python - nltk.DecisionTreeClassifier.train doesn't work -

i attempting run following code provided example in following book chapter: http://www.nltk.org/book/ch06.html [see 1.4 part-of-speech tagging]

import nltk nltk.corpus import brown  # suffix_fdist = nltk.freqdist() word in brown.words():     word = word.lower()     suffix_fdist[word[-1:]] += 1     suffix_fdist[word[-2:]] += 1     suffix_fdist[word[-3:]] += 1  common_suffixes = [suffix (suffix,count) in suffix_fdist.most_common(100)] print(common_suffixes)  def pos_features(word):     features={}     suffix in common_suffixes:         features['endswith({})'.format(suffix)] = word.lower().endswith(suffix)     return features  tagged_words = brown.tagged_words(categories='news') featuresets = [(pos_features(n), g) (n,g) in tagged_words]  size = int(len(featuresets)*0.1) train_set, test_set = featuresets[size:], featuresets[:size]  classifier = nltk.decisiontreeclassifier.train(train_set) nltk.classify.accuracy(classifier, test_set) 

when run code idle seems go infinite loop. problem appears occur within nltk.decisiontreeclassifier.trai method call. idea why happening , how might go fixing highly appreciated.

mike