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Tue Feb 17 09:27:43 EST 2004 · http://mail.python.org/pipermail/python-dev/2004-February/042593.html

At 08:55 AM 2/17/04 +0100, Martin v. Löwis wrote:
>Phillip J. Eby wrote:
>>Actually, it might be even better to start with equality.  Hashing is 
>>only meaningful for objects that can be tested for equality.
>
>However, in Python, all objects can be tested for equality. So hashing
>is meaningful for all objects?
>
>It is not: it is only meaningful for objects which compare equal to the
>same other objects over their lifespan.

That's not valid logic; "X is only meaningful for Y" means that X implies 
Y, not the other way around.  The fact that Y is a tautology doesn't imply 
that X is true, in fact if Y is a tautology then it only proves that "X 
implies Y" is true, because everything implies Y.

However, I see your point that it's therefore silly to talk about things 
that imply a tautology.  :)  Anyway, if you read the rest of my post, you'd 
see that I explained the "lifespan" issue with a bit more precision than 
you have stated above.  For example, I pointed out that it isn't necessary 
for an object to compare equal to the same other objects over its lifespan, 
only over the lifespan following its first __hash__ invocation.

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[Python-Dev] bool does not want to be subclassed?