python identity dictionary [duplicate]

Question

Possible Duplicate:
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?

Answer 1

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
>>> 

Answer 2

you mean something like the following?

value = dictionary.get(key, key)

Answer 3

class Dict(dict):
    def __getitem__(self, key):
        try:
            return super(Dict, self).__getitem__(key)
        except KeyError:
            return key

>>> a = Dict()
>>> a[1]
1
>>> a[1] = 'foo'
>>> a[1]
foo

This works if you have to support Python < 2.5 (which added the __missing__ method mentioned by @katrielalex).