On 17 February 2014 11:43, M.-A. Lemburg <m...@egenix.com> wrote: > On 17.02.2014 12:23, Gustavo Carneiro wrote: > > On 17 February 2014 11:14, M.-A. Lemburg <m...@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. > > This doesn't only apply to numeric comparisons. In Python 2 you > can compare None with any kind of object and it always sorts first, > based on the intuition that nothing is less than anything :-) > > I still think that relying on your intuition is not the right way for Python. Refuse the temptation to guess.
> FWIW, I don't think we need to invent a new name for it, just add > an appropriate tp_richcompare slot to the PyNoneType or readd the > special case to Object/object.c. This would also aid in porting > existing Python 2 code to Python 3. > Based on your comment, SortsFirst and SortsLast sound like good names ;-) These would be "universal sortable objects", that could be compared to any other type. > > -- > Marc-Andre Lemburg > eGenix.com > > Professional Python Services directly from the Source (#1, Feb 17 2014) > >>> Python Projects, Consulting and Support ... http://www.egenix.com/ > >>> mxODBC.Zope/Plone.Database.Adapter ... http://zope.egenix.com/ > >>> mxODBC, mxDateTime, mxTextTools ... http://python.egenix.com/ > ________________________________________________________________________ > 2014-02-12: Released mxODBC.Connect 2.0.4 ... http://egenix.com/go53 > > ::::: Try our mxODBC.Connect Python Database Interface for free ! :::::: > > eGenix.com Software, Skills and Services GmbH Pastor-Loeh-Str.48 > D-40764 Langenfeld, Germany. CEO Dipl.-Math. Marc-Andre Lemburg > Registered at Amtsgericht Duesseldorf: HRB 46611 > http://www.egenix.com/company/contact/ > -- Gustavo J. A. M. Carneiro Gambit Research LLC "The universe is always one step beyond logic." -- Frank Herbert
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