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Groups > comp.lang.python > #5816

hash values and equality

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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)
...
--> one = Wierd(1)
--> two = Wierd(2)
--> three = Wierd(3)
--> one
<Wierd object at 0x00BFE710>
--> one == 1
True
--> one == 2
False
--> two == 2
True
--> three == 3
True
--> d = dict()
--> d[one] = '1'
--> d[two] = '2'
--> d[three] = '3'
--> d
{<Wierd object at 0x00BFE710>: '1',
  <Wierd object at 0x00BFE870>: '3',
  <Wierd object at 0x00BFE830>: '2'}
--> d[1] = '1.0'
--> d[2] = '2.0'
--> d[3] = '3.0'
--> d
{<Wierd object at 0x00BFE870>: '3',
  1: '1.0',
  2: '2.0',
  3: '3.0',
  <Wierd object at 0x00BFE830>: '2',
  <Wierd object at 0x00BFE710>: '1'}
--> d[2]
'2.0'
--> d[two]
'2'

All information greatly appreciated!

~Ethan~

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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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