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'}