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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: a.index(float('nan')) fails
Date: Fri, 26 Oct 2012 14:49:51 -0400
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On 10/26/2012 11:26 AM, Steven D'Aprano wrote:
> 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:

The quote precedes and refers to Python code.

>
> "... 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.

Disagreement about what Python should do has been expressed on the lists 
and even on the tracker. There was one discussion on python-ideas within 
the last month, another a year or so ago.

Python does not implement the full IEEE standard with signalling and 
non-signalling nans and multiple bit patterns.

When a nan is put in a Python collection, it is in effect treated as if 
it were equal to itself.
See the discussion in http://bugs.python.org/issue4296
including "I'm not sure that Python should be asked to guarantee
anything more than "b == b" returning False when b is
a NaN.  It wouldn't seem unreasonable to consider
behavior of nans in containers (sets, lists, dicts)
as undefined when it comes to equality and identity
checks."



-- 
Terry Jan Reedy