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 :-) 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. -- 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/ _______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
