Type IObserver

object --+
         |
        IObserver

IObserver () -> IObserver

A class matching a observer interface of the observer pattern.

The IObserver class is an interface, that should be implemented by
classes, that should be able to register themselves on concrete
Subject implementations. It declares a single method, update(),
which receives the state change details of the Subject.

A short example for reacting upon state changes of Subject:

class MyObserver (IObserver):
    ...
    def update (self, subject, attribute, oldval, newval):
        # Look for the certain subject.
        if subject == 'UniqueSubjectName':
            # Which state changed?
            if attribute == 'value':
                self.do_some_action (oldval, newval)
        ...