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Groups > comp.lang.python > #5868

Re: hash values and equality

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Ulrich Eckhardt wrote:
> 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?
> 
> If you were familiar with what a hash map is, you wouldn't ask. The thing is 
> that the hash is used to look up the place in the map where the thing is 
> stored. 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.

> If this is something you don't 
> care about, and all you care about is identity, then I'd derive the hash 
> from each object's ID.

This won't work, as objects of the same type that compare equal should 
(and do, in my code) hash equal.

> This ID has another property which is something that is assumed for hashes, 
> and your code seems a bit to get that wrong, too, and that is that the hash 
> must not change. 

The hash does not change on the instances, and is the same for all 
instances of my type that compare equal.

~Ethan~

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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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