The collections module in Python

This module implements a number of classes, which are very flexible. Can be used to replace Python's built-in dict, list, tuple, set type. And some features are not available for these built-in types.

On the network to find some information, focusing on the collections module deque, Defaultdict, Counter class

1. Class Deque
Similar to Python's built-in list, but it's a two-way list. Can be manipulated at any end

Help (Collections.deque)

Class Deque (__builtin__.object)
Deque ([iterable[, MaxLen]])--Deque Object

The deque parameter should be a iterable.

Example:

In [1]: import collectionsin [2]: d = collections.deque (' abcdef ') In [3 ]: print ddeque ([' A ',  ' B ',  ' C ',  ' d ',  ' e ',  ' F ']) in [4]: d.append (' MM ')           #追加元素In  [5]: print ddeque ([' A ',  ' B ',  ' C ',  ' d ',  ' e ',  ' f ',  ' mm ']) in [6]: d.appendleft (' GG ')     #从左追加元素In  [7]: print ddeque ([' GG ',  ' a ',  ' B ',  ' C ',  ' d ',  ' e ',  ' f ',   ' mm ']) In [8]: d.pop ()     # pop  one element out[8]:  ' mm ' in [9]:  print ddeque ([' GG ',  ' a ',  ' B ',  ' C ',  ' d ',  ' e ',  ' F ']) in [10]:  D.popleft ()   #从左pop   An element out[10]:  ' GG ' in [11]: print ddeque ([' A ',  ' B ',  ' C ',  ' d ',  ' e ',  ' F ']) in [12]: d.extend (' xyz ')     #扩展In  [13]:  Print ddEque ([' A ',  ' B ',  ' C ',  ' d ',  ' e ',  ' F ',  ' x ',  ' y ',  ' z ']) in [14]:  D.extendleft (' 123 ')    #左扩展In  [15]: print ddeque ([' 3 ',  ' 2 ',  ' 1 ',  ' a ',   ' B ',  ' C ',  ' d ',  ' e ',  ' F ',  ' x ',  ' y ',  ' z ']) in [16]: d.reverse ( )   #反转In  [17]: print ddeque ([' Z ',  ' y ',  ' x ',  ' F ',  ' e ',  ' d ',  ' C ',  ' B ',  ' A ',  ' 1 ',  ' 2 ',  ' 3 ']) in [22]: d.extend (' 1111 ') in [23]:  D.count (' 1 ')        #统计一个元素的个数Out [23]: 5in [24]: print ddeque ( [' Z ',  ' y ',  ' x ',  ' F ',  ' e ',  ' d ',  ' C ',  ' B ',  ' A ',  ' 1 ',  ' 2 ',   ' 3 ',  ' 1 ',  ' 1 ',  ' 1 ',  ' 1 ']) in [27]: d.rotate (2)    #右旋转In  [28]:  print ddeque ([' 1 ',  ' 1 ',  ' z ',  ' y ',  ' x ',  ' F ',  ' e ',  ' d ',  ' C ',   ' B ',  ' a ',   ' 1 ',  ' 2 ',  ' 3 ',  ' 1 ',  ' 1 ']) in [29]: d.rotate ( -2)   #左旋转In  [30]: print  ddeque ([' Z ',  ' y ',  ' x ',  ' F ',  ' e ',  ' d ',  ' C ',  ' B ',  ' A ',  ' 1 ',   ' 2 ',  ' 3 ',  ' 1 ',  ' 1 ',  ' 1 ',  ' 1 ']) in [31]: d.clear ()   #清空In  [32] :  print ddeque ([])

In addition, a maxlen parameter is found in the Deque function. The main limitation is the size of this two-way list

In [the]: D = collections.deque (xrange), 5) in [+]: Print Ddeque ([5, 6, 7, 8, 9], maxlen=5) in [36]:

2. Class Defaultdict

Python's built-in Dict type has a SetDefault method, which means. Gets the value corresponding to the key in a dict. If this key does not exist in the dict, then in this dict set key, the key corresponding to the value of defaultdict set the corresponding values

Example:

in [+]: D = {' name ': ' God ', ' age ': 28}in []: D.setdefault (' name ') # This key exists in Dict and therefore returns the corresponding valueout[52]: ' God ' in [48]: D.setdefault (' Test ') # This key does not exist in Dict, so it is set to Dict, no value is specified, value is Nonein [+]: dout[49]: {' age ': ' ' Name ': ' G Od ', ' Test ': none}in [[]: D.setdefault (' test2 ', ' Test2value ') # This key does not exist in Dict, so it is set to Dict, specifying value as "Test2value" Ou T[50]: ' Test2value ' in [Wuyi]: dout[51]: {' age ': +, ' Name ': ' God ', ' Test ': None, ' test2 ': ' Test2value '}

The class defaultdict is similar to the SetDefault function in Dict. It just sets the default value when it initializes the Dict.

Example:

#! /usr/bin/env pythonimport collectionsdef default_factory (): Return "Defualt value" #在dict存在一个d [' foo '] = ' bar ' value. If you get a key that does not exist in Dict, the return value of #则返回default_factory function d = collections.defaultdict (default_factory,foo= ' bar ') print ' d: ', dprint "Foo=>", D.get (' foo ') print "foo=>", d[' foo ']print "var=>", d[' bar ']print ' * ' * 20# defined dict, if key does not exist in Dict, Then put this key in Dict, # and assign this key to LISTD = Collections.defaultdict (list) print ' d: ', Dprint ' foo=> ', D.get (' foo ') print foo= > ", d[' foo ']print" var=> ", d[' bar]

The results of the implementation are as follows:

D:defaultdict (<function default_factory at 0x7f230849e8c0>, {' foo ': ' Bar '}) foo=> barfoo=> barvar=> Defualt value********************d:defaultdict (<type ' list ';, {}) foo=> nonefoo=> []var=> []

3. Class Counter

This article is from the "Learning Notes" blog, so be sure to keep this source http://unixman.blog.51cto.com/10163040/1651979

The collections module in Python