Another newbie question

[Mike Meyer]

aleax at mail.comcast.net (Alex Martelli) writes:
> Mike Meyer <mwm at mired.org> wrote:
>> > "In addition to the full set of methods which operate on the coordinate as
>> > a whole, you can operate on the individual ordinates via instance.x and
>> > instance.y which are floats."
>> That's an API which makes changing the object more difficult. It may
>> be the best API for the case at hand, but you should be aware of the
>> downsides.
> Since x and y are important abstractions of a "2-D coordinate", I
> disagree that exposing them makes changing the object more difficult, as
> long of course as I can, if and when needed, change them into properties
> (or otherwise obtain similar effects -- before we had properties in
> Python, __setattr__ was still quite usable in such cases, for example,
> although properties are clearly simpler and more direct).

Exposing them doesn't make making changes more difficult. Allowing
them to be used to manipulate the object makes some changes more
difficult. Properties makes the set of such changes smaller, but it
doesn't make them vanish.

Take our much-abused coordinate example, and assume you've exposed the
x and y coordinates as attributes.

Now we have a changing requirement - we want to get to make the polar
coordinates available. To keep the API consistent, they should be
another pair of attributes, r and theta. Thanks to Pythons nice
properties, we can implement these with a pair of getters, and compute
them on the fly.

If x and y can't be manipulated individually, you're done. If they
can, you have more work to do. If nothing else, you have to decide
that you're going to provide an incomplete interface, in that users
will be able to manipulate the object with some attributes but not
others for no obvious good reason. To avoid that, you'll have to add
code to run the coordinate transformations in reverse, which wouldn't
otherwise be needed. Properties make this possible, which is a great
thing.

          <mike
-- 
Mike Meyer <mwm at mired.org>			http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.