Observer Pattern in Python

Well here are the first things i got from "Head First Design Patterns" book :) I was working on a big web project and have also some small open source projects i try to finish. I stoppped codding on that projects immediately after read the first chapter of the book :) I realized that have big gaps in my programming skills about patterns. What i know now is :

- Knowing OO principles and rules doesnt make u a OO guru.
- Inheritance is not the best way of code reuse.
- Inherit the static parts and use composition for things that can change in the future.
- Less dependency between classes is alway better.

Will add to those more in the future i think.
Ok lets go back to the title of that entry. Sometimes we need to send some notifications to other classes from a specific class. What i would do before knowing the observer pattern would be sth like that. I would define a notifier() method and will call the classes i want to notify :

1. def notify(self):


2.    classA.update(args)


3.    classB.update(args)


4.    classC.update(args)


5.  

Ok it seems good for now but wat happens if you want to notify the classes D and E in the future u have to add them to the list above. That means altering the existing code which may not be a good idea. Ok now the classB doesnt want anymore to be notified , so what happens now ? It is very clear that the structure above is not the best for that situation. What we need is a dynamic srtucture which will allow us to add and remove new observers during the runtime without altering the existing code, so that is where the observer pattern comes into the scene ...

Here is the code i wrote to make a simple scenarion for observer pattern on my personal git_repo : LINK
Now we can do sth like that easily :

from pyalgorithm.dp.observer import *



print "I creatte the our subject"

c_s = ConcreteSubject()

c_s.my_x = 22

c_s.my_y =33



print "I notify my subscribers :"

c_s.values_changed() #that can be done dynamic but it is cool for now



print "Lets add a new XObserver"

x_o = ObserverX()

g_o = ObserverGeneral()

c_s.add_observer(x_o)



print "I notify my subscribers :"

c_s.values_changed() #that can be done dynamic but it is cool for now





print "Lets add a new GeneralObserver"

c_s.add_observer(g_o)

print "I notify my subscribers :"

c_s.values_changed() #that can be done dynamic but it is cool for now



print "The X observer doesnt want to know anything about us remove him"

c_s.remove_observer(x_o)



print "I notify my subscribers :"

c_s.values_changed() #that can be done dynamic but it is cool for now

The output for that will be as follow :

I creatte the our subject
I notify my subscribers :
Lets add a new XObserver
I notify my subscribers :
Im X observer and got : 22
Lets add a new GeneralObserver
I notify my subscribers :
Im X observer and got : 22
I got all of them Here is my values 22 and 33
The X observer doesnt want to know anything about us remove him
I notify my subscribers :
I got all of them Here is my values 22 and 33

Well i think it became pretty dynamic ,i have lots of ideas where i can use that pattern. As a conclusion what i can say is that "Head First Design patterns" book is changing my vision to OO programming.

TO BE CONTINUED ...