float("nan") in set or as key

[Grant Edwards]

On 2011-05-29, Wolfgang Rohdewald <wolfgang at rohdewald.de> wrote:
> On Sonntag 29 Mai 2011, Tim Delaney wrote:
>> There's a second part the mystery - sets and dictionaries (and
>> I think lists) assume that identify implies equality (hence
>> the second result). This was recently discussed on
>> python-dev, and the decision was to leave things as-is.
>
> On Sonntag 29 Mai 2011, Grant Edwards wrote:
>> Even if they are the same nan, it's still not equal to itself.
>
> if I understand this thread correctly, they are not equal to itself
> as specified by IEEE

And Python follows that convention.

> but Python treats them equal in sets and dictionaries for performance
> reasons

It treats them as identical (not sure if that's the right word).  The
implementation is checking for ( A is B or A == B ).  Presumably, the
assumpting being that all objects are equal to themselves.  That
assumption is not true for NaN objects, so the buggy behavior is
observed.

-- 
Grant