若字典不会增加字段，只读/原字段修改
使用ConstDict可节省内存
1、Dict扩容机制，预留内存空间
2、Dict也是一个对象，内部会动态维护__dict__，增加slot类属性可以节省内容
python版本：python2.7
例子：
>>> test_dic = {
... "m_HP": 1,
... "m_MP": 2,
... "m_Attack": 3,
... "m_Defense": 4,
... "m_Speed": 5,
... "m_Dodge": 6,
... "m_Hit": 7,
... "m_Double": 8,
... }
>>>
>>> class MonsterDict(constdict.ConstDict):
... __slots__ = test_dic.keys()
>>> const_dic = MonsterDict(test_dic)
>>> print(asizesof(test_dic))
(1192,)
>>> print(asizesof(const_dic))
(584,)
>>> print(const_dic)
{'m_Defense': 4, 'm_Speed': 5, 'm_Hit': 7, 'm_Double': 8, 'm_Attack': 3, 'm_HP': 1, 'm_Dodge': 6, 'm_MP': 2}
>>> print(const_dic.keys())
['m_HP', 'm_Defense', 'm_Speed', 'm_Attack', 'm_Dodge', 'm_MP', 'm_Hit', 'm_Double']
>>> print(const_dic.values())
[1, 4, 5, 3, 6, 2, 7, 8]
>>> const_dic["m_Dodge"] = 12456
>>> print(const_dic["m_Dodge"])
12456
>>> print(const_dic.iteritems())
<generator object <genexpr> at 0x00000000094FA2C8>