Python Design Patterns - Observer

In this pattern, objects are represented as observers that wait for an event to trigger. An observer attaches to the subject once the specified event occurs. As the event occurs, the subject tells the observers that it has occurred.

The following UML diagram represents the observer pattern −

How to implement the observer pattern?

Let us now see how to implement the observer pattern.

import threading
import time
import pdb

class Downloader(threading.Thread):
   
   def run(self):
      print  downloading 
      for i in range(1,5):
         self.i = i
         time.sleep(2)
			print  unfunf 
         return  hello world 

class Worker(threading.Thread):
   def run(self):
      for i in range(1,5):
         print  worker running: %i (%i)  % (i, t.i)
         time.sleep(1)
         t.join()

         print  done 

t = Downloader()
t.start()

time.sleep(1)

t1 = Worker()
t1.start()

t2 = Worker()
t2.start()

t3 = Worker()
t3.start()

Output

The above program generates the following output −

Explanation

The above code explains the procedure of downloading a particular result. As per the observer pattern logic, every object is treated as observer. It prints the output when event is triggered.