[Python-3000] Please re-add __cmp__ to python 3000

[Steven Bethard]

On 10/30/07, Adam Olsen <rhamph at gmail.com> wrote:
> On 10/30/07, Steven Bethard <steven.bethard at gmail.com> wrote:
> > On 10/30/07, Adam Olsen <rhamph at gmail.com> wrote:
> > > cmp and __cmp__ are doomed, due to unorderable types now raising exceptions:
> > >
> > > >>> cmp(3, 'hello')
> > > Traceback (most recent call last):
> > >   File "<stdin>", line 1, in <module>
> > > TypeError: unorderable types: int() < str()
> > > >>> 3 == 'hello'
> > > False
> > >
> > > A mixin for __cmp__ would be sufficient for scalars (where you can
> > > avoid this exception and your size is constant), but not for
> > > containers (which need to avoid inappropriate types and wish to avoid
> > > multiple passes.)
> >
> > I don't understand this conclusion.  If you start comparing things
> > that are unorderable, you'll get an exception.  But cmp() still makes
> > sense when you compare other things::
> >
> >     >>> cmp((1, 'a', 4.5), (1, 'a', 6.2))
> >     -1
> >     >>> cmp([6, 5, 4], [6, 4, 5])
> >     1
> >
> > I definitely don't want any cmp/__cmp__ implementation that swallows
> > exceptions when the types don't align, e.g.::
> >
> >     >>> cmp((1, 'a'), ('a', 1))
> >     Traceback (most recent call last):
> >       File "<stdin>", line 1, in <module>
> >     TypeError: unorderable types: int() < str()
>
> What I meant is that you can't use a mixin to map __eq__ to __cmp__,
> as you'll get TypeError even though == is defined for those types.

I wasn't suggesting that, though I don't see why a mixin would fail
here assuming you have both __eq__ and __lt__.  Just to the __lt__
comparison first.

I'm actually currently in favor of keeping __cmp__ as it is in Python
2.5.  If a class defines only __cmp__, Python will do the appropriate
dance to make <, >, ==, etc. work right.  If a class defines only
__eq__, __lt__, etc. Python will do the appropriate dance to make
cmp() work right.

Steve
-- 
I'm not *in*-sane. Indeed, I am so far *out* of sane that you appear a
tiny blip on the distant coast of sanity.
        --- Bucky Katt, Get Fuzzy