Now that we’ve seen how sockets figure into the Internet picture, let’s move on to explore the tools that Python provides for programming sockets with Python scripts. This section shows you how to use the Python socket interface to perform low-level network communications. In later chapters, we will instead use one of the higher-level protocol modules that hide underlying sockets. Python’s socket interfaces can be used directly, though, to implement custom network dialogs and to access standard protocols directly.
The basic socket interface in Python is the standard library’s
socket
module. Like the os
POSIX module, Python’s socket
module is just a thin wrapper
(interface layer) over the underlying C library’s socket calls. Like
Python files, it’s also object-based—methods of a socket
object implemented by this module call out to the
corresponding C library’s operations after data conversions. For
instance, the C library’s send
and
recv
function calls become methods
of socket objects in Python.
Python’s socket
module
supports socket programming on any machine that supports BSD-style
sockets—Windows, Macs, Linux, Unix, and so on—and so provides a
portable socket interface. In addition, this module supports all
commonly used socket types—TCP/IP, UDP, datagram, and Unix domain—and
can be used as both a network interface API and a general IPC
mechanism between processes running on the same machine.
Beyond basic data communication tasks, this module also includes a variety of more advanced tools. For instance, it has calls for:
Converting bytes to a standard network ordering (ntohl
, htonl
)
Wrapping socket objects in a file object interface (sockobj.makefile
)
Making socket calls nonblocking (sockbj.setblocking
)
Setting socket timeouts (sockobj.settimeout
)
and more. Provided your Python was compiled with Secure Sockets
Layer (SSL) support, this module now also supports encrypted transfers
with its socket.ssl
. This call is
used in turn by other standard library modules to support the HTTPS
secure web site protocol (httplib
,
urllib
, and urllib2
), secure email transfers (poplib
and smtplib
), and more. We’ll meet some of these
other modules later in this part of the book, but we won’t study all
of the socket
module’s advanced
features in this text; see the Python library manual for usage details
omitted here.
Although we won’t get into advanced socket use in this
chapter, basic socket transfers are remarkably easy to code in
Python. To create a connection between machines, Python programs
import the socket
module, create
a socket object, and call the object’s methods to establish
connections and send and receive data.
Sockets are inherently bidirectional in nature, and socket
object methods map directly to socket calls in the C library. For
example, the script in Example
13-1 implements a program that simply listens for a
connection on a socket, and echoes back over a socket whatever it
receives through that socket, adding 'Echo=>'
string prefixes.
Example 13-1. PP3EInternetSocketsecho-server.py
############################################################################ # Server side: open a TCP/IP socket on a port, listen for a message from # a client, and send an echo reply; this is a simple one-shot listen/reply # conversation per client, but it goes into an infinite loop to listen for # more clients as long as this server script runs; the client may run on # a remote machine, or on same computer if it uses 'localhost' for server ############################################################################ from socket import * # get socket constructor and constants myHost = '' # server machine, '' means local host myPort = 50007 # listen on a non-reserved port number sockobj = socket(AF_INET, SOCK_STREAM) # make a TCP socket object sockobj.bind((myHost, myPort)) # bind it to server port number sockobj.listen(5) # listen, allow 5 pending connects while True: # listen until process killed connection, address = sockobj.accept( ) # wait for next client connect print 'Server connected by', address # connection is a new socket while True: data = connection.recv(1024) # read next line on client socket if not data: break # send a reply line to the client connection.send('Echo=>' + data) # until eof when socket closed connection.close( )
As mentioned earlier, we usually call programs like this that listen for incoming connections servers because they provide a service that can be accessed at a given machine and port on the Internet. Programs that connect to such a server to access its service are generally called clients. Example 13-2 shows a simple client implemented in Python.
Example 13-2. PP3EInternetSocketsecho-client.py
############################################################################ # Client side: use sockets to send data to the server, and print server's # reply to each message line; 'localhost' means that the server is running # on the same machine as the client, which lets us test client and server # on one machine; to test over the Internet, run a server on a remote # machine, and set serverHost or argv[1] to machine's domain name or IP addr; # Python sockets are a portable BSD socket interface, with object methods # for the standard socket calls available in the sytstem's C library; ############################################################################ import sys from socket import * # portable socket interface plus constants serverHost = 'localhost' # server name, or: 'starship.python.net' serverPort = 50007 # non-reserved port used by the server message = ['Hello network world'] # default text to send to server if len(sys.argv) > 1: serverHost = sys.argv[1] # or server from cmd line arg 1 if len(sys.argv) > 2: # or text from cmd line args 2..n message = sys.argv[2:] # one message for each arg listed sockobj = socket(AF_INET, SOCK_STREAM) # make a TCP/IP socket object sockobj.connect((serverHost, serverPort)) # connect to server machine and port for line in message: sockobj.send(line) # send line to server over socket data = sockobj.recv(1024) # receive line from server: up to 1k print 'Client received:', repr(data) # make sure it is quoted, was 'x' sockobj.close( ) # close socket to send eof to server
Before we see these programs in action, let’s take a minute to explain how this client and server do their stuff. Both are fairly simple examples of socket scripts, but they illustrate the common call patterns of most socket-based programs. In fact, this is boilerplate code: most socket programs generally make the same socket calls that our two scripts do, so let’s step through the important points of these scripts line by line.
Programs such as Example 13-1 that provide services for other programs with sockets generally start out by following this sequence of calls:
sockobj = socket(AF_INET,
SOCK_STREAM)
Uses the Python socket module to create a TCP socket
object. The names AF_INET
and SOCK_STREAM
are
preassigned variables defined by and imported from the socket
module; using them in combination means “create a TCP/IP
socket,” the standard communication device for the Internet.
More specifically, AF_INET
means the IP address
protocol, and SOCK_STREAM
means the TCP transfer protocol.
If you use other names in this call, you can instead
create things like UDP connectionless sockets (use SOCK_DGRAM
second) and Unix domain
sockets on the local machine (use AF_UNIX
first), but we won’t do so
in this book. See the Python library manual for details on
these and other socket module options. Using other socket
types is mostly a matter of using different forms of
boilerplate code.
sockobj.bind((myHost,
myPort))
Associates the socket object to an address—for IP addresses, we pass a server machine name and port number on that machine. This is where the server identifies the machine and port associated with the socket. In server programs, the hostname is typically an empty string (“”), which means the machine that the script runs on and the port is a number outside the range 0 to 1023 (which is reserved for standard protocols, described earlier).
Note that each unique socket dialog you support must
have its own port number; if you try to open a socket on a
port already in use, Python will raise an exception. Also
notice the nested parentheses in this call—for the AF_INET
address protocol socket
here, we pass the host/port socket address to bind
as a two-item tuple object
(pass a string for AF_UNIX
). Technically, bind
takes a tuple of values
appropriate for the type of socket created (but see the next
Note box about the older and deprecated convention of
passing values to this function as distinct
arguments).
sockobj.listen(5)
Starts listening for incoming client connections and allows for a backlog of up to five pending requests. The value passed sets the number of incoming client requests queued by the operating system before new requests are denied (which happens only if a server isn’t fast enough to process requests before the queues fill up). A value of 5 is usually enough for most socket-based programs; the value must be at least 1.
At this point, the server is ready to accept connection
requests from client programs running on remote machines (or the
same machine), and falls into an infinite loop—while True:
, or the equivalent while 1:
on older Pythons—waiting for
them to arrive:
connection, address =
sockobj.accept( )
Waits for the next client connection request to occur;
when it does, the accept
call returns a brand-new socket object over which data can
be transferred from and to the connected client. Connections
are accepted on sockobj
,
but communication with a client happens on connection
, the new socket. This
call actually returns a two-item tuple—address
is the connecting client’s
Internet address. We can call accept
more than one time, to
service multiple client connections; that’s why each call
returns a new, distinct socket for talking to a particular
client.
Once we have a client connection, we fall into another loop to receive data from the client in blocks of 1,024 bytes at a time, and echo each block back to the client:
data =
connection.recv(1024)
Reads at most 1,024 more bytes of the next message sent from a client (i.e., coming across the network), and returns it to the script as a string. We get back an empty string when the client has finished—end-of-file is triggered when the client closes its end of the socket.
connection.send('Echo=>' +
data)
Sends the latest data block back to the client
program, prepending the string 'Echo=>'
to it first. The
client program can then recv
what we send
here—the next reply line.
Technically this call sends as much data as possible, and
returns the number of bytes actually sent. To be fully
robust, programs need to resend unsent portions or use
connection.sendall
to
force all bytes to be sent.
connection.close(
)
Shuts down the connection with this particular client.
After talking with a given client, the server goes back to its infinite loop and waits for the next client connection request.
On the other hand, client programs like the one shown in Example 13-2 follow simpler call sequences. The main thing to keep in mind is that the client and server must specify the same port number when opening their sockets, and the client must identify the machine on which the server is running (in our scripts, server and client agree to use port number 50007 for their conversation, outside the standard protocol range):
sockobj = socket(AF_INET,
SOCK_STREAM)
Creates a Python socket object in the client program, just like the server.
sockobj.connect((serverHost,
serverPort))
Opens a connection to the machine and port on which
the server program is listening for client connections. This
is where the client specifies the string name of the service
to be contacted. In the client, we can either specify the
name of the remote machine as a domain name (e.g.,
starship.python.net
)
or numeric IP address. We can also give the server name as
localhost
(or the
equivalent IP address 127.0.0.1
) to specify that the
server program is running on the same machine as the client;
that comes in handy for debugging servers without having to
connect to the Net. And again, the client’s port number must
match the server’s exactly. Note the nested parentheses
again—just as in server bind
calls we really pass the
server’s host/port address to connect
in a tuple object.
Once the client establishes a connection to the server, it falls into a loop, sending a message one line at a time and printing whatever the server sends back after each line is sent:
sockobj.send(line)
Transfers the next message line to the server over the socket.
data =
sockobj.recv(1024)
Reads the next reply line sent by the server program. Technically, this reads up to 1,024 bytes of the next reply message and returns it as a string.
sockobj.close(
)
Closes the connection with the server, sending it the end-of-file signal.
And that’s it. The server exchanges one or more lines of text with each client that connects. The operating system takes care of locating remote machines, routing bytes sent between programs across the Internet, and (with TCP) making sure that our messages arrive intact. That involves a lot of processing too—our strings may ultimately travel around the world, crossing phone wires, satellite links, and more along the way. But we can be happily ignorant of what goes on beneath the socket call layer when programming in Python.
In older Python code, you may see the AF_INET
server address passed to the
server-side bind
and
client-side connect
socket
methods as two distinct arguments, instead of as a two-item
tuple:
soc.bind(host,port) vs soc.bind((host,port)) soc.connect(host,port) vs soc.connect((host,port))
This two-argument form is now deprecated, and only worked at all due to a shortcoming in earlier Python releases (unfortunately, the Python library manual’s socket example used the two-argument form too!). The tuple server address form is preferred and, in a rare Python break with full backward-compatibility, will likely be the only one that will work in future Python releases.
Okay, let’s put this client and server to work. There are two ways to run these scripts—on either the same machine or two different machines. To run the client and the server on the same machine, bring up two command-line consoles on your computer, start the server program in one, and run the client repeatedly in the other. The server keeps running and responds to requests made each time you run the client script in the other window.
For instance, here is the text that shows up in the MS-DOS console window where I’ve started the server script:
C:...PP3EInternetSockets>python echo-server.py
Server connected by ('127.0.0.1', 1025)
Server connected by ('127.0.0.1', 1026)
Server connected by ('127.0.0.1', 1027)
The output here gives the address (machine IP name and port number) of each connecting client. Like most servers, this one runs perpetually, listening for client connection requests. This server receives three, but I have to show you the client window’s text for you to understand what this means:
C:...PP3EInternetSockets>python echo-client.py
Client received: 'Echo=>Hello network world' C:...PP3EInternetSockets>python echo-client.py localhost spam Spam SPAM
Client received: 'Echo=>spam' Client received: 'Echo=>Spam' Client received: 'Echo=>SPAM' C:...PP3EInternetSockets>python echo-client.py localhost Shrubbery
Client received: 'Echo=>Shrubbery'
Here, I ran the client script three times, while the server script kept running in the other window. Each client connected to the server, sent it a message of one or more lines of text, and read back the server’s reply—an echo of each line of text sent from the client. And each time a client is run, a new connection message shows up in the server’s window (that’s why we got three).
It’s important to notice that client and server are running on the same machine here (a Windows PC). The server and client agree on the port number, but they use the machine names “” and localhost, respectively, to refer to the computer on which they are running. In fact, there is no Internet connection to speak of. Sockets also work well as cross-program communications tools on a single machine.
To make these scripts talk over the Internet rather than on a single machine, we have to do some extra work to run the server on a different computer. First, upload the server’s source file to a remote machine where you have an account and a Python. Here’s how I do it with FTP; your server name and upload interface details may vary, and there are other ways to copy files to a computer (e.g., email, web page post forms, etc.):[*]
C:...PP3EInternetSockets>ftp starship.python.net
Connected to starship.python.net. User (starship.python.net:(none)):lutz
331 Password required for lutz. Password: 230 User lutz logged in. ftp>put echo-server.py
200 PORT command successful. 150 Opening ASCII mode data connection for echo-server.py. 226 Transfer complete. ftp: 1322 bytes sent in 0.06Seconds 22.03Kbytes/sec. ftp>quit
Once you have the server program loaded on the other
computer, you need to run it there. Connect to that computer and
start the server program. I usually Telnet into my server machine
and start the server program as a perpetually running process from
the command line.[*] The &
syntax
in Unix/Linux shells can be used to run the server script in the
background; we could also make the server directly executable with
a #!
line and a chmod
command (see Chapter 3 for details). Here is the
text that shows up in a Window on my PC that is running a Telnet
session connected to the Linux server where I have an account
(minus a few deleted informational lines):
C:...PP3EInternetSockets>telnet starship.python.net
Red Hat Linux release 6.2 (Zoot) Kernel 2.2.14-5.0smp on a 2-processor i686 login:lutz
Password: [lutz@starship lutz]$python echo-server.py &
[1] 4098
Now that the server is listening for connections on the Net, run the client on your local computer multiple times again. This time, the client runs on a different machine than the server, so we pass in the server’s domain or IP name as a client command-line argument. The server still uses a machine name of “” because it always listens on whatever machine it runs on. Here is what shows up in the server’s Telnet window:
[lutz@starship lutz]$ Server connected by ('166.93.68.61', 1037) Server connected by ('166.93.68.61', 1040) Server connected by ('166.93.68.61', 1043) Server connected by ('166.93.68.61', 1050)
And here is what appears in the MS-DOS console box where I run the client. A “connected by” message appears in the server Telnet window each time the client script is run in the client window:
C:...PP3EInternetSockets>python echo-client.py starship.python.net
Client received: 'Echo=>Hello network world' C:...PP3EInternetSockets>python echo-client.py starship.python.net ni Ni NI
Client received: 'Echo=>ni' Client received: 'Echo=>Ni' Client received: 'Echo=>NI' C:...PP3EInternetSockets>python echo-client.py starship.python.net Shrubbery
Client received: 'Echo=>Shrubbery' C:...PP3EInternetSockets>ping starship.python.net
Pinging starship.python.net [208.185.174.112] with 32 bytes of data: Reply from 208.185.174.112: bytes=32 time=311ms TTL=246ctrl-C
C:...PP3EInternetSockets>python echo-client.py 208.185.174.112 Does she?
Client received: 'Echo=>Does' Client received: 'Echo=>she?'
The ping
command can be
used to get an IP address for a machine’s domain name; either
machine name form can be used to connect in the client. This
output is perhaps a bit understated—a lot is happening under the
hood. The client, running on my Windows laptop, connects with and
talks to the server program running on a Linux machine perhaps
thousands of miles away. It all happens about as fast as when
client and server both run on the laptop, and it uses the same
library calls; only the server name passed to clients
differs.
Before we move on, there are three practical usage details you should know. First, you can run the client and server like this on any two Internet-aware machines where Python is installed. Of course, to run the client and server on different computers, you need both a live Internet connection and access to another machine on which to run the server. You don’t need a big, expensive Internet link, though—a simple modem and dial-up Internet account will do for clients. When sockets are opened, Python is happy to use whatever connectivity you have, be it a dedicated T1 line or a dial-up modem account.
On a laptop PC with just dial-up access, for instance,
Windows automatically dials out to your ISP when clients are
started or when Telnet server sessions are opened. If a broadband
connection is available, that is utilized instead. In this book’s
examples, server-side programs that run remotely are executed on a
machine called starship.python.net
. If you don’t have
an account of your own on such a server, simply run client and
server examples on the same machine, as shown earlier; all you
need then is a computer that allows sockets, and most do.
Second, the socket module generally raises exceptions if you ask for something invalid. For instance, trying to connect to a nonexistent server (or unreachable servers, if you have no Internet link) fails:
C:...PP3EInternetSockets>python echo-client.py www.nonesuch.com hello
Traceback (innermost last):
File "echo-client.py", line 24, in ?
sockobj.connect((serverHost, serverPort)) # connect to server machine...
File "<string>", line 1, in connect
socket.error: (10061, 'winsock error')
Finally, also be sure to kill the server process before restarting it again, or else the port number will still be in use, and you’ll get another exception:
[lutz@starship uploads]$ps -x
PID TTY STAT TIME COMMAND 5570 pts/0 S 0:00 -bash 5570 pts/0 S 0:00 -bash5633 pts/0 S 0:00 python echo-server.py
5634 pts/0 R 0:00 ps -x [lutz@starship uploads]$python echo-server.py
Traceback (most recent call last): File "echo-server.py", line 14, in ? sockobj.bind((myHost, myPort)) # bind it to server port number socket.error: (98, 'Address already in use')
A series of Ctrl-Cs will kill the server on Linux (be sure
to type fg
to bring it to the
foreground first if started with an &
):
[lutz@starship uploads]$python echo-server.py
ctrl-c
Traceback (most recent call last): File "echo-server.py", line 18, in ? connection, address = sockobj.accept( ) # wait for next client connect KeyboardInterrupt
A Ctrl-C kill key combination won’t kill the server on my
Windows machine, however. To kill the perpetually running server
process running locally on Windows, you may need to type a
Ctrl-Alt-Delete key combination, and then end the Python task by
selecting it in the process listbox that appears. Closing the
window in which the server is running will also suffice on
Windows, but you’ll lose that window’s command history. You can
also usually kill a server on Linux with a kill -9
pid
shell command if it is running in another window or in the
background, but Ctrl-C requires less typing.
To see how the server handles the load, let’s fire
up eight copies of the client script in parallel using the script
in Example 13-3 (see
the end of Chapter 5 for
details on the launchmodes
module used here to spawn clients).
Example 13-3. PP3EInternetSockets estecho.py
import sys from PP3E.launchmodes import QuietPortableLauncher numclients = 8 def start(cmdline): QuietPortableLauncher(cmdline, cmdline)( ) # start('echo-server.py') # spawn server locally if not yet started args = ' '.join(sys.argv[1:]) # pass server name if running remotely for i in range(numclients): start('echo-client.py %s' % args) # spawn 8? clients to test the server
To run this script, pass no arguments to talk to a server listening on port 50007 on the local machine; pass a real machine name to talk to a server running remotely. On Windows, the clients’ output is discarded when spawned from this script:
C:...PP3EInternetSockets>python testecho.py C:...PP3EInternetSockets>python testecho.py starship.python.net
If the spawned clients connect to a server run locally, connection messages show up in the server’s window on the local machine:
C:...PP3EInternetSockets>python echo-server.py
Server connected by ('127.0.0.1', 1283)
Server connected by ('127.0.0.1', 1284)
Server connected by ('127.0.0.1', 1285)
Server connected by ('127.0.0.1', 1286)
Server connected by ('127.0.0.1', 1287)
Server connected by ('127.0.0.1', 1288)
Server connected by ('127.0.0.1', 1289)
Server connected by ('127.0.0.1', 1290)
If the server is running remotely, the client connection messages instead appear in the window displaying the Telnet connection to the remote computer:
[lutz@starship lutz]$python echo-server.py
Server connected by ('166.93.68.61', 1301)
Server connected by ('166.93.68.61', 1302)
Server connected by ('166.93.68.61', 1308)
Server connected by ('166.93.68.61', 1309)
Server connected by ('166.93.68.61', 1313)
Server connected by ('166.93.68.61', 1314)
Server connected by ('166.93.68.61', 1307)
Server connected by ('166.93.68.61', 1312)
Keep in mind, however, that this works for our simple
scripts only because the server doesn’t take a long time to
respond to each client’s requests—it can get back to the top of
the server script’s outer while
loop in time to process the next incoming client. If it could not,
we would probably need to change the server to handle each client
in parallel, or some might be denied a connection. Technically,
client connections would fail after five clients are already
waiting for the server’s attention, as specified in the server’s
listen
call. We’ll see how
servers can handle multiple clients robustly in the next
section.
It’s also important to know that this client and server engage in a proprietary sort of discussion, and so use the port number 50007 outside the range reserved for standard protocols (0 to 1023). There’s nothing preventing a client from opening a socket on one of these special ports, however. For instance, the following client-side code connects to programs listening on the standard email, FTP, and HTTP web server ports on three different server machines:
C:...PP3EInternetSockets>python >>>from socket import *
>>>sock = socket(AF_INET, SOCK_STREAM)
>>>sock.connect(('mail.rmi.net', 110))
# talk to RMI POP mail server >>>print sock.recv(40)
+OK Cubic Circle's v1.31 1998/05/13 POP3 >>>sock.close( )
>>>sock = socket(AF_INET, SOCK_STREAM)
>>>sock.connect(('www.python.org', 21))
# talk to Python FTP server >>>print sock.recv(40)
220 python.org FTP server (Version wu-2. >>>sock.close( )
>>>sock = socket(AF_INET, SOCK_STREAM)
>>>sock.connect(('starship.python.net', 80))
# starship HTTP web server >>>sock.send('GET / ')
# fetch root web page 7 >>>sock.recv(60)
'<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">