Collection Module
collections Python is a collection of built-in module that provides many useful collections.
>>> import collections
Counter class
CounterIt is a help to the object count hashable dict subclass . It is an unordered collection of elements wherein the storage object is hashable dictionary key, stores the count value thereof dictionary, count may be any integer, including zero and negative.
Counter example : Hits certain word appears in Python LICENSE file.
>>> from collections import Counter
>>> import re
>>> path = '/usr/lib/python3.5/LICENSE.txt'
>>> words = re.findall('\w+', open(path).read().lower())
>>> Counter(words).most_common(10)
[('the', 80), ('or', 78), ('1', 66), ('of', 61), ('to', 50), ('and', 48), ('python', 46), ('in', 38), ('license', 37), ('any', 37)]
Object Counter elements()method, which returns the sequence, counting the number of repeated following the same element, the element order is disordered.
>>> c = Counter(a=4, b=2, c=0, d=-2)
>>> list(c.elements())
['b','b','a', 'a', 'a', 'a']
most_common()Returns the most common elements and their count, the order of most common to least
>>> Counter('abracadabra').most_common(3)
[('a', 5), ('r', 2), ('b', 2)]
defaultdict class
defaultdictIt is built dictsubclass of the class which overrides a method and adds a writable instance variable. The remaining functions of the same dictionary.
Using the same function defaultdictthan dict.setdefaultfaster method.
>>> from collections import defaultdict
>>> s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
>>> d = defaultdict(list)
>>> for k, v in s:
... d[k].append(v)
...
>>> d.items()
dict_items([('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])])
Can see that even defaultdict objects of a key does not exist, it will automatically create an empty list.
namedtuple class
Named tuple tuple helps give meaning to each location, and make the code more readable and self-documenting nature.
Create a named tuple to show for the save location information for each tuple:
>>> from collections import namedtuple
>>> Point = namedtuple('Point', ['x', 'y']) # 定义命名元组
>>> p = Point(10, y=20) # 创建一个对象
>>> p
Point(x=10, y=20)
>>> p.x + p.y
30
>>> p[0] + p[1] # 像普通元组那样访问元素
30
>>> x, y = p # 元组拆封
>>> x
10
>>> y
20
Source: laboratory building
