Python dictionary that defaults to key?

Question

Is there a way to get a defaultdict to return the key by default? Or some data structure with equivalent behavior? I.e., after initializing dictionary d,

>>> d['a'] = 1 >>> d['a'] 1 >>> d['b'] 'b' >>> d['c'] 'c' 

I've only seen default dictionaries take functions that don't take parameters, so I'm not sure if there's a solution other than creating a new kind of dictionary .

Answer 1

I'd override the __missing__ method of dict:

>>> class MyDefaultDict(dict): ...     def __missing__(self, key): ...         self[key] = key ...         return key ... >>> d = MyDefaultDict() >>> d['joe'] 'joe' >>> d {'joe': 'joe'}