Type Subject

object --+
         |
        Subject

Subject (name) -> Subject

A class matching the subscriptable Subject of the observer pattern.

The Subject class allows other classes to be notified whenever its
state(s) change. It has an unique name which identifies itself upon
notification of the registered classes.

The unique name of the concrete subject implementation can be
adjusted using the 'name' attribute and set_name() method.

subject.name = 'MySubject'
subject.set_name ('AnotherSubject')

Observers will be notified about state changes through an update()
method, they have to implement. The IObserver interface class
contains details about its signature.

To notify observers about a state change, the Subject implementation
must invoke its notify() method with the state, that changed, its
old state value and the newly set value.

class MySubject (Subject):
    def __init__ (self):
        Subject.__init__ (self, 'UniqueSubjectName')
        self._value = None
    
    def set_value (self, value):
        # Preserve old value.
        oldval = self._value

        # Set new value.
        self._value = value

        # Notify observers
        self.notify ('value', old, value)

A listening observer which should react upon changes of that value
could implement the reaction the following way:

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)
        ...

Attributes:
name      - The name of the Subject.
listeners - List containing the attached observers.