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

Question

I want to create a python dictionary that returns me the key value for the keys are missing from the dictionary.

Usage example:

dic = smart_dict() dic['a'] = 'one a' print(dic['a']) # >>> one a print(dic['b']) # >>> b 

Answer 1

dicts have a __missing__ hook for this:

class smart_dict(dict):     def __missing__(self, key):         return key 

Could simplify it as (since self is never used):

class smart_dict(dict):     @staticmethod     def __missing__(key):         return key 

Answer 2

Why don't you just use

dic.get('b', 'b') 

Sure, you can subclass dict as others point out, but I find it handy to remind myself every once in a while that get can have a default value!

If you want to have a go at the defaultdict, try this:

dic = defaultdict() dic.__missing__ = lambda key: key dic['b'] # should set dic['b'] to 'b' and return 'b' 

except... well: AttributeError: ^collections.defaultdict^object attribute '__missing__' is read-only, so you will have to subclass:

from collections import defaultdict class KeyDict(defaultdict):     def __missing__(self, key):         return key  d = KeyDict() print d['b'] #prints 'b' print d.keys() #prints []