python collection module

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Collection module of python

collections The module provides several additional data types based on the built-in data types, such as int, str, list, dict, etc.

1.namedtuple(): Generate a tuple subclass that can use names to access element content
2.deque: Double-ended queue, which can quickly append and add from the other side Export object
3.Counter: Counter, mainly used for counting
4.OrderedDict: Ordered dictionary
5.defaultdict: Dictionary with default values

One: namedtuple(), named tuples.

tuple is an immutable collection. namedtuple is a function. It is used to construct a custom tuple object and specifies the number of tuple elements. Certain elements of the tuple can be referenced through attributes rather than indexes. elements. We can easily customize a data type through namedtuple, which has the immutable attributes of tuple and can be referenced based on the attributes.

from collections import namedtuple
piont=namedtuple('piont',['x','y'])
p=piont(2,1)
print(p)
print("x =",p.x)
print("Y =",p.y)

>>>isinstance（p,piont）
True
>>>isinstance(p,tuple)
True

类似的创建一个圆，通过坐标，半径r。用namedtuple构建
Cirle=namedtup（‘Circle’,['x','y','r']）


2、有序字典(orderedDict )
dict中key是无序的，在做迭代时，无法确认key的顺序。
orderdDict是对字典类型的补充，他记住了字典元素添加的顺序

from collections import OrderedDict
od=OrderedDict([('a',1),('z',2),('c',3)])// 有序字典顺序是插入进去的顺序排列的
print(od)
d=dict([('a',1),('z',2),('c',3)])  //字典是无序的，key的顺序是变化的
print(d)

3、默认字典（defaultdict）
defaultdict是对字典的类型的补充，他默认给字典的值设置了一个类型。

我们都知道，在使用Python原生的数据结构dict的时候，如果用 d[key] 这样的方式访问， 当指定的key不存在时，是会抛出KeyError异常的。但是，如果使用defaultdict，只要你传入一个默认的工厂方法，那么请求一个不存在的key时， 便会调用这个工厂方法使用其结果来作为这个key的默认值。

4、计数器Counter
Counter是对字典类型的补充，用于追踪值的出现次数。
具备字典的所有功能+自己的功能

from collections import Counter
c=Counter(['11','22','11','33','11','44','55'])
c2=Counter("abcdefdasfdsafaf")
print(c)
print(c2)

most_common(self, n=None),数量从大到小排列，获取前N个元素。

elements(self) 计数器中的所有元素。注：此处非所有元素集合，而是包含所有元素集合的迭代器sorted(c.elements())

update(self,iterable=None,**keds):更新计数器，其实是增加计数器，如果没有则新建。

c.update('witch')

subtract(self, iterable=None, **kwds):相减，原来的计数器中的每一个元素的数量减去后添加的元素的数量c.subtract('witch')

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