Python对句子中的单词出现次数排序

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出现次数相同的单词按照单词字母排序 

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"""Count words."""

def count_words(s, n):
    """Return the n most frequently occuring words in s."""
    words = s.split(" ")
    # TODO: Count the number of occurences of each word in s
    counters = {}
    for word in words:
        if word in counters:
            counters[word] += 1
        else:
            counters[word] = 1
    # TODO: Sort the occurences in descending order (alphabetically in case of ties)
    top = sorted(counters.iteritems(), key=lambda d:(-d[1],d[0]))
    ##lambda中"-"为反序，意思是先把单词出现次数从大到小排列，
    ##然后再以单词的字母从小到大排列
    # TODO: Return the top n words as a list of tuples (<word>, <count>)
    top_n = top[:n]##返回前n个单词
    return top_n

def test_run():
    """Test count_words() with some inputs."""
    print count_words("cat bat mat cat bat cat", 3)
    print count_words("betty bought a bit of butter but the butter was bitter", 3)

if __name__ == "__main__":
    test_run()