[Python-3000] None in Comparisons: None vs. float("inf")

[Jeffrey Yasskin]

Be glad you're not programming in C++ then, where trying to sort NaN
can cause segfaults!

More seriously, I think using the following function as the sort key
will make sort do what you want:

def SortNoneFirstAndNanLast(x):
  if x is None:
    return (1, x)
  if isnan(x):
    return (3, x)
  return (2, x)

No need to modify either sort() or <.

On Thu, Nov 13, 2008 at 11:15 AM, Bruce Leban <bruce at leapyear.org> wrote:
> I think the behavior of NaN in comparisons is more confusing:
>>>> sorted([1,nan,2])
> [1, nan, 2]
>>>> sorted([2,nan,1])
> [2, nan, 1]
>>>> sorted([2,None,1])
> Traceback (most recent call last):
>   File "<pyshell#28>", line 1, in <module>
>     sorted([2,None,1])
> TypeError: unorderable types: NoneType() < int()
> At least the third case is clear that I shouldn't have done that. The way
> nan works, the results of sorting where one of the values is nan is
> unpredictable and useless.
> Yes, I know the rules about how NaN values behave in comparisons.
> Notwithstanding that, sorting could use a different comparison rule imposing
> a total ordering: -inf, ..., inf, nan as some other systems do.
> --- Bruce
>
> On Thu, Nov 13, 2008 at 10:55 AM, Tim Peters <tim.peters at gmail.com> wrote:
>>
>> [M.-A. Lemburg]
>> > ...
>> > So far, I haven't heard a single argument for why not having None
>> > participate in an ordering scheme is a good strategy to use, except
>> > that it's pure.
>>
>> I've tracked down plenty of program logic errors that would have been
>> discovered more easily if comparing None to (mostly, but among others)
>> integers and strings had raised an exception instead of returning a
>> meaningless true/false result.  Perhaps you haven't.  For those who
>> have, the attraction to making comparisons with None refuse to return
>> nonsense silently is both obvious and visceral.
>>
>>
>> > IMHO, practicality beats purity in this special case.
>>
>> If hiding program logic errors is practical, sure ;-)
>>
>> there-is-no-behavior-no-matter-how-bizarre-someone-won't
>> come-to-rely-on-ly y'rs  - tim
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-- 
Namasté,
Jeffrey Yasskin
http://jeffrey.yasskin.info/