Dictionary bidirectional

[Ken Starks]

Dennis Lee Bieber wrote:
> On Sun, 13 Jul 2008 16:21:11 -0700 (PDT), Kless
> <jonas.esp at googlemail.com> declaimed the following in comp.lang.python:
> 
>> I need a dictionary where get the result from a 'key' (on left), but
>> also from a 'value' (on right), how to get it?
>>
>> I know that dictionaries aren't bidirectional, but is there any way
>> without use two dictionaries?
>>
> 	Just out of curiosity... What do you expect to have returned from...
> 
> aDict = { "one" : "two",
> 		"three" : "four",
> 		"What?" : "two"	}
> 
> when looking for the value "two"?
> 
> 	In a dictionary, the /keys/ are unique... but the /values/ can be
> duplicates.

I wonder if anyone has implemented an 'equivalence class' class (for
finite sets) based on this.

Obviously the relation defined by
    k1~k2  iff D[k1] == D[k2]
does partition the set of all keys as an equivalence class.

So ... as a kind of inverse you could return a set, a subset of the
keys. How you would get a canonical representative of that set is
a different matter, of course. Unless, as in the OP's case, it is
a singleton set.

It would seem more efficient to do this when a key-value pair is
added or removed from the original dictionary rather than iterating
over all the keys each time you used it.