Re: hash values and equality

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From	Peter Otten <__peter__@web.de>
Newsgroups	comp.lang.python
Subject	Re: hash values and equality
Followup-To	comp.lang.python
Date	Fri, 20 May 2011 08:38:30 +0200
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Ethan Furman wrote:

> Several folk have said that objects that compare equal must hash equal,
> and the docs also state this
> http://docs.python.org/dev/reference/datamodel.html#object.__hash__
> 
> I'm hoping somebody can tell me what horrible thing will happen if this
> isn't the case?  Here's a toy example of a class I'm thinking of writing
> that will compare equal with int's, but hash differently:
> 
> --> class Wierd():
> ...     def __init__(self, value):
> ...         self.value = value
> ...     def __eq__(self, other):
> ...         return self.value == other
> ...     def __hash__(self):
> ...         return hash((self.value + 13) ** 3)
> ...

Try this:

>>> d = {Wierd(1): 0}
>>> 1 in d
False
>>> 1 in d.keys()
True

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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