python collections模块

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最新推荐文章于 2020-08-05 20:27:43 发布

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本文链接：https://blog.youkuaiyun.com/qq_29681777/article/details/106461792

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本文深入探讨了Python标准库中collections模块的各种高效数据结构和工具，包括双端队列deque、默认字典defaultdict、有序字典OrderedDict、计数器Counter及命名元组namedtuple。这些工具提供了解决常见编程问题的高级解决方案。

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collections.deque

双端队列，可以在队列的两端执行添加和弹出元素的操作

In [1]: from collections import deque

In [2]: q = deque()

In [3]: q.append(1)

In [4]: q.append(2)

In [5]: q.append(3)

In [6]: q
Out[6]: deque([1, 2, 3])

In [7]:  q.appendleft(4)

In [8]: q
Out[8]: deque([4, 1, 2, 3])

In [9]: q.pop()
Out[9]: 3

In [10]: q.popleft()
Out[10]: 4

In [11]: q
Out[11]: deque([1, 2])

使用 deque(maxlen=N) 构造函数会新建一个固定大小的队列。当新的元素加入并且这个队列已满的时候，最老的元素会自动被移除掉。

In [1]: from collections import deque

In [2]: q = deque(maxlen=3)

In [3]: q.append(1)

In [4]: q.append(2)

In [5]: q.append(3)

In [6]: q
Out[6]: deque([1, 2, 3])

In [7]: q.append(4)

In [8]: q
Out[8]: deque([2, 3, 4])

collections.defaultdict

defaultdict 会自动初始化每个 key 刚开始对应的值

In [12]: from collections import defaultdict

In [13]: d = defaultdict(list)

In [14]: d['a'].append(1)

In [15]: d['a'].append(2)

In [16]: d['b'].append(4)

In [17]: d
Out[17]: defaultdict(list, {'a': [1, 2], 'b': [4]})

在普通的字典上使用setdefault() 方法可以实现同样的效果

In [18]: d = {}

In [19]: d.setdefault('a', []).append(1)
    ...: d.setdefault('a', []).append(2)
    ...: d.setdefault('b', []).append(4)

In [20]: d
Out[20]: {'a': [1, 2], 'b': [4]}

collections.OrderedDict

OrderedDict类在迭代操作的时候它会保持元素被插入时的顺序

In [22]: from collections import OrderedDict

In [23]: d = OrderedDict()

In [24]: d['foo'] = 1
    ...: d['bar'] = 2
    ...: d['spam'] = 3
    ...: d['grok'] = 4


In [26]: for key in d:
            print(key, d[key])
foo 1
bar 2
spam 3
grok 4

collections.Counter

将元素数量统计，然后计数返回一个字典，键为元素，值为元素个数

most_common() 可以获取出现次数最多的n个元素和对应次数

In [32]: words = [
    ...: 'look', 'into', 'my', 'eyes', 'look', 'into', 'my', 'eyes',
    ...: 'the', 'eyes', 'the', 'eyes', 'the', 'eyes', 'not', 'around', 'the',
    ...: 'eyes', "don't", 'look', 'around', 'the', 'eyes', 'look', 'into',
    ...: 'my', 'eyes', "you're", 'under'
    ...: ]

In [33]: from collections import Counter

In [34]: word_counts = Counter(words)

In [35]: top_three = word_counts.most_common(3)

In [36]: print(top_three)
[('eyes', 8), ('the', 5), ('look', 4)]

update方法可以继续添加元素，然后统计

In [37]: morewords = ['why','are','you','not','looking','in','my','eyes']

In [38]: word_counts.update(morewords)

In [39]: top_three = word_counts.most_common(3)

In [40]: print(top_three)
[('eyes', 9), ('the', 5), ('look', 4)]

collections.namedtuple

命名元组

In [41]: from collections import namedtuple

In [42]: Subscriber = namedtuple('Subscriber', ['addr', 'joined'])

In [43]: sub = Subscriber('jonesy@example.com', '2012-10-19')

In [44]: sub
Out[44]: Subscriber(addr='jonesy@example.com', joined='2012-10-19')

In [45]: sub.addr
Out[45]: 'jonesy@example.com'

In [46]: addr, joined = sub

In [47]: addr
Out[47]: 'jonesy@example.com'