Comparisons of incompatible types

[John Nagle]

On 12/8/2010 7:56 PM, geremy condra wrote:
> On Wed, Dec 8, 2010 at 1:01 PM, John Nagle<nagle at animats.com>  wrote:
>> On 12/7/2010 3:59 PM, Mark Wooding wrote:
>>>>
>>>> Exactly one of
>>>>>
>>>>>        a > b
>>>>>        a = b
>>>>>        a < b
>>>>>
>>>>>   is true, or an type exception must be raised.

    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.

It's not good to break trichotomy.

				John Nagle