Re: 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

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