[Python-Dev] Re: Re: lists v. tuples

Guido van Rossum guido@python.org
Sun, 16 Mar 2003 08:06:02 -0500

> > Guido> Yes.  And I'm still hoping to remove __cmp__; there should be
> > Guido> only one way to overload comparisons.

> > Moreover, for some data structures, the __cmp__ approach can be
> > expensive.  For example, if you're comparing sequences of any kind,
> > and you know that the comparison is for == or !=, you have your answer
> > immediately if the sequences differ in length.  If you don't know
> > what's being tested, as you wouldn't inside __cmp__, you may spend a
> > lot more time to obtain a result that will be thrown away.

> Yes.  OTOH, as long as cmp() is in the language, these same situations
> are more efficiently done by a __cmp__ implementation than by calling
> __lt__ and then __eq__ or similar (it's hard to decide which order is
> best).  So cmp() should be removed at the same time as __cmp__.

I realized the first sentence wasn't very clear.  I meant that
implementing cmp() is inefficient without __cmp__ for some types
(especially container types).  Example:

 cmp(range(1000)+[1], range(1000)+[0])

If the list type implements __cmp__, each of the pairs of items is
compared once.  OTOH, if the list type only implemented __lt__, __eq__
and __gt__, cmp() presumably would have to try one of those first, and
then another one.  If it picked __lt__ followed by __eq__, it would
get two False results in a row, meaning it could return 1 (cmp()
doesn't really expect incomparable results :-), but at the cost of
comparing each pair of items twice.  If cmp() picked another set of
two operators to try, I'd simply adjust the example.

--Guido van Rossum (home page: http://www.python.org/~guido/)