[Python-Dev] python 3 niggle: None < 1 raises TypeError

[Gustavo Carneiro]

On 17 February 2014 11:14, M.-A. Lemburg <mal at egenix.com> wrote:

> On 15.02.2014 07:03, Stephen J. Turnbull wrote:
> > M.-A. Lemburg writes:
> >
> >  > IMO, it was a mistake to have None return a TypeError in
> >  > comparisons, since it makes many typical data operations
> >  > fail, e.g.
> >
> > I don't understand this statement.  The theory is that they *should*
> > fail.
> >
> > The example of sort is a good one.  Sometimes you want missing values
> > to be collected at the beginning of a list, sometimes at the end.
> > Sometimes you want them treated as top elements, sometimes as bottom.
> > And sometimes it is a real error for missing values to be present.
> > Not to mention that sometimes the programmer simply hasn't thought
> > about the appropriate policy.  I don't think Python should silently
> > impose a policy in that case, especially given that the programmer may
> > have experience with any of the above treatments in other contexts.
>
> None is special in Python and has always (and intentionally) sorted
> before any other object. In data processing and elsewhere in Python
> programming, it's used to signal: no value available.
>
> Python 3 breaks this notion by always raising an exception when
> using None in an ordered comparison, making it pretty much useless
> for the above purpose.
>
> Yes, there are ways around this, but none of them are intuitive.
>
> Here's a particularly nasty case:
>
> >>> l = [(1, None), (2, None)]
> >>> l.sort()
> >>> l
> [(1, None), (2, None)]
>
> >>> l = [(1, None), (2, None), (3, 4)]
> >>> l.sort()
> >>> l
> [(1, None), (2, None), (3, 4)]
>
> >>> l = [(1, None), (2, None), (3, 4), (2, 3)]
> >>> l.sort()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: unorderable types: int() < NoneType()
>
>
Maybe Python 3 should have a couple of None-like objects that compare the
way you want: AlwaysComparesLess and AlwaysComparesGreater, but with better
names (maybe 'PlusInfinity' and 'MinusInfinity'?).  Just leave None alone,
please.




> --
> Marc-Andre Lemburg
> eGenix.com
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-- 
Gustavo J. A. M. Carneiro
Gambit Research LLC
"The universe is always one step beyond logic." -- Frank Herbert
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