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Date: Fri, 20 May 2011 11:38:41 -0700
From: Ethan Furman <ethan@stoneleaf.us>
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To: Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com>
Subject: 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~