float("nan") in set or as key

[Albert Hopkins]

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.

Two "nan"s are not equal in the manner that 1.0 and 1.0 are equal:

>>> 1.0 == 1.0
True
>>> float("nan") == float("nan")
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.