Home Original page

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

Andrew Koenig ark@research.att.com
Sun, 16 Mar 2003 11:02:13 -0500 (EST) 
Guido> I realized the first sentence wasn't very clear.  I meant that
Guido> implementing cmp() is inefficient without __cmp__ for some types
Guido> (especially container types).  Example:

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

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

Yes.  If you want to present a 3-way comparison to users, an
underlying 3-way comparison is the fastest way to do it.  The trouble
is that a 3-way comparison is definitely not the fastest way to
present a 2-way comparison to users.

So if you want users to see separate 2-way and 3-way comparisons,
I think the fastest way to implement them is not to try to force
commonality where none exists.