[Python-Dev] PyObject_RichCompareBool identity shortcut

[Glenn Linderman]

On 4/27/2011 6:11 PM, Ethan Furman wrote:
> Mark Dickinson wrote:
>> On Wed, Apr 27, 2011 at 10:37 AM, Hrvoje Niksic 
>> <hrvoje.niksic at avl.com> wrote:
>>> The other day I was surprised to learn this:
>>>
>>>>>> nan = float('nan')
>>>>>> nan == nan
>>> False
>>>>>> [nan] == [nan]
>>> True                  # also True in tuples, dicts, etc.
>>
>> That one surprises me a bit too:  I knew we were using
>> identity-then-equality checks for containment (nan in [nan]), but I
>> hadn't realised identity-then-equality was also used for the
>> item-by-item comparisons when comparing two lists.  It's defensible,
>> though: [nan] == [nan] should presumably produce the same result as
>> {nan} == {nan}, and the latter is a test that's arguably based on
>> containment (for sets s and t, s == t if each element of s is in t,
>> and vice versa).
>>
>> I don't think any of this should change.  It seems to me that we've
>> currently got something approaching the best approximation to
>> consistency and sanity achievable, given the fundamental
>> incompatibility of (1) nan breaking reflexivity of equality and (2)
>> containment being based on equality.  That incompatibility is bound to
>> create inconsistencies somewhere along the line.
>>
>> Declaring that 'nan == nan' should be True seems attractive in theory,
>> but I agree that it doesn't really seem like a realistic option in
>> terms of backwards compatibility and compatibility with other
>> mainstream languages.
>
> Totally out of my depth, but what if the a NaN object was allowed to 
> compare equal to itself, but different NaN objects still compared 
> unequal?  If NaN was a singleton then the current behavior makes more 
> sense, but since we get a new NaN with each instance creation is there 
> really a good reason why the same NaN can't be equal to itself?

 >>> n1 = float('NaN')
 >>> n2 = float('NaN')
 >>> n3 = n1

 >>> n1
nan
 >>> n2
nan
 >>> n3
nan

 >>> [n1] == [n2]
False
 >>> [n1] == [n3]
True

This is the current situation: some NaNs compare equal sometimes, and 
some don't.  And unless you are particularly aware of the identity of 
the object containing the NaN (not the list, but the particular NaN 
value) it is surprising and confusing, because the mathematical 
definition of NaN is that it should not be equal to itself.
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