python – 与dict.fromkeys()和类似dict的对象的KeyError

In [1]: d = {'a': 1,'b': 2}

In [2]: dict.fromkeys(d)
Out[2]: {'a': None,'b': None}

我尝试用类似dict的对象做同样的事情,但这总是引发一个KeyError,例如：

In [1]: class SemiDict:
   ...:     def __init__(self):
   ...:         self.d = {}
   ...:
   ...:     def __getitem__(self,key):
   ...:         return self.d[key]
   ...:
   ...:     def __setitem__(self,key,value):
   ...:         self.d[key] = value
   ...:
   ...:

In [2]: sd = SemiDict()

In [3]: sd['a'] = 1

In [4]: dict.fromkeys(sd)
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)

C:binConsole2&;ipython console> in <module>()

C:binConsole2&;ipython console> in __getitem__(self,key)

KeyError: 0

到底发生了什么？除了使用像dict.fromkeys(sd.d)这样的东西之外,还能解决吗？