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

Andrew Koenig ark@research.att.com
Tue, 15 Apr 2003 17:29:24 -0400 (EDT)

>> 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.

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


Guido> And then we should also change list.sort(), as Tim points out.  Maybe
Guido> we can start introducing this earlier by using keyword arguments:

Guido>   list.sort(lt=function)     sorts using a < implementation
Guido>   list.sort(cmp=function)    sorts using a __cmp__ implementation

The keyword argument might not be necessary:  It is always possible for
a function such as sort to figure out whether a comparison function is
2-way or 3-way (assuming it matters) by doing only one extra comparison.