The select module

This module allows you to check for incoming data on one or more sockets, pipes, or other compatible stream objects.

You can pass one or more sockets to the select function, to wait for them to become readable, writable, or signal an error.

  • A socket becomes ready for reading when 1) someone connects after a call to listen (which means that accept won’t block), or 2) data arrives from the remote end, or 3) the socket is closed or reset (in this case, recv will return an empty string).
  • A socket becomes ready for writing when 1) the connection is established after a non-blocking call to connect, or 2) data can be written to the socket.
  • A socket signals an error condition when the connection fails after a non-blocking call to connect.

Example: Using the select module to wait for data arriving over sockets

# File:

import select
import socket
import time

PORT = 8037

TIME1970 = 2208988800L

service = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
service.bind(("", PORT))

print "listening on port", PORT

while 1:
    is_readable = [service]
    is_writable = []
    is_error = []
    r, w, e =, is_writable, is_error, 1.0)
    if r:
        channel, info = service.accept()
        print "connection from", info
        t = int(time.time()) + TIME1970
        t = chr(t>>24&255) + chr(t>>16&255) + chr(t>>8&255) + chr(t&255)
        channel.send(t) # send timestamp
        channel.close() # disconnect
        print "still waiting"

listening on port 8037
still waiting
still waiting
connection from ('', 1469)
still waiting
connection from ('', 1470)

In this example, we wait for the listening socket to become readable, which indicates that a connection request has arrived. We treat the channel socket as usual, since it’s not very likely that writing the four bytes will fill the network buffers. If you need to send larger amounts of data to the client, you should add it to the is_writable list at the top of the loop, and write only when select tells you to.

If you set the socket in non-blocking mode (by calling the setblocking method), you can use select also to wait for a socket to become connected. But the asyncore module (see the next section) provides a powerful framework which handles all this for you, so I won’t go into further details here.