Roy Smith roy at
Sat Nov 23 03:15:10 CET 2013

In article <b8d42424-e0ab-4595-9c87-25e5c1b53349 at>,
 Bhanu Karthik <bhanukarthik2002 at> wrote:

> please help me.. what does the following line do?
> read_sockets,write_sockets,error_sockets = 

This is a little tricky.

First,read the docs at  
There's a lot of complicated stuff there, but just concentrate on the 
description of the call for now.

Imagine a process which has a lot of network connections open.  A great 
example would be something like a MUD (Multi User Dungeon).  You've got 
one server process(*) and a bunch of clients which have all made TCP 
connections over individual sockets.

Each client will be sending commands asynchronously, and the server 
needs to handle this.  You need some way to figure out which of those 
sockets have something that's been sent to you (which you need to 
process) and which are just sitting idle.  That's where select() comes 
in.  It gives you a way to say, "Here's a list of sockets.  Sleep until 
one of them has something available for me to read, and let me know 
which one."

One bit of complication is that you can also check for sockets which are 
ready to be written on, and sockets which have some sort of error 
condition.  That's why the call returns a 3-tuple.  But, for now, let's 
just concentrate on reading.

Here's a very simplistic server which uses select():

import socket
import select

sock = socket.socket()
sock.bind(('localhost', 23000))

# Accept four connections.
connections = []
for i in range(4):
    s, addr = sock.accept()
    print "Got connection from %s" % str(addr)

while True:
    readable, _, _ =, [], [])
    print "ready for reading: %s" % readable
    for s in readable:
        data = s.recv(1024)
        print "Read from %s: %s" % (s, data)

You can write a little client which connects to this (I've got one I 
used for testing, but I'll leave it to you to write one yourself as an 
exercise).  Connect four clients, and have them send some input in 
random order.

Actually, this server has a bug (which you'll discover as soon as you 
close one of the four connection), but it should serve to illustrate the 
basic concept.

(*) I'm not sure if real MUDs are programmed this way, but it's a 
plausible architecture.  For simplicity sake, I'm assuming a 
single-threaded server.

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