Path: csiph.com!optima2.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!1.eu.feeder.erje.net!newsfeed.xs4all.nl!newsfeed7.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: Looking up a dictionary _key_ by key?
Date: Wed, 24 Jun 2015 08:44 +0200
Organization: None
References: <CAGGBd_pd9SA7SczanGARFr-+Z9SxP9Mkhfo-RivpcJLrnuexng@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 7Bit
User-Agent: KNode/4.13.3
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.6.1435128250.3674.python-list@python.org>
Lines: 40
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:93064

Dan Stromberg wrote:

> I know that sounds strange: usually we look up values by key, not keys.
> 
> But suppose you have a strange key type that despite being "equal", is
> not identical in some fields, and you need to see those fields.
> 
> Is there a way of getting the key used by the dictionary, short of
> storing a reference to it in the value, or using a second dictionary?

$ cat grab_key.py
class GrabKey:
    def __init__(self, key):
        self.key = key

    def __eq__(self, other):
        if self.key == other:
            self.dict_key = other
            return True
        return False

    def __hash__(self):
        return hash(self.key)


def grab_key(k, d):
    g = GrabKey(k)
    d[g]
    return g.dict_key


if __name__ == "__main__":
    d = {1: int, 2.0: float}
    print(grab_key(1.0, d))
    print(grab_key(2, d))
$ python3 grab_key.py 
1
2.0