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:
A socket becomes ready for reading when someone connects after a call to
listen(which means thatacceptwon’t block) when data arrives from the remote end, or when the socket is closed or reset (in this case,recvwill return an empty string).A socket becomes ready for writing when the connection is established after a non-blocking call to
connector when 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 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.