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Date: Fri, 20 May 2011 16:50:02 +0100
From: MRAB <python@mrabarnett.plus.com>
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Subject: Re: hash values and equality
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On 20/05/2011 07:33, 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.
[snip]
Is this strictly true? I thought that the hash value, an integer, is
moduloed (Is that how you spell it? Looks weird!) with the number of
array elements to give an index into the array, so different hashes
could give the same index, and objects with different hashes could be
stored in the same 'bucket'.