特殊函数一般以__methodname__的形式命名，如：__init__（构造方法）, __getitem__、 __setitem__（subscriptable所需method）, __delitem__（del obj[key]所需method）, __len__（len(…)所需method）等；
以下以什么都不做的Something类，结合lambda表达式，来说明这些特殊函数；
>>> class Something:
... pass
...
>>> s = Something()
>>> s['key1']
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'Something' object is not subscriptable
与subscriptable相关的函数是 __getitem__、 __setitem__，顾名思义，两者分别用于获取和设置相应的key的值；
Something.__getitem__ = lambda self, key: key
__getitem__简单的返回key；
>>> s['key1']
'key1'
>>> s[1]
1
>>> s[(1,2,3)]
(1, 2, 3)
注意：不可变类，都可以作为key；
>>> s[1] = 2
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'Something' object does not support item assignment
增加__setitem__方法，简单的打印key-value对；
>>> Something.__setitem__ = lambda self, key, value: print(repr(key) + ":" + repr(value))
>>> s[1] = 33
1:33
>>> s[(1,2,3)] = "333"
(1, 2, 3):'333'
>>> del s[1]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: __delitem__
增加__delitem__方法，简单打印
>>> Something.__delitem__ = lambda self, key : print(repr(key) + " is deleted")
>>> del s[23]
23 is deleted
>>> len(s)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: object of type 'Something' has no len()
增加__len__方法，简单返回1
>>> Something.__len__ = lambda self : 1
>>> len(s)
1
