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To: python-list@python.org
From: Christoph Groth <cwg@falma.de>
Subject: Re: dict: retrieve the original key by key
Date: Sun, 15 May 2011 11:11:41 +0200
References: <874o4wwenu.fsf@falma.de> <BANLkTikszH_r-PAuL0pJ8reMc+bGafuRew@mail.gmail.com>
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Chris Rebert <clp2@rebertia.com> writes:

> On Sun, May 15, 2011 at 1:28 AM, Christoph Groth <cwg@falma.de> wrote:
>> I use a huge python dictionary where the values are lists of that
>> dictionary's keys (yes, a graph).  Each key is thus referenced
>> several times.
>>
>> As the keys are rather large objects, I would like to save memory by
>> re-using key objects wherever possible, instead of having several
>> equal objects in memory.
>>
>> There does not seem to be a way to retrieve the original key from a
>> python dictionary.  Is there a technical reason for this?  (Other
>> than that such functionality was not considered to be useful enough.)
>
> Define "original key".

def original_key(dictionary, key):
    for k in dictionary:
        if k == key:
            return k
    raise KeyError(key)

But this is not efficient.

I would like to avoid having _multiple_ objects which are equal (a == b)
but not the same (a is not b).  This would save a lot of memory.