collections.md

collections 学习笔记

collections模块集结了Python中的高性能的容器数据类型，这些数据类型主要包括：namedtuple()、deque、Counter、OrderedDiect、defaultdict。他们作为替换内建（built-in）容器数据类型dict、list、set、tuple的可选方案。

####namedtuple()
快速,轻量级的attribute-style方式访问tuple.举例:
假设你有一个元组包含用户名和密码,访问用户名和密码需要通过元组的索引如:

credential = ('zhangsan', '123456') 
print ('username:'+ credential[0])
print ('password:'+credential[1])

这段代码没有任何错误,但是你想理解元组的含义你就必须读文档查看这两个参数的具体含义,那么这个时候namedtuple就可以派上用场了.

import collections
Credential = collections.namedtuple('Credential', 'username, password')
credential = Credential(username='zhangsan', password='123456')
print ('username:'+credential.username)
print ('password:'+credential.password)

namedtuple是在python2.6中开始出现的,在collections模块下面,它扩展的基础的tuple数据结构,tuple的每个索引位置被一个名字来代替,这样就可以通过名字来访问tuple中的元素了,同时你仍然还是可以通过索引来访问元素.它的好处就是可读性更强一些.另外只需更少的内存.

语法是这样的:

collections.namedtuple(typename, filed_names[, verbose])

地一个参数指定新类型的名字,第二个参数是字符串(用空格或者逗号隔开)它构成了这个类型的域. 如果verbose等于True.那么就会打印出类生成信息.

>>> People = namedtuple("People",'age name', True)
class People(tuple):
    'People(age, name)'

    __slots__ = ()

    _fields = ('age', 'name')

    def __new__(_cls, age, name):
        'Create new instance of People(age, name)'
        return _tuple.__new__(_cls, (age, name))

    @classmethod
    def _make(cls, iterable, new=tuple.__new__, len=len):
        'Make a new People object from a sequence or iterable'
        result = new(cls, iterable)
        if len(result) != 2:
            raise TypeError('Expected 2 arguments, got %d' % len(result))
        return result

    def __repr__(self):
        'Return a nicely formatted representation string'
        return 'People(age=%r, name=%r)' % self

    def _asdict(self):
        'Return a new OrderedDict which maps field names to their values'
        return OrderedDict(zip(self._fields, self))

    def _replace(_self, **kwds):
        'Return a new People object replacing specified fields with new values'
        result = _self._make(map(kwds.pop, ('age', 'name'), _self))
        if kwds:
            raise ValueError('Got unexpected field names: %r' % kwds.keys())
        return result

    def __getnewargs__(self):
        'Return self as a plain tuple.  Used by copy and pickle.'
        return tuple(self)

    __dict__ = _property(_asdict)

    def __getstate__(self):
        'Exclude the OrderedDict from pickling'
        pass

    age = _property(_itemgetter(0), doc='Alias for field number 0')

    name = _property(_itemgetter(1), doc='Alias for field number 1')


>>> 
>>> 
>>> 
>>> p = People(22,'zhsan')
>>> p[0]
22
>>> p.age
22
>>> 

类方法_make可以创建一个新实例

People._make(30, 'lisi')

http://stackoverflow.com/questions/2970608/what-are-named-tuples-in-python http://docs.python.org/2/library/collections.html?highlight=collections#collections.namedtuple