Lambda函数导致TypeError：'int'对象不可迭代

我刚刚开始学习Python。我正在使用API​​来构建IDF模型，但是我遇到了无法解决的lambda函数错误。生成IDF的此类：

class Idfs(DocumentFrequencies, Model):

    def build(self, corpus):
        log.info('Counting documents in corpus...')
        N = float(corpus.count())
        dfs = super(Idfs, self).build(corpus)

        log.info('Building idf model: N=%i', N)
        return dfs\
           .map(lambda (term, (df,rank)): (term, df))\  
           .mapValues(lambda df: math.log(N/df))

    @staticmethod
    def format_item((term, idf)):
        return {
        '_id': term,
        'idf': idf,
        }

这是计算DF的类：

class DocumentFrequencies(ModelBuilder):

    def __init__(self, lowercase=False, max_ngram=1, min_df=2):
        self.lowercase = lowercase
        self.max_ngram = max_ngram
        self.min_df = min_df

    def build(self, docs):
        m = docs.map(lambda d: d['text'])
        if self.lowercase:
            m = m.map(lambda text: text.lower())

        return m\
            .flatMap(lambda text: set(ngrams(text, self.max_ngram)))\
            .map(lambda t: (t, 1))\
            .reduceByKey(add)\
            .filter(lambda (k,v): v > self.min_df)

错误出现在此行.map(lambda (term, (df, rank)): (term, df))\，这是错误消息：

TypeError: 'int' object is not iterable

这是我打电话时得到的DocumentFrequencies.collect()：

Out[5]:                                                                         
[(u'fawn', 3),
 (u'1,800', 31),
 (u'clotted', 3),
 (u'comically', 11),
 (u'Adjusting', 3),
 (u'O(log', 6),
 (u'unnecessarily', 15),
 (u'evangelical', 53),
 (u'naturopathic', 3),
 (u'grenadiers', 4),
 (u'stipulate', 4),
 (u'Vikrant', 3),
 (u'fractal', 18),

我不知道到底是哪个参数导致了错误。我正在使用python 2.7、8 GB 1600 MHz DDR和2核。这些是pyspark配置：

conf = pyspark.SparkConf().setAll([('spark.executor.memory', '8g'),('spark.driver.memory','8g'),('spark.network.timeout','100000000s'),('spark.executor.heartbeatInterval','10000000s'),('spark.driver.maxResultSize','8g'),('spark.driver.cores','2')])

提前致谢，