1.3.5 OrderedDict 有序字典
OrderedDict是dict的子类，它记住了内容添加的顺序。
import collections
print 'Regular dictionary:'
d = {}
d['a'] = 'A'
d['b'] = 'B'
d['c'] = 'C'
for k, v in d.items():
print k, v
print '\nOrderedDict:'
d = collections.OrderedDict()
d['a'] = 'A'
d['b'] = 'B'
d['c'] = 'C'
for k, v in d.items():
print k, v
执行结果：
# ./collections_ordereddict_iter.py
Regular dictionary:
a A
c C
b B
OrderedDict:
a A
b B
c C
OrderedDict要内容和顺序完全相同才会视为相等。
import collections
print 'dict :',
d1 = {}
d1['a'] = 'A'
d1['b'] = 'B'
d1['c'] = 'C'
d2 = {}
d2['c'] = 'C'
d2['b'] = 'B'
d2['a'] = 'A'
print d1 == d2
print 'OrderedDict:',
d1 = collections.OrderedDict()
d1['a'] = 'A'
d1['b'] = 'B'
d1['c'] = 'C'
d2 = collections.OrderedDict()
d2['c'] = 'C'
d2['b'] = 'B'
d2['a'] = 'A'
print d1 == d2
执行结果：
# ./collections_ordereddict_equality.py
dict : True
OrderedDict: False
定义：
class collections.OrderedDict([items])
注意顺序以添加顺序为准，和修改的顺序无关。
特殊方法：OrderedDict.popitem(last=True) 。last为True是LIFO,即为堆栈，反之是FIFO，即为队列。还支持排序：reversed().
有序字典和有序字典的相等比较，是顺序相关的；和其他映射类型比较，是顺序无关的。
手册中的实例：
>>> # regular unsorteddictionary
>>> d = {'banana': 3, 'apple':4,'pear': 1, 'orange': 2}
>>> # dictionary sorted by key
>>> OrderedDict(sorted(d.items(),key=lambda t: t[0]))
OrderedDict([('apple', 4), ('banana', 3),('orange', 2), ('pear', 1)])
>>> # dictionary sorted by value
>>> OrderedDict(sorted(d.items(),key=lambda t: t[1]))
OrderedDict([('pear', 1), ('orange', 2),('banana', 3), ('apple', 4)])
>>> # dictionary sorted bylength of the key string
>>> OrderedDict(sorted(d.items(),key=lambda t: len(t[0])))
OrderedDict([('pear', 1), ('apple', 4),('orange', 2), ('banana', 3)])
如果想根据插入顺序排序：
classLastUpdatedOrderedDict(OrderedDict):
'Store items in the order the keys were last added'
def __setitem__(self, key, value):
if key in self:
del self[key]
OrderedDict.__setitem__(self, key, value)
上面已经存在的元素再次插入，就会移动到最后。
还可以与Counter结合，是Counter记住第一次元素出现的顺序。
classOrderedCounter(Counter, OrderedDict):
'Counter that remembers the order elementsare first encountered'
def __repr__(self):
return '%s(%r)' % (self.__class__.__name__,OrderedDict(self))
def __reduce__(self):
return self.__class__, (OrderedDict(self),)
参考资料：
SeeAlso:
collections(http://docs.python.org/library/collections.html) The standard library
documentationfor this module.