Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!aioe.org!feeder.news-service.com!newsfeed.xs4all.nl!newsfeed6.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
Date: Fri, 20 May 2011 14:48:27 -0700
From: Ethan Furman <ethan@stoneleaf.us>
User-Agent: Thunderbird 1.5.0.10 (Windows/20070221)
MIME-Version: 1.0
To: Peter Otten <__peter__@web.de>
Subject: Re: hash values and equality
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$1@solani.org> <4DD6AB96.2040307@stoneleaf.us> <ir6ing$et2$1@dough.gmane.org>
In-Reply-To: <ir6ing$et2$1@dough.gmane.org>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Cc: python-list@python.org
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.1862.1305927371.9059.python-list@python.org>
Lines: 55
NNTP-Posting-Host: 82.94.164.166
Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:5883

Peter Otten wrote:
> 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__
>>>>
>>>> --> 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

Ah!!  The light finally dawned!  Many thanks for everyone's input.

So if Wierd has a need to compare equal to some other type, it should 
implement a .equals() method.  Gotcha.

Likewise, if two different type's instances can compare equal, then for 
the most part they should be interchangeable.

~Ethan~