Re: hash values and equality

From	Gregory Ewing <greg.ewing@canterbury.ac.nz>
Newsgroups	comp.lang.python
Subject	Re: hash values and equality
Date	2011-05-21 20:24 +1200
Message-ID	<93pb5nF3gtU1@mid.individual.net> (permalink)
References	(5 earlier) <BANLkTikx8U4jWwLXXazpmtsL6MasDmyVyg@mail.gmail.com> <BANLkTi=_GqrNntRU7pv7x=kj3gHurk-Gaw@mail.gmail.com> <mailman.1822.1305870290.9059.python-list@python.org> <0deha8-6u9.ln1@satorlaser.homedns.org> <mailman.1850.1305915987.9059.python-list@python.org>

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Ethan Furman wrote:
> Ulrich Eckhardt wrote:
> 
>> If two equal objects have different hashes, they 
>> will be stored in different places in the hash map. Looking for 
>> object1 will then not turn up with object2, even though they are equal. 
> 
> In this case this is the behavior I want.

You can't rely on it, though. The hash value gets reduced
modulo the size of the dict, so even if two objects have
different hashes, in some cases they will land on the same
dict slot anyway.

So an object such as you're postulating would behave
unpredictably when used as a dict key. Sometimes a lookup
using a different but equal object would find it, and
sometimes not, seemingly at random.

-- 
Greg

Thread

hash values and equality Ethan Furman <ethan@stoneleaf.us> - 2011-05-19 22:43 -0700
  Re: hash values and equality Peter Otten <__peter__@web.de> - 2011-05-20 08:38 +0200
    Re: hash values and equality Ethan Furman <ethan@stoneleaf.us> - 2011-05-20 10:57 -0700
    Re: hash values and equality Peter Otten <__peter__@web.de> - 2011-05-20 22:25 +0200
    Re: hash values and equality Ethan Furman <ethan@stoneleaf.us> - 2011-05-20 14:48 -0700
  Re: hash values and equality Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2011-05-20 08:33 +0200
    Re: hash values and equality MRAB <python@mrabarnett.plus.com> - 2011-05-20 16:50 +0100
    Re: hash values and equality Chris Angelico <rosuav@gmail.com> - 2011-05-21 02:20 +1000
    Re: hash values and equality Ian Kelly <ian.g.kelly@gmail.com> - 2011-05-20 11:59 -0600
    Re: hash values and equality Ethan Furman <ethan@stoneleaf.us> - 2011-05-20 11:38 -0700
      Re: hash values and equality Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-05-21 20:24 +1200
        Re: hash values and equality Peter Otten <__peter__@web.de> - 2011-05-21 11:24 +0200
    Re: hash values and equality Christian Heimes <lists@cheimes.de> - 2011-05-20 21:01 +0200
    Re: hash values and equality MRAB <python@mrabarnett.plus.com> - 2011-05-20 21:17 +0100
      Re: hash values and equality Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-21 00:47 +0000
        Re: hash values and equality MRAB <python@mrabarnett.plus.com> - 2011-05-21 02:02 +0100
          Re: hash values and equality Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-21 01:55 +0000
    Re: hash values and equality John Nagle <nagle@animats.com> - 2011-05-21 15:55 -0700
      Re: hash values and equality Irmen de Jong <irmen.NOSPAM@xs4all.nl> - 2011-05-22 01:07 +0200
      Re: hash values and equality Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-22 00:24 +0000

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