collections.Counter类统计列表元素出现次数

# 使用collections.Counter类统计列表元素出现次数

 from collections import Counter

 names = ["Stanley", "Lily", "Bob", "Well", "Peter", "Bob", "Well", "Peter", "Well", "Peter", "Bob",
     "Stanley", "Lily", "Bob", "Well", "Peter", "Bob", "Bob", "Well", "Peter", "Bob", "Well"]

 names_counts = Counter(names)  # 实例化Counter对象，可接收任何hashable序列，Counter对象可以像字典一样访问元素并返回出现的次数
 print(names_counts["Stanley"])
 #
 print(names_counts)
 # Counter({'Bob': 7, 'Well': 6, 'Peter': 5, 'Stanley': 2, 'Lily': 2})

 top_three = names_counts.most_common(3)  # 取出出现次数最多的三个元素
 print(top_three)
 # [('Bob', 7), ('Well', 6), ('Peter', 5)]

 more_names = ["Stanley", "Lily", "Bob", "Well"]
 names_counts.update(more_names)  # 使用update方法新增需要统计的序列
 top_three = names_counts.most_common(3)  # 取出出现次数最多的三个元素
 print(top_three)
 # [('Bob', 8), ('Well', 7), ('Peter', 5)]

 names_counts = Counter(names)
 more_names_counts = Counter(more_names)
 print(names_counts)
 # Counter({'Bob': 7, 'Well': 6, 'Peter': 5, 'Stanley': 2, 'Lily': 2})
 print(more_names_counts)
 # Counter({'Stanley': 1, 'Lily': 1, 'Bob': 1, 'Well': 1})
 print(names_counts - more_names_counts) # 减去次数
 # Counter({'Bob': 6, 'Well': 5, 'Peter': 5, 'Stanley': 1, 'Lily': 1})
 print(names_counts + more_names_counts) # 合并次数
 # Counter({'Bob': 8, 'Well': 7, 'Peter': 5, 'Stanley': 3, 'Lily': 3})

参考资料：
　　Python Cookbook, 3rd edition, by David Beazley and Brian K. Jones (O’Reilly).