collections.Counter类接收任意由可哈希(hashable)元素构成的序列对象。在底层实现上一个Counter对象就是一个字典，将元素映射在其出现的次数上，十分适合统计序列元素数量：

    1. from collections import Counter
    3. words = [
    4.     "look", "into", "my", "eyes", "look", "into", "my", "eyes",
    5.     "the", "eyes", "the", "eyes", "the", "eyes", "not", "around", "the",
    6.     "eyes", "don't", "look", "around", "the", "eyes", "look", "into",
    7.     "my", "eyes", "you're", "under"
    8. ]
    10. word_counts = Counter(words)
    11. # 出现频率最高的3个单词
    12. top_three = word_counts.most_common(3)
    13. print(top_three)  # [("eyes", 8), ("the", 5), ("look", 4)]
    15. print(word_counts["eyes"])  # 8
    17. # 使用update方法更新
    18. more_words = ["eyes", "the"]
    19. word_counts.update(more_words)
    20. print(word_counts["eyes"])  # 9
    21. print(word_counts["the"])  # 6

    Counter实例进行数学运算：

    1. from collections import Counter
    3. words = [
    4.     "eyes", "look", "into", "eyes", "the",
    5.     "eyes", "the", "eyes", "the", "eyes", "the"
    6. ]
    8. more_words = ["eyes", "the"]
    10. a = Counter(words)
    11. b = Counter(more_words)
    13. add_words = a + b
    14. print(add_words)  # Counter({"eyes": 6, "the": 5, "look": 1, "into": 1})
    16. sub_words = a - b
    17. print(sub_words)  # Counter({"eyes": 4, "the": 3, "look": 1, "into": 1})