float("nan") in set or as key

[Steven D'Aprano]

On Sun, 29 May 2011 22:19:49 +0100, Nobody wrote:

> On Sun, 29 May 2011 10:29:28 +0000, Steven D'Aprano wrote:
> 
>>>     The correct answer to "nan == nan" is to raise an exception,
>>>     because
>>> you have asked a question for which the answer is nether True nor
>>> False.
>> 
>> Wrong.
> 
> That's overstating it. There's a good argument to be made for raising an
> exception. 

If so, I've never heard it, and I cannot imagine what such a good 
argument would be. Please give it.

(I can think of *bad* arguments, like "NANs confuse me and I don't 
understand the reason for their existence, therefore I'll give them 
behaviours that make no sense and aren't useful". But you did state there 
is a *good* argument.)



> Bear in mind that an exception is not necessarily an error,
> just an "exceptional" condition.

True, but what's your point? Testing two floats for equality is not an 
exceptional condition.


>> The correct answer to "nan == nan" is False, they are not equal.
> 
> There is no correct answer to "nan == nan". 

Why on earth not?


> Defining it to be false is just the "least wrong" answer.

So you say, but I think you are incorrect.


> Arguably, "nan != nan" should also be false,
> but that would violate the invariant "(x != y) == !(x == y)".

I cannot imagine what that argument would be. Please explain.



-- 
Steven