float("nan") in set or as key

[Grant Edwards]

On 2011-05-29, Albert Hopkins <marduk at letterboxes.org> wrote:
> On Sun, 2011-05-29 at 00:41 +0100, MRAB wrote:
>> Here's a curiosity. float("nan") can occur multiple times in a set or as 
>> a key in a dict:
>> 
>>  >>> {float("nan"), float("nan")}
>> {nan, nan}
>> 
> These two nans are not equal (they are two different nans)
>
>> except that sometimes it can't:
>> 
>>  >>> nan = float("nan")
>>  >>> {nan, nan}
>> {nan}
>
> This is the same nan, so it is equal to itself.

No, it's not.

>>> x = float("nan")
>>> y = x
>>> x is y
True
>>> x == y
False

> I can't cite this in a spec, but it makes sense (to me) that two things
> which are nan are not necessarily the same nan.

Even if they _are_ the same nan, it's still not equal to itself.

-- 
Grant