TypeError:classify()缺少1个必需的位置参数:“featureset”

这是我调用方法classify()的代码:

def sentiment(text):
    feats = find_features(text)
    return voted_classifier.classify(feats),voted_classifier.confidence(feats)

find_features()方法的定义:

def find_features(document):
    words = word_tokenize(document)
    features = {}
    for w in word_features:
        features[w] = (w in words)

    return features

我得到错误:

TypeError: classify() missing 1 required positional argument: 'featureset'

其中特征集为:

featuresets_f = open("pickled_algos/featuresets.pickle", "rb")
featuresets = pickle.load(featuresets_f)
featuresets_f.close()

random.shuffle(featuresets)
print(len(featuresets))

testing_set = featuresets[8000:]
training_set = featuresets[:8000]

(注意:我正在Ubuntu 14.04上使用Python 3.4、nltk进行twitter情绪分析)