Identity dictionnary

Bob Ippolito bob at
Sun Feb 29 03:31:40 CET 2004

On 2004-02-28 21:05:48 -0500, Jean-Sebastien Roy <js at> said:

> I'm trying to create an object to be used as a key in a dictionary, 
> where comparison (for key retrieval) would be done by 'is' instead of 
> __eq__.
> (__eq__ is defined in this object, but serve another purpose and does 
> not return a boolean).
> Is defining __hash__ as id(self) sufficient to guarantee that __eq__ 
> will never be called by the dict implementation ? (it seems to work in 
> practice)
> If not, how to ensure __eq__ will never be called ?

I don't believe your assumption about dict is correct.  A typical dict 
is not going to have 2**32 slots in it, so you will eventually get a 
collision, which will call __eq__ unless the key you're getting happens 
to be the first item in that slot (it think uses object identity first 
as an optimization and for sanity, though I have not looked at the 

What you might want is a special kind of container, not a special kind 
of object to use as keys to a dict:

class identitymap(object):
    def __init__(self):
        self._dict = {}

    def __getitem__(self, item):
        return self._dict[id(item)][1]

    def __setitem__(self, item, value):
        self._dict[id(item)] = (item, value)

... etc.

I have used a data structure such as this before for similar reasons 
(an attempt at pure python transparent object persistence..).


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