Comparisons of incompatible types

[Terry Reedy]

On 12/9/2010 2:58 AM, Steven D'Aprano wrote:
> On Wed, 08 Dec 2010 20:16:57 -0800, John Nagle wrote:
>
>>      Here's an example where this issue produces invalid results in
>>      Python.
>>
>>   >>>  NaN = float("nan")
>>   >>>  arr = [1.0, 4.0, 3.0, 2.0, 5.0, NaN, 6.0, 3.0, NaN, 0.0, 1.0, 4.0,
>> 3.0, 2.0, 5.0, NaN, 6.0, 3.0, NaN, 0.0]
>>   >>>  sorted(arr)
>> [0.0, 0.0, 1.0, 1.0, 2.0, 2.0, 3.0, 3.0, 3.0, 3.0, 4.0, 5.0, nan, 5.0,
>> 6.0, nan, 4.0, nan, 6.0, nan]
>>
>> The sorted numerical values aren't in order. Note the 4.0 near the end,
>> after the 6.0.  "sort" has failed because it assumes that a<  b and b<
>> c implies a<  c. But that's not a valid assumption here.

This is transitivity.

>> It's not good to break trichotomy.

I believe that is that exactly one of <,=.> are true.



-- 
Terry Jan Reedy