Удаление объекта

#!/usr/bin/python2.7
import threading
import time
import sys
 
class f():
 
    num = 0
    stop = 0
    threads = []
        
    def __init__(self, _num):
        self.num = _num
        print(str(self.num)," created")
        self.threads.append(threading.Thread(target=self.listen, name="audio", args=()))
        self.threads.append(threading.Thread(target=self.listen, name="video", args=()))
        
        
    def listen(self):
        while stop == 0:
            print (self.num)
            time.sleep(2)
    
    def stop(self):
        self.stop = 1
        stop_flag = len(self.threads)
        print(str(self.num)," closing...")
        while stop_flag > 0:
            for th in self.threads:
                if not th.is_alive():
                    stop_flag -= 1
                    del th
                else:
                    print(str(self.num)," thread named " + th.getName() + " still alive!")  
 
        self.threads.clear()
        self.threads = None
        
        del self    
        
    def __del__(self):
        print("Avaible links to this object: " + str(sys.getrefcount(self)))
        print("Object destroyed")
        
pipes = []
        
for i in range(0,5):
    pipes.append(f(i))
 
print("Deleting objects...")
for x in range(len(pipes)-1, -1, -1):
    pipes[x].stop()
    del pipes[x]
    
print ("Now pipes: ", len(pipes))       
print("2 seconds to terminate...")      
 
for i in range(0,2):
    time.sleep(1)
 
print ("Now pipes: ", len(pipes))