Observer design pattern example. Increase in student record to a database/record is notified to the concerned departments.

Observer.py

"""
@Author Sujayyendhiren Ramarao
@Description Observer Design pattern
"""
import abc

class Subject(object):
    """ Subject class that notifies the increase or decrease in student count to all the departments (observers)."""

    def __init__(self):
        self.studentCount = 0 # Student count in college
        self.observers = [] # List of all the observers

    def getState(self):
        return self.studentCount

    def setState(self, incOrDec):
        if( incOrDec == True ):
            self.studentCount = self.studentCount + 1
        else:
            self.studentCount = self.studentCount - 1
        print("Setting count: " + str(self.studentCount))
        self.notifyAllObservers()

    def notifyAllObservers(self):
        for observer in self.observers:
            observer.update()

    def attach(self,newObserver):
        self.observers.append(newObserver)

class Observer( object ):
    """ Abstract class for Observer"""
    __metaclass__ = abc.ABCMeta
    @abc.abstractmethod
    def update(self):
        return

class ObserverCompSci(Observer):
    """Computer Science department"""
    def __init__(self, subject):
        self.subject = subject
        self.subject.attach(self)
        self.totalStudents = 0;

    def update(self):
        self.totalStudents = self.subject.getState()
        print("Computer science department has " +  str(self.totalStudents) +" students")

class ObserverAdminDept(Observer):
    """Admin dept."""
    def __init__(self, subject):
        self.subject = subject
        self.subject.attach(self)
        self.totalStudents = 0;

    def update(self):
        self.totalStudents = self.subject.getState()
        print("Administrative department has " +  str(self.totalStudents) +" students")

if __name__ == "__main__":
    subject = Subject()

    obsCS = ObserverCompSci(subject)
    obsAdn = ObserverAdminDept(subject)

    subject.setState( True )
    subject.setState( True )
    subject.setState( True )

    subject.setState( False )