Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!selfless.tophat.at!newsfeed.xs4all.nl!newsfeed6.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: hash values and equality
Date: Fri, 20 May 2011 22:25:27 +0200
Organization: None
References: <4DD2C2A5.3080403@stoneleaf.us> <BANLkTimvzZeN5dmm2xHP5xV8Kpw2Nb9kuQ@mail.gmail.com> <4DD2D89D.4000303@stoneleaf.us> <BANLkTintqgBLFtBx8+1b+R10nywuKdKHOw@mail.gmail.com> <4DD2F661.2050005@stoneleaf.us> <BANLkTikx8U4jWwLXXazpmtsL6MasDmyVyg@mail.gmail.com> <BANLkTi=_GqrNntRU7pv7x=kj3gHurk-Gaw@mail.gmail.com> <mailman.1822.1305870290.9059.python-list@python.org> <ir5292$3md$1@solani.org> <4DD6AB96.2040307@stoneleaf.us>
Mime-Version: 1.0
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 7Bit
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.1860.1305923134.9059.python-list@python.org>
Lines: 52
NNTP-Posting-Host: 82.94.164.166
Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:5881

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