Basic Python Question

[Christopher T King]

On Thu, 22 Jul 2004, Richard Spooner wrote:

> I have created a threaded class which sets up a socket and then binds to a
> port.  When I make a new instance I send the port number and I would like
> the __init__ routine to set up the socket and then attempt to bind it to a
> port.
> 
> If I already have an instance of this class running then obviously I'll
> already have a port bound.  So if I try and create another instance with the
> same port I'd like the program to flag this error and inform the user that
> it is instead using the original instance..

To get the effect you want (return the previous instance), you'll need to 
do some trickery with __new__, the function which is responsible for 
creating a class (as opposed to initialization, which is handled by 
__init__):

import threading
import socket
import struct

class dataretriever(threading.Thread):
    __bound = {}	# this holds data of the form port:class_instance
    __opened = False	# used to stop multiple __init__s

    def __new__(cls,port):

        # try to get an existing class instance bound to port
        try:
            c = cls.__bound[port]

        # if that fails, make a new instance
        except KeyError:
            c = threading.Thread.__new__(cls, port)
            # and store it in our class variable
            cls.__bound[port] = c

        # this is executed if the try: succeeded
        else:
            print 'Port %d already bound, returning previous instance!' % port

        # __new__ must return an instance of cls
        return c

    def __init__(self, port):

        # only initialize ourselves once
	if self.__opened: return True
        self.__opened=True
	
        threading.Thread.__init__(self)
        self.setDaemon(1)
        self.resultQueue = resultsQueue
        self.s = socket.socket( socket.AF_INET, socket.SOCK_DGRAM )
        try:
            self.s.bind(( '', port))
        except:
            print "Error binding worker"
 
 
        self.start()

    def run(self):
        while 1:
            pass
            #playing with the data I recieve


>>> x = dataretriever(9999)
>>> z = dataretriever(10000)
>>> z is x
False
>>> y = dataretriever(9999)
Port 9999 lready bound, returning previous instance!
>>> y is x
True

Hope this all makes some sense :P