python学习笔记： collections模块

1. collections模块介绍

collections模块在原生python数据类型的基础上提供了新的数据类型：

• namedtuple: 生成可以使用名字来访问元素内容的tuple子类
• deque: 双端队列，可以快速的从另外一侧追加和推出对象
• Counter: 计数器，主要用来计数
• OrderedDict: 有序字典
• defaultdict: 带有默认值的字典

2. 使用介绍

2.1 namedtuple

namedtuple主要用来产生可以使用名称来访问元素的数据对象，通常用来增强代码的可读性，例如：

In [1]: from collections import namedtuple

In [2]: websites = [
   ...:     ('google', 'http://www.google.com/', 'search engine'),
   ...:     ('Sina', 'http://www.sina.com.cn/', 'blog'),
   ...:     ('taobao', 'http://www.taobao.com/', 'shopping store')
   ...: ]

In [3]: Website = namedtuple('Website', ['name', 'url', 'remark'])

In [4]: for website in websites:
   ...:     website = Website._make(website)
   ...:     print website
   ...:
Website(name='google', url='http://www.google.com/', remark='search engine')
Website(name='Sina', url='http://www.sina.com.cn/', remark='blog')
Website(name='taobao', url='http://www.taobao.com/', remark='shopping store')

In [5]: p = Website('w3', 'www.w3school.com.cn', 'study')

In [6]: p
Out[6]: Website(name='w3', url='www.w3school.com.cn', remark='study')

In [7]: d = p._asdict()

In [8]: d['name']
Out[8]: 'w3'

In [9]: d['url']
Out[9]: 'www.w3school.com.cn'

2.2 deque

deque是双端队列，可以从队列的头部增加或取出对象，相较于list，在数据量大的情况下能提高运行速度。

In [1]: from collections import deque

In [2]: p = deque([1, 2, 3, 4])

In [3]: p.append(5)

In [4]: p.appendleft(0)

In [5]: p
Out[5]: deque([0, 1, 2, 3, 4, 5])

2.3 defaultdict

defaultdict可传入一个工厂函数，在请求字典内不存在的key时，将调用工厂函数方法，将结果作为key的默认值，避免标准字典类型的KeyError错误。

In [1]: from collections import

In [2]: d = defaultdict(lambda: 'None')

In [3]: d[1]
Out[4]: 'None'

In [5]: members = [
    ...:     ['male', 'John'],
    ...:     ['male', 'Jack'],
    ...:     ['female', 'Lily'],
    ...:     ['male', 'Pony'],
    ...:     ['female', 'Lucy'],
    ...: ]

In [6]: result = defaultdict(list)

In [7]: for sex, name in members:
    ...:     result[sex].append(name)
    ...:

In [8]: print result
defaultdict(<type 'list'>, {'male': ['John', 'Jack', 'Pony'], 'female': ['Lily', 'Lucy']})

2.4 OrderedDict

OrderedDict不同于Dict, 它是一个有序的字典对象。

In [1]: from collections import OrderedDict

In [2]: items = (
   ...:     ('A', 1),
   ...:     ('B', 2),
   ...:     ('C', 3),
   ...:     ('D', 4)
   ...: )

In [3]: regular_dict = dict(items)

In [4]: ordered_dict = OrderedDict(items)

In [5]: for k, v in regular_dict.iteritems():
   ...:     print "regular dict %s = %s" % (k, v)
   ...:
regular dict A = 1
regular dict C = 3
regular dict B = 2
regular dict D = 4

In [6]: for k, v in ordered_dict.iteritems():
   ...:     print "ordered dict %s = %s" % (k, v)
   ...:
ordered dict A = 1
ordered dict B = 2
ordered dict C = 3
ordered dict D = 4

2.5 Counter

Counter是一个计数器，可以用来统计字符出现次数

In [1]: from collections import Counter

In [2]: c = Counter()

In [3]: for s in '111345eeabab':
   ...:     c[s] = c[s] + 1
   ...:

In [4]: c
Out[4]: Counter({'1': 3, '3': 1, '4': 1, '5': 1, 'a': 2, 'b': 2, 'e': 2})