This select module, shown in Example 7-6, 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:
Example 7-6. Using the select Module to Wait for Data Arriving Over Sockets
File: select-example-1.py import select import socket import time PORT = 8037 TIME1970 = 2208988800L service = socket.socket(socket.AF_INET, socket.SOCK_STREAM) service.bind(("", PORT)) service.listen(1) print "listening on port", PORT while 1: is_readable = [service] is_writable =  is_error =  r, w, e = select.select(is_readable, 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 else: print "still waiting" listening on port 8037 still waiting still waiting connection from ('127.0.0.1', 1469) still waiting connection from ('127.0.0.1', 1470) ...
In Example 7-6, 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 the data to the is_writable list at the top of the loop and write only when told to by select.
If you set the socket in non-blocking mode (by calling the setblocking method), you can use select to wait for a socket to become connected, but the asyncore module (see the next section) provides a powerful framework that handles all this for you, so I won't go into further detail here.
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