Error : Only one usage of each socket address (protocol/network address/port) is normally permitted

I am trying to write a TCP client-server program in Python. The server echoes back (all CAPS) whatever the client sends to the server, once a connection is established. The server should be able to handle more than one request at a time in parallel by using threads.

However, as soon as I run the server.py file, I get the error :

Only one usage of each socket address (protocol/network address/port) is normally permitted.

I have tried changing the port number, and also tried using setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)

I am still getting the error. Please Help. Here is my code so far.

#!/usr/bin/python

import socket 
import sys 
import threading

class Server: 
    def __init__(self): 
        self.host = ''
        self.port = 12345 
        self.backlog = 5 
        self.size = 1024 
        self.server = None 
        self.threads = []       #initalizes a list of client threads

    def open_socket(self): 
        try: 
            self.server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
            self.server.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
            self.server.bind((self.host,self.port)) 
            self.server.listen(5) 
        except socket.error, (value,message): 
            if self.server: 
                self.server.close() 
            print "Could not open socket: " + message 
            sys.exit(1) 

    def run(self): 
        self.open_socket() 
        running = 1 
        while running:
            # handle the server socket 
            c = Client(self.server.accept()) 
            c.start() 
            self.threads.append(c) 

        #close all threads 

        self.server.close() 
        for c in self.threads: 
            c.join()

class Client(threading.Thread): 
    def __init__(self,(client,address)): 
        threading.Thread.__init__(self) 
        self.client = client 
        self.address = address 
        self.size = 1024

    def run(self):
        running = 1
        while running:
            data = client.recv(size)
            if data == 'Quit':
                break
            else:
                print 'Received : ', data
                newdata = data.upper()
                client.send(newdata)
        print '\nConnection Closed'
        client.close()
        running = 0

if __name__ == "__main__": 
    s = Server() 
    s.run()