a.index(float('nan')) fails

[Steven D'Aprano]

On Fri, 26 Oct 2012 03:54:02 -0400, Terry Reedy wrote:

> On 10/25/2012 10:44 PM, Steven D'Aprano wrote:
>> On Thu, 25 Oct 2012 22:04:52 -0400, Terry Reedy wrote:
>>
>>> It is a consequence of the following, which some people (but not all)
>>> believe is mandated by the IEEE standard.
>>>
>>>   >>> nan = float('nan')
>>>   >>> nan is nan
>>> True
>>
>> The IEEE 754 standard says nothing about object identity. It only
>> discusses value equality.
>>
>>>   >>> nan == nan
>>> False
>>
>> IEEE 754 states that all NANs compare unequal to everything, including
>> NANs with the same bit value. It doesn't make an exception for
>> comparisons with itself.
>>
>> I'm not entirely sure why you suggest that there is an argument about
>> what IEEE 754 says about NANs.
> 
> I did not do so.

I'm afraid you did. Your quote is shown above, and repeated here:

"... some people (but not all) believe is mandated by the IEEE standard"

This suggests that there is a disagreement -- an argument -- about what 
the IEEE standard mandates about NANs. I don't know why you think this 
disagreement exists, or who these "some people" are. The standard is not 
ambiguous, and while it is not readily available at no cost, it is widely 
described by many secondary sources.

Every NAN must compare unequal to every float, including itself.


> There has been disagreement about whether the standard mandates that
> Python behave the way it does. That is a fact, but I have no interest in
> discussing the issue.

I'm not entirely sure which behaviour of Python you are referring to 
here. If you choose not to reply, of course I can't force you to. It's 
your right to make ambiguous statements and then refuse to clarify what 
you are talking about.

If you are referring to *identity comparisons*, the IEEE 754 says nothing 
about object identity, so it has no bearing on Python's `is` operator.

If you are referring to the fact that `nan != nan` in Python, that is 
mandated by the IEEE 754 standard. I can't imagine who maintains that the 
standard doesn't mandate that; as I said, the disagreement that I have 
seen is whether or not to follow the standard, not on what the standard 
says.

If you are referring to something else, I don't know what it is.



-- 
Steven