[Persistence-sig] Observing object changes

[Lamy Jean-Baptiste]

Hi everyone,

I've just see the new persistance-SIG and i realize i have already worked
on observing object changes. I use it for updating graphical interfaces
when objects are changed, and for debugging, but it appears it can be
usefull for OO database too -- so such a feature would rock !


Here is the API i use:

addevent   (obj, func) -- Add    the event FUNC (=an observer) to OBJ
removeevent(obj, func) -- Remove the event FUNC from OBJ

FUNC is a callable which is then called each time OBJ is modified. It takes
4 arguments: the modified object, the name of the modified attribute, the
new value and the old value.


Example of use:

class Point:
  def __init__(self, x, y): self.x, self.y = x, y

def event(obj, attr, new, old):
  print attr, "was", old, "is now", new

p = Point(0.0, 0.0)

addevent(p, event)

p.x = 1.0
# print "x was 0.0 is now 1.0"


I have already written a pure Python implementation of that (feel free to
ask for the code if you're interested); it works well but it is really an
ugly hack ! The principle is to change the class of the observed object to
a new class that define a __setattr__ method.

However, the interest of this approach is that it work for ANY object
(including list and dict, and already existent object); the object doesn't
have to be designed to fit it (e.g. the Point class above was not written
especially to fit a particular observation framework).

I wonder if it is possible to write a more cleaner version of it in C...?


I hope this helps. Please don't strike me if i am off-topic ;-)

Jiba