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.
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