificador()
        corpus = self.DatosTesteo        

        #Las funciones de NLTK usan sets.
        #Construyo sets de referencia y testeo para positivos y negativos.
        refSet = {CLASE_POSITIVO:set(), CLASE_NEGATIVO:set()}   #Tiene los valores reales
        testSet = {CLASE_POSITIVO:set(), CLASE_NEGATIVO:set()}  #Tiene los valores luego de clasificar.
       
        #Valores para la matriz de conf.
        refList = []
        testList = []
        
        #Tengo que construir conjuntos de referencia y testeo a partir de los de testeo, para poder usar las funciones de nltk
        for i, c in enumerate(corpus):
            refSet[c[1]].add(i) #Lo agrega en los positivos o negativos segun su clase.
            clasificado = clasificador.classify(c[0])
            testSet[clasificado].add(i)
        
            refList.append(c[1])
            testList.append(clasificado)


        positivos = ( precision(refSet[CLASE_POSITIVO], testSet[CLASE_POSITIVO]), recall(refSet[CLASE_POSITIVO], testSet[CLASE_POSITIVO]) )
        negativos = ( precision(refSet[CLASE_NEGATIVO], testSet[CLASE_NEGATIVO]), recall(refSet[CLASE_NEGATIVO], testSet[CLASE_NEGATIVO]) )

        return (positivos, negativos, ConfusionMatrix(refList, testList))

 def benchmarking(self, classifier,_test_set,all_f_measure=[],all_precision=[],all_recall=[]):
     from nltk import classify
     accuracy = classify.accuracy(classifier, _test_set)
     
     print("accuracy:",accuracy)
     
     from nltk.metrics import precision
     from nltk.metrics import recall
     from nltk.metrics import f_measure
     
     import collections
     refsets = collections.defaultdict(set)
     testsets = collections.defaultdict(set)
     for i, (feats, label) in enumerate(_test_set):
         refsets[label].add(i)
         observed = classifier.classify(feats)
         testsets[observed].add(i)
         
     prec=precision(refsets['class'], testsets['class'])
     rec=recall(refsets['class'], testsets['class'])
     f1=f_measure(refsets['class'], testsets['class'])
     print('precision:', prec)
     print('recall:', rec)
     print('F-measure:', f1)
             
     all_f_measure.append(f1)
     all_precision.append(prec)
     all_recall.append(rec)
     print('========Show top 10 most informative features========')
     classifier.show_most_informative_features(10)

def precision_and_recall(classifier, testfeats):
    #Finds precision and recall on that big booty classifier.
    refsets = collections.defaultdict(set)
    testsets = collections.defaultdict(set)
    
    
    #Feats is the dictionary of words
    #label is the label, pos or neg
    for i, (feats, label) in enumerate(testfeats):
        
        #a mapping of which entries are pos and negative
        #ex refsets[pos] = {1,2,3,4,6,7,11,78}
        refsets[label].add(i)
        
        #Classifies something as pos or neg given its feats
        observed = classifier.classify(feats)
        
        #a mapping of entries and their classifications
        #ex testsets[pos] = {1,2,3,4,5,8,11}
        testsets[observed].add(i)
        
        prec = {}
        rec = {}
        
    for label in classifier.labels():
        prec[label] = precision(refsets[label], testsets[label])
        rec[label] = recall(refsets[label], testsets[label])
    
    return prec, rec

def multi_metrics(multi_classifier, test_feats):
	mds = []
	refsets = collections.defaultdict(set)
	testsets = collections.defaultdict(set)
	
	for i, (feat, labels) in enumerate(test_feats):
		for label in labels:
			refsets[label].add(i)
		
		guessed = multi_classifier.classify(feat)
		
		for label in guessed:
			testsets[label].add(i)
		
		mds.append(metrics.masi_distance(set(labels), guessed))
	
	avg_md = sum(mds) / float(len(mds))
	precisions = {}
	recalls = {}
	
	for label in multi_classifier.labels():
		precisions[label] = metrics.precision(refsets[label], testsets[label])
		recalls[label] = metrics.recall(refsets[label], testsets[label])
	
	return precisions, recalls, avg_md

def calculate(classifier, feature_set):
    refsets = collections.defaultdict(set)
    testsets = collections.defaultdict(set)

    print("Calculating refsets for precision and recall")
    for i, (feats, label) in enumerate(feature_set):
        refsets[label].add(i)
        observed = classifier.classify(feats)
        testsets[observed].add(i)

    print('country precision:', metrics.precision(refsets['country'], testsets['country']))
    print('country recall:', metrics.recall(refsets['country'], testsets['country']))

    print('religion precision:', metrics.precision(refsets['religion'], testsets['religion']))
    print('religion recall:', metrics.recall(refsets['religion'], testsets['religion']))

    print('astronomy precision:', metrics.precision(refsets['astronomy'], testsets['astronomy']))
    print('astronomy recall:', metrics.recall(refsets['astronomy'], testsets['astronomy']))

def print_results(classifier, featureset, results, name):
    print '''
    %s classifier results:
    Classifier accuracy: %s
    B Precision: %s
    B Recall: %s
    I Precision: %s
    I Recall: %s
    O Precision: %s
    O Recall: %s
    ''' % (name, 
       accuracy(classifier, featureset), 
       precision(results[0]['B-SNP'], results[1]['B-SNP']), 
       recall(results[0]['B-SNP'], results[1]['B-SNP']), 
       precision(results[0]['I-SNP'], results[1]['I-SNP']),
       recall(results[0]['I-SNP'], results[1]['I-SNP']),
       precision(results[0]['O'], results[1]['O']),
       recall(results[0]['O'], results[1]['O']))

def eval_stats(results):
	'''
	Compute recall, precision, and f-measure from passed results.
	The expected format for results is a dictionary whose keys=<name of article>
	and values=tuple (<test category>, <reference category>, <scores>), where:
	test=category suggested by classifier, reference=pre-classified gold
	category, scores=can be None or dictionary whose keys=category names and
	values=matching score for this article.
	'''
	# Calculate number of correct matches
	correct = 0
	missed = defaultdict(tuple)
	for article_name, (suggested, real, scores) in results.iteritems():
		if suggested==real:
			correct += 1
		else:
			missed[article_name] = (suggested, real)
	success_ratio = correct / float(len(results))
	print "Ratio: %0.3f" % success_ratio
	
	# Print wrong matches
	for name, (suggested, real) in missed.iteritems():
		print "%s\t%s\t%s" % (name, suggested, real)
	
	# Create sets of references / test classification for evaluation
	cat_ref = defaultdict(set)
	cat_test= defaultdict(set)
	for name, (test_category, ref_category, scores) in results.iteritems():
		cat_ref[ref_category].add(name) 		# gold-tagged categories
		cat_test[test_category].add(name) 	# suggested categories

	# Precision, recall, f-measure, support (num of reference articles in
	# each category) for each category
	print "\nCategory\tPrecision\tRecall\tF-measure\tSupport" 
	measures = defaultdict(tuple)
	for category in cat_ref.keys():
		cat_prec = metrics.precision(cat_ref[category], cat_test[category])
		cat_rec = metrics.recall(cat_ref[category], cat_test[category])
		cat_f = metrics.f_measure(cat_ref[category], cat_test[category])
		cat_support = len(cat_ref[category])
		measures[category] = (cat_prec, cat_rec, cat_f, cat_support)
		print "%s\t%0.3f\t%0.3f\t%0.3f\t%d" % \
		(category, cat_prec, cat_rec, cat_f, cat_support)
	
	# Calculate precision, recall, f-measure for entire corpus:
	# This is a weighted average of the values of separate categories
	# SUM(product of all precisions, product of all supports)/sum(total number of supports)
	avg_prec = weighted_average([(cat_measure[0], cat_measure[3]) for \
		cat_measure in measures.values()])
	avg_rec = weighted_average([(cat_measure[1], cat_measure[3]) for \
		cat_measure in measures.values()])
	avg_f = weighted_average([(cat_measure[2], cat_measure[3]) for \
		cat_measure in measures.values()])
	total_support = sum([cat_support[3] for cat_support in measures.values()])
	
	print "%s\t%0.3f\t%0.3f\t%0.3f\t%d" % ("Total", avg_prec, avg_rec, avg_f, total_support)

def evaluate_features(feature_extractor, N, only_acc=False):
    from nltk.corpus import movie_reviews
    from nltk.classify import NaiveBayesClassifier as naive
    from nltk.classify.util import accuracy
    from nltk.metrics import precision, recall, f_measure
    from sys import stdout
    
    negative = movie_reviews.fileids('neg')
    positive = movie_reviews.fileids('pos')
    negfeats = [(feature_extractor(movie_reviews.sents(fileids=[f])),
                 'neg') for f in negative]

    posfeats = [(feature_extractor(movie_reviews.sents(fileids=[f])),
                 'pos') for f in positive]
    negtrain, negtest = stratifiedSamples(negfeats, N)
    postrain, postest = stratifiedSamples(posfeats, N)

    trainfeats = negtrain + postrain
    testfeats = negtest + postest
    classifier = naive.train(trainfeats)
    if only_acc: return accuracy(classifier, testfeats)
    print 'accuracy: {}'.format(accuracy(classifier, testfeats))

    # Precision, Recall, F-measure
    from collections import defaultdict
    refsets = defaultdict(set)
    testsets = defaultdict(set)

    for i, (feats, label) in enumerate(testfeats):
        refsets[label].add(i)
        observed = classifier.classify(feats)
        testsets[observed].add(i)
        
    print 'pos precision:', precision(refsets['pos'], testsets['pos'])
    print 'pos recall:', recall(refsets['pos'], testsets['pos'])
    print 'pos F-measure:', f_measure(refsets['pos'], testsets['pos'])
    print 'neg precision:', precision(refsets['neg'], testsets['neg'])
    print 'neg recall:', recall(refsets['neg'], testsets['neg'])
    print 'neg F-measure:', f_measure(refsets['neg'], testsets['neg'])
    stdout.flush()
    classifier.show_most_informative_features()
    return classifier

 def evaluate_features(self, feature_select):
     #reading pre-labeled input and splitting into lines
     posSentences = open('rt-polarity-pos.txt', 'r')
     negSentences = open('rt-polarity-neg.txt', 'r')
     posSentences = re.split(r'\n', posSentences.read())
     negSentences = re.split(r'\n', negSentences.read())
   
     posFeatures = []
     negFeatures = []
     #breaks up the sentences into lists of individual words (as selected by the input mechanism) and appends 'pos' or 'neg' after each list
     for i in posSentences:
         posWords = re.findall(r"[\w']+|[.,!?;]", i)
         posWords = [feature_select(posWords), 'pos']
         posFeatures.append(posWords)
     for i in negSentences:
         negWords = re.findall(r"[\w']+|[.,!?;]", i)
         negWords = [feature_select(negWords), 'neg']
         negFeatures.append(negWords)
      
     #selects 3/4 of the features to be used for training and 1/4 to be used for testing
     posCutoff = int(math.floor(len(posFeatures)*3/4))
     negCutoff = int(math.floor(len(negFeatures)*3/4))
     trainFeatures = posFeatures[:posCutoff] + negFeatures[:negCutoff]
     testFeatures = posFeatures[posCutoff:] + negFeatures[negCutoff:]
     #Training Phase: 
     classifier = NaiveBayesClassifier.train(trainFeatures)
      
     referenceSets = collections.defaultdict(set)
     testSets = collections.defaultdict(set)    
      
     #Testing Phase:
     for i, (features, label) in enumerate(testFeatures):
         referenceSets[label].add(i)
         predicted = classifier.classify(features)
         testSets[predicted].add(i)
          
     print 'Trained on %d instances, Tested on %d instances' % (len(trainFeatures), len(testFeatures))
     print 'Accuracy:', nltk.classify.util.accuracy(classifier, testFeatures)
     print 'Positive Precision:', precision(referenceSets['pos'], testSets['pos'])
     print 'Positive Recall:', recall(referenceSets['pos'], testSets['pos'])
     print 'Negative Precision:', precision(referenceSets['neg'], testSets['neg'])
     print 'Negative Recall:', recall(referenceSets['neg'], testSets['neg'])

def calcAllClassesRecall(classSet, refsets, testsets):
    rSum = 0.0
    denominator = 0
    for category in classSet:
        num = recall(refsets[category], testsets[category])
        if num is None:
            continue
        rSum += num
        denominator += 1
    
    return rSum/denominator

def main():
    global best_words
    tweets = get_tweets_from_db()
    tweet_list = tweets[1000:1599000]
    test_list = tweets[:1000]+ tweets[1599000:]
    word_scores = create_word_scores()
    best_words = find_best_words(word_scores, 500000)
    f = open('bestwords.pickle', 'wb')
    pickle.dump(best_words, f)
    f.close()
    training_set = classify.apply_features(best_word_features, tweet_list)
    print "extracted features"
    # train the classifier with the training set
    classifier = NaiveBayesClassifier.train(training_set)
    print "trained classifier"
    # create the pickle file
    f = open('NBclassifier_new.pickle', 'wb')
    pickle.dump(classifier, f)
    f.close()
    print "created pickle"
    # test for precision and recall
    refsets = collections.defaultdict(set)
    testsets = collections.defaultdict(set)

    test_set = classify.apply_features(best_word_features, test_list)
 
    for i, (feats, label) in enumerate(test_set):
        refsets[label].add(i)
        observed = classifier.classify(feats)
        testsets[observed].add(i)
     
    print 'neg precision:', metrics.precision(refsets['0'], testsets['0'])
    print 'neg recall:', metrics.recall(refsets['0'], testsets['0'])
    print 'pos precision:', metrics.precision(refsets['4'], testsets['4'])
    print 'pos recall:', metrics.recall(refsets['4'], testsets['4'])
    # test_set = classify.apply_features(extract_features, test_list)
    # print "extracted features"
    print classify.accuracy(classifier, test_set)
    print classifier.show_most_informative_features(30)

def precision_recall(classifier, testfeats):
	refsets = collections.defaultdict(set)
	testsets = collections.defaultdict(set)
	for i, (feats, label) in enumerate(testfeats):
		refsets[label].add(i)
		observed = classifier.classify(feats)
		testsets[observed].add(i)
	precisions = {}
	recalls = {}
	for label in classifier.labels():
		precisions[label] = metrics.precision(refsets[label], testsets[label])
		recalls[label] = metrics.recall(refsets[label], testsets[label])
	return precisions, recalls

def precision_recall(classifier, testfeats):
    #gives precision and recall of classifiers
    #precision = lack of false positives
    #recall = lack of false negatives
    refsets = collections.defaultdict(set)
    testsets = collections.defaultdict(set)
    
    for i, (feats, label) in enumerate(testfeats):
        refsets[label].add(i)
        observed = classifier.classify(feats)
        testsets[observed].add(i)

    precisions = {}
    recalls = {}
    for label in classifier.labels():
        precisions[label] = precision(refsets[label], testsets[label])
        recalls[label] = recall(refsets[label], testsets[label])
    return precisions, recalls

    def recall(self, reference):
        """
        Return the recall of an aligned sentence with respect to a
        "gold standard" reference ``AlignedSent``.

        :type reference: AlignedSent or Alignment
        :param reference: A "gold standard" reference aligned sentence.
        :rtype: float or None
        """
        # Get alignments in set of 2-tuples form
        # The "sure" recall is used so we don't penalize for missing an
        # alignment that was only marked as "possible".

        align = self.alignment
        if isinstance(reference, AlignedSent):
            sure = reference.alignment
        else:
            sure  = Alignment(reference)

        # Call NLTKs existing functions for recall
        return recall(sure, align)

def train_classifiers(posFeatures,negFeatures):
    
    #selects 3/4 of the features to be used for training and 1/4 to be used for testing
    posCutoff = int(math.floor(len(posFeatures)*3/4))
    negCutoff = int(math.floor(len(negFeatures)*3/4))
    trainFeatures = posFeatures[:posCutoff] + negFeatures[:negCutoff]
    testFeatures = posFeatures[posCutoff:] + negFeatures[negCutoff:]
    
    #trains a Naive Bayes Classifier
    print ("----------------Naive Bayes Classifier-----------")
    classifier = NaiveBayesClassifier.train(trainFeatures)	
    
    #initiates referenceSets and testSets
    referenceSets = collections.defaultdict(set)
    testSets = collections.defaultdict(set)	
    
    #puts correctly labeled sentences in referenceSets and the predictively labeled version in testsets
    for i, (features, label) in enumerate(testFeatures):
    	referenceSets[label].add(i)
    	predicted = classifier.classify(features)
    	testSets[predicted].add(i)	
    
    #prints metrics to show how well the feature selection did
    print ('train on %d instances, test on %d instances' % (len(trainFeatures), len(testFeatures)))
    print ('Original Naive Bayes Accuracy:', (nltk.classify.util.accuracy(classifier, testFeatures))*100)
    print ('pos precision:', precision(referenceSets['pos'], testSets['pos']))
    print ('pos recall:', recall(referenceSets['pos'], testSets['pos']))
    print ('neg precision:',precision(referenceSets['neg'], testSets['neg']))
    print ('neg recall:', recall(referenceSets['neg'], testSets['neg']))
    classifier.show_most_informative_features(10)

    #Pickle the algorithm for future use
    save_classifier = open("pickled_algos/originalnaivebayes.pickle","wb")
    pickle.dump(classifier, save_classifier)
    save_classifier.close()    
     
    
    MNB_classifier = SklearnClassifier(MultinomialNB())
    MNB_classifier.train(trainFeatures)
    print("MNB_classifier accuracy percent:", (nltk.classify.accuracy(MNB_classifier, testFeatures))*100)

    #Pickle the algorithm for future use    
    save_classifier = open("pickled_algos/MNB_classifier.pickle","wb")
    pickle.dump(MNB_classifier, save_classifier)
    save_classifier.close()   

    
    BernoulliNB_classifier = SklearnClassifier(BernoulliNB())
    BernoulliNB_classifier.train(trainFeatures)
    print("BernoulliNB_classifier accuracy percent:", (nltk.classify.accuracy(BernoulliNB_classifier, testFeatures))*100)
    
    #Pickle the algorithm for future use     
    save_classifier = open("pickled_algos/BernoulliNB_classifier.pickle","wb")
    pickle.dump(BernoulliNB_classifier, save_classifier)
    save_classifier.close()    
    
    
    LogisticRegression_classifier = SklearnClassifier(LogisticRegression())
    LogisticRegression_classifier.train(trainFeatures)
    print("LogisticRegression_classifier accuracy percent:", (nltk.classify.accuracy(LogisticRegression_classifier, testFeatures))*100)
    
    #Pickle the algorithm for future use 
    save_classifier = open("pickled_algos/LogisticRegression_classifier.pickle","wb")
    pickle.dump(LogisticRegression_classifier, save_classifier)
    save_classifier.close()

    LinearSVC_classifier = SklearnClassifier(LinearSVC())
    LinearSVC_classifier.train(trainFeatures)
    print("LinearSVC_classifier accuracy percent:", (nltk.classify.accuracy(LinearSVC_classifier, testFeatures))*100)
    
    #Pickle the algorithm for future use    
    save_classifier = open("pickled_algos/LinearSVC_classifier.pickle","wb")
    pickle.dump(LinearSVC_classifier, save_classifier)
    save_classifier.close()

    
    SGDC_classifier = SklearnClassifier(SGDClassifier())
    SGDC_classifier.train(trainFeatures)
    print("SGDClassifier accuracy percent:",nltk.classify.accuracy(SGDC_classifier, testFeatures)*100)
    
    #Pickle the algorithm for future use 
    save_classifier = open("pickled_algos/SGDC_classifier.pickle","wb")
    pickle.dump(SGDC_classifier, save_classifier)
    save_classifier.close()
    
    Dec_Tree_Classifier = SklearnClassifier(DecisionTreeClassifier())    
    Dec_Tree_Classifier.train(trainFeatures)
    print("DecisionTreeClassifier Accuracy:",(nltk.classify.accuracy(Dec_Tree_Classifier,testFeatures))*100)
    
    
    #Pickle the algorithm for future use 
    save_classifier = open("pickled_algos/decision_tree.pickle","wb")
    pickle.dump(Dec_Tree_Classifier, save_classifier)
    save_classifier.close()    
    
    """
    
#    Grad_Boost_Classifier = SklearnClassifier(GradientBoostingClassifier())
#    Grad_Boost_Classifier.train(trainFeatures)
#    print("Gradient Boosting Classifier Accuracy:", (nltk.classify.accuracy(Grad_Boost_Classifier,testFeatures))*100)    
#.........这里部分代码省略.........

    trainfeats = negfeats[:4000] + posfeats[:4000]
    testfeats = negfeats[4000:] + posfeats[4000:]
    print("train on %d instances, test on %d instances" % (len(trainfeats), len(testfeats)))
    classifier = NaiveBayesClassifier.train(trainfeats)
    refsets = collections.defaultdict(set)
    testsets = collections.defaultdict(set)
    for i, (feats, label) in enumerate(testfeats):
        refsets[label].add(i)
        observed = classifier.classify(feats)
        testsets[observed].add(i)

    # cross validation  3-fold
    feats = negfeats + posfeats
    M = math.floor(len(feats) / 3)
    result = []
    for n in range(3):
        val_set = feats[n * M :][:M]
        train_set = feats[(n + 1) * M :] + feats[: n * M]
        classifier = nltk.NaiveBayesClassifier.train(train_set)
        result.append("{:.4f}".format(round(nltk.classify.accuracy(classifier, val_set) * 100, 4)))

    print("cross_validation:", result)

    print("pos precision:", precision(refsets["pos"], testsets["pos"]))
    print("pos recall:", recall(refsets["pos"], testsets["pos"]))
    print("pos F-measure:", f_measure(refsets["pos"], testsets["pos"]))
    print("neg precision:", precision(refsets["neg"], testsets["neg"]))
    print("neg recall:", recall(refsets["neg"], testsets["neg"]))
    print("neg F-measure:", f_measure(refsets["neg"], testsets["neg"]))
    classifier.show_most_informative_features()

# Now create the data structure for model evaluation
#
refsets = collections.defaultdict(set)
testsets = collections.defaultdict(set)
for i, (feats, label) in enumerate(testfeats):
    refsets[label].add(i)
    observed = classifier.classify(feats)
    testsets[observed].add(i)
#print len(refsets)
#print len(testsets)
#print refsets
precisions = {}
recalls = {}
for label in classifier.labels():
    precisions[label] = metrics.precision(refsets[label],testsets[label])
    recalls[label] = metrics.recall(refsets[label], testsets[label])
#
# Let us calculate Precision & Recall and compare with nltk
#
# Luckily the data structures are symmetric
#
c_00=len(refsets[labels[0]].intersection(testsets[labels[0]]))
c_01=len(refsets[labels[0]].intersection(testsets[labels[1]]))
c_10=len(refsets[labels[1]].intersection(testsets[labels[0]]))
c_11=len(refsets[labels[1]].intersection(testsets[labels[1]]))
#
print '  |   H   |   S   |'
print '--|-------|-------|'
print 'H | %5d | %5d |' % (c_00,c_01)
print '--|-------|-------|'
print 'S | %5d | %5d |' % (c_10,c_11)