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Re: hash values and equality

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Ethan Furman wrote:

> Peter Otten 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?  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
>> 
> 
> My apologies -- I'm trying this in Python3:

Then you have to convert the keys to a list explicitly:

>>> class Weird:
...     def __init__(self, value):
...             self.value = value
...     def __eq__(self, other):
...             return self.value == other
...     def __hash__(self):
...             return hash((self.value + 13) **3)
...
>>> d = {Weird(1): 0}
>>> 1 in d
False
>>> 1 in d.keys()
False
>>> 1 in list(d.keys())
True

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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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