为什么80%的码农都做不了架构师？>>> 
OrderedDict可以实现一个FIFO（先进先出）的dict，当容量超出限制时，先删除最早添加的Key：
#!/usr/bin/env python
#-*- coding: utf8 -*-
from collections import OrderedDict
class LastUpdatedOrderedDict(OrderedDict):
def __init__(self, capacity):
#super(LastUpdatedOrderedDict, self).__init__()
OrderedDict.__init__(self)#注意这里有self
self._capacity = capacity
def __setitem__(self, key, value):
containsKey = 1 if key in self else 0
if len(self) - containsKey >= self._capacity:
last = self.popitem(False)
print 'remove:', last
if containsKey:
del self[key]
print 'set:', (key, value)
else:
print 'add:', (key, value)
super(LastUpdatedOrderedDict, self).__setitem__(key, value)
#OrderedDict.__setitem__(self, key, value)
FIFO = LastUpdatedOrderedDict(5)
FIFO.__setitem__('huangyi', 'lz')
FIFO.__setitem__(1, 1)
FIFO.__setitem__(2, 2)
FIFO.__setitem__(3, 3)
FIFO.__setitem__(4, 4)super(LastUpdatedOrderedDict, self).__init__()等价于OrderedDict.__init__(self)，super返回LastUpdatedOrderedDict的父类OrderedDict。注意前面参数不带self，后面参数带self。
last = self.popitem(False)的popitem不带参数(或带True)时返回最后一个元素，带参数False时返回第一个元素。
References
[1].https://docs.python.org/3/library/stdtypes.html?highlight=popitem#dict.popitem
[2].http://www.cnblogs.com/lovemo1314/archive/2011/05/03/2035005.html