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How to make a python dictionary that returns key for keys missing from the dictionary instead of raising KeyError?

I need something like a defaultdict. However, for any key that isn't in the dictionary, it should return the key itself.

What is the best way to do that?

Use the magic __missing__ method:

>>> class KeyDict(dict):
...     def __missing__(self, key):
...             return key
... 
>>> x = KeyDict()
>>> x[2]
2
>>> x[2]=0
>>> x[2]
0
>>>