#!/usr/bin/python

import objc
from Foundation import NSObject, NSKeyValueObservingOptionNew, NSKeyValueChangeNewKey

class MyClass(NSObject):
    base = objc.ivar("base", objc._C_INT)
    power = objc.ivar("power", objc._C_INT)
    result = objc.ivar("result", objc._C_INT)

    def result(self):
        return self.base ** self.power

MyClass.setKeys_triggerChangeNotificationsForDependentKey_(
    [u"base", u"power"],
    u"result"
)

class Observer(NSObject):
    def observeValueForKeyPath_ofObject_change_context_(self, path, object, changeDescription, context):
        print 'path "%s" was changed to "%s".' % (path, changeDescription[NSKeyValueChangeNewKey])

myInstance = MyClass.new()
observer = Observer.new()

myInstance.addObserver_forKeyPath_options_context_(observer, "result", NSKeyValueObservingOptionNew, 0)
myInstance.addObserver_forKeyPath_options_context_(observer, "base", NSKeyValueObservingOptionNew, 0)
myInstance.addObserver_forKeyPath_options_context_(observer, "power", NSKeyValueObservingOptionNew, 0)

myInstance.setValue_forKey_(2, "base")
myInstance.power = 4

print "%d ** %d == %d" % (myInstance.base, myInstance.valueForKey_("power"), myInstance.result())