from UserDict import UserDict
class RecursiveDict(UserDict):
    def __init__(self,*args,**kwargs):
        UserDict.__init__(self,*args,**kwargs)
    def __getitem__(self,key):
        if key in self.data:
            return self.data[key]
        else:
            self.__setitem__(key,dict())
            return self.data[key]
        if hasattr(self.__class__, "__missing__"):
            return self.__class__.__missing__(self, key)
        raise KeyError(key)
    def __setitem__(self, key, item):
         self.data[key] = item
my_dict = RecursiveDict()
my_dict['P'] = 6
my_dict['K']['D'] = 7
print my_dict