Foundations of Python Network Programming 978-1-4302-3004-5

CHAPTER 2 ■ UDP
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
MAX = 65535
PORT = 1060
if sys.argv[1:] == ['server']:
» s.bind(('127.0.0.1', PORT))
» print 'Listening at', s.getsockname()
» while True:
» » data, address = s.recvfrom(MAX)
» » print 'The client at', address, 'says', repr(data)
» » s.sendto('Your data was %d bytes' % len(data), address)
elif sys.argv[1:] == ['client']:
» print 'Address before sending:', s.getsockname()
» s.sendto('This is my message', ('127.0.0.1', PORT))
» print 'Address after sending', s.getsockname()
» data, address = s.recvfrom(MAX) # overly promiscuous - see text!
» print 'The server', address, 'says', repr(data)
else:
» print >>sys.stderr, 'usage: udp_local.py server|client'
You should be able to run this script right on your own computer, even if you are not currently in
the range of a network, because both server and client use only the “localhost” IP address. Try running
the server first:
$ python udp_local.py server
Listening at ('127.0.0.1', 1060)
After printing this line of output, the server hangs and waits for an incoming message. In the source
code, you can see that it took three steps for the server to get up and running.
It first created a plain socket with the socket() call. This new socket has no name, is not yet
connected to anything, and will raise an exception if you attempt any communications with it. But the
socket is, at least, marked as being of a particular type: its family is AF_INET, the Internet family of
protocols, and it is of the SOCK_DGRAM datagram type, which means UDP. (The term “datagram” is the
official term for an application-level block of transmitted data. Some people call UDP packets
“datagrams”—like Candygrams, I suppose, but with data in them instead.)
Next, this simple server uses the bind() command to request a UDP network address, which you can
see is a simple tuple containing an IP address (a hostname is also acceptable) and a UDP port number.
At this point, an exception could be raised if another program is already using that UDP port and the
server script cannot obtain it. Try running another copy of the server—you will see that it complains:
$ python udp_local.py server
Traceback (most recent call last):
...
socket.error: [Errno 98] Address already in use
Of course, there is some very small chance that you got this error the first time you ran the server,
because port 1060 was already in use on your machine. It happens that I found myself in a bit of a bind
when choosing the port number for this first example. It had to be above 1023, of course, or you could
not have run the script without being a system administrator—and, while I really do like my little
example scripts, I really do not want to encourage anyone running them as the system administrator! I
could have let the operating system choose the port number (as I did for the client, as we will see in a
moment) and had the server print it out and then made you type it into the client as one of its
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