[Python-Dev] Oberver Pattern

Raymond Hettinger python@rcn.com
Fri, 10 May 2002 19:19:38 -0400

From: "Andrew P. Lentvorski" <bsder@allcaps.org>
> How do you reconcile the range of subject/observer coupling and the
> complexity of the notify object?  At one end, the subject sends out all
> the necessary data for the observer to update its view without querying
> the subject at all.  At the other end, the subject merely sends out a
> minimal notify and the observer has to figure out what changed.

Like the GoF pattern, I had intended a minimal notify with self as a
parameter; however, there is certainly merit in PyNotify(self, keyOrIndex).

That would help the  observer know what part of the container changed.

> Do you
> intend to allow this flexibility or do you intend to impose one particular
> implementation?

Hmm.  Flexibility would be great; however,  I should really start with
the simplest possible implementation as a proof-of-concept and to
see how it is received.

> While I would love a formalized, reviewed set of python modules
> implementing patterns, I don't understand why you would want to do this at
> the C level.  The normal advantage to going to C is performance.

The other advantage of going to C is to make something possible that
cannot be done at the pure Python level.  In this case, there is currently
no way to tell whether a mutuable container has changed without reviewing
its entire content.

> As a
> rule of thumb, the observer pattern and performance are mutually
> exclusive.

Agreed :(

I may have to implement the simplified version (GvR's idea for having
only a single observer) in order to find-out for certain whether
observation is cheap or expensive.

Raymond Hettinger