float("nan") in set or as key

[Erik Max Francis]

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}
> 
> except that sometimes it can't:
> 
>  >>> nan = float("nan")
>  >>> {nan, nan}
> {nan}

It's fundamentally because NaN is not equal to itself, by design. 
Dictionaries and sets rely on equality to test for uniqueness of keys or 
elements.

 >>> nan = float("nan")
 >>> nan == nan
False

In short, don't do that.

-- 
Erik Max Francis && max at alcyone.com && http://www.alcyone.com/max/
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