OrderedDict 有序字典
字典是无序的，可以通过OrderedDict使其有序。当对字典做迭代时，它会严格按照元素的初始的添加的顺序进行。
In [60]: o1 = collections.OrderedDict()
In [79]: o1['foo'] = 1
In [80]: o1['bar'] = 2
In [81]: o1['spam'] = 3
In [82]: o1['grok'] = 4
In [83]: o1['aa'] = 5
In [84]: o1
Out[84]: OrderedDict([('foo', 1), ('bar', 2), ('spam', 3), ('grok', 4), ('aa', 5)])
In [87]: for key in o1:
...: print key, o1[key]
...:
foo 1
bar 2
spam 3
grok 4
aa 5
由于有序字典会记住其插入顺序，因此可以与排序结合使用以创建排序字典
>>> # regular unsorted dictionary
>>> 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 by length of the key string
>>> OrderedDict(sorted(d.items(), key=lambda t: len(t[0])))
OrderedDict([('pear', 1), ('apple', 4), ('orange', 2), ('banana', 3)])
