计算可迭代集合中所有项目的所有出现。计数器

from collections import Counter

c = Counter(["a", "b", "c", "d", "a", "b", "a", "c", "d"])
c
# Out: Counter({'a': 3, 'b': 2, 'c': 2, 'd': 2})
c["a"]
# Out: 3

c[7]     # not in the list (7 occurred 0 times!)
# Out: 0

collections.Counter 可用于任何迭代，并计算每个元素的每次出现次数。

注意 ：一个例外是如果给出 dict 或类似 collections.Mapping 的类，那么它将不计算它们，而是创建一个具有这些值的 Counter：

Counter({"e": 2})
# Out: Counter({"e": 2})

Counter({"e": "e"})        # warning Counter does not verify the values are int
# Out: Counter({"e": "e"})