[Tutor] detecting data member changes

[Kent Johnson]

Luke Paireepinart wrote:
> Supposing I have a class like this:
> 
> class wiimote(object):
>     def __init__(self):
>         self.leds = [0,0,0,0]
>     def updateLEDs(self):
>         do_stuff()#write correct output report to wiimote to toggle LEDs.
> 
> So basically what I want to do is that whenever the self.leds variable 
> is changed, the updateLEDs method is called.
> ( I can't call it constantly in a loop because 1) the wii remote freaks 
> out, and 2) that seems pretty ineffiicent.)

Make leds a property:

class wiimote(object):
     def __init__(self):
         self._leds = [0, 0, 0, 0]

     def _set_leds(self, leds):
         self._leds = leds
         self._updateLEDs()

     def _get_leds(self):
         return self._leds

     leds = property(_get_leds, _set_leds)

     def _updateLEDs(self):
         print self._leds

w = wiimote()
print w.leds
w.leds = [1, 2, 3, 4]
print w.leds

I renamed _updateLEDs because it doesn't seem like this should be part 
of the public interface.

If you don't like having _set_leds and _get_leds in the namespace of 
wiimote, use one of the recipes here:
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/205183
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/410698

> I know I could use a __setattr__ but then I'd have to access the leds 
> using a dictionary with an 'LED' key mapped to the status array.
> It's not necessarily a bad thing for my own use, but it seems like it'd 
> be off-putting to other people using my library.

I don't understand this part. It seems to me this would work:
def __setattr__(self, name, value):
   object.__setattr__(self, name, value)
   if name == 'leds':
     self._updateLEDs()

Kent