Sockets programming in Python

Before you start

About this tutorial

Python is a popular object-oriented scripting language with a simple syntax and a large developer base. It is a general purpose language and can be used in a variety of settings. It’s also popular as a beginner’s programming language, much like the BASIC language of the 1970s.

This tutorial demonstrates the Python language with emphasis on networking programming. I define the basic sockets features of Python in addition to some of Python’s other classes that provide asynchronous sockets. I also detail Python’s application-layer protocol classes, showing how to build Web clients, mail servers and clients, and more.

I also demonstrate a simple chat server to illustrate the power of Python for sockets applications.

You should have a basic understanding of the standard BSD Sockets API and some experience with the GNU/Linux® environment. Some familiarity with object-oriented concepts is also beneficial.


This tutorial and the examples demonstrated in it rely on version 2.4 of Python. You can download this version from the Python Web site (see Resources for a link). To build the Python interpreter, you need the GNU C compiler (gcc) and the configure/make utilities (which are part of any standard GNU/Linux distribution).

You should have a basic understanding of the standard BSD Sockets API and some experience with the GNU/Linux environment. Some familiarity with object-oriented concepts is also beneficial.

Introducing Python

First, I’ll provide a taste of Python.

What is Python

Python is a general purpose object-oriented scripting language that can be applied to a large variety of problems. It was created in the early 1990s at CWI in Amsterdam and continues to evolve today under the Python Software Foundation.

Python is amazingly portable and can be found in almost all operating systems.

Python is interpreted and is easy to extend. You can extend Python by adding new modules that include functions, variables, or types through compiled C or C++ functions.

You can also easily embed Python within C or C++ programs, allowing you to extend an application with scripting capabilities.

One of the most useful aspects of Python is its massive number of extension modules. These modules provide standard functions such as string or list processing, but there are also application-layer modules for video and image processing, audio processing, and yes, networking.

A taste of Python

I’ll give you a sense of what Python is all about.

As an interpreted language, it’s easy to use Python to try out ideas and quickly prototype software. Python programs can be interpreted as a whole or line by line.

You can test the following snippets of Python code by first running Python, then typing each line one at a time. After Python is invoked, a prompt (>>>) appears to allow you to type commands. Note that indentation is important in Python, so the preceding spaces in a line must not be ignored:

Listing 1. Some Python samples to try out
#Open a file, read each line, and print it out
for line in open('file.txt'):
  print line

#Create a file and write to it
file = open("test.txt", "w")
file.write("test line\n")

#Create a small dictionary of names and ages and manipulate
family = {'Megan': 13, 'Elise': 8, 'Marc': 6}

#results in 8
#Remove the key/value pair
del family'Elise'

#Create a list and a function that doubles its input.  Map the
#   function to each of the elements of the list (creating a new
#   list as a result).
arr = 1, 2, 3, 4, 5def double(x): return x*x
map(double, arr)

#Create a class, inherit by another, and then instantiate it and
#   invoke its methods.
class Simple:
  def init(self, name): = name

  def hello(self):
    print" says hi."

class Simple2(Simple):
  def goodbye(self):
    print" says goodbye."

me = Simple2("Tim")

Why use Python

The number one reason to learn and use Python is its popularity. The size of its user base and the growing number of applications built with Python make it a worthwhile investment.

You find Python in several development areas — it’s used to build system utilities, as a glue language for program integration, for Internet applications, and for rapid prototyping.

Python also has some advantages over other scripting languages. It has a simple syntax and is conceptually clear, making it easy to learn. Python is also easier and more descriptive when using complex data structures (such as lists, dictionaries, and tuples). Python can also extend languages and be extended by languages in turn.

I find that Python’s syntax makes it more readable and maintainable than Perl but less so than Ruby. Python’s advantage over Ruby is the large number of libraries and modules that are available. Using these, you can build feature-rich programs with little custom code.

Python’s use of indentation to identify block scope can be rather annoying, but its simplicity tends to make up for this minor flaw.

Now, let’s dig into sockets programming in Python.

Python sockets modules

Basic Python sockets modules

Python offers two basic sockets modules. The first, Socket, provides the standard BSD Sockets API. The second, SocketServer, provides a server-centric class that simplifies the development of network servers. It does this in an asynchronous way in which you can provide plug-in classes to do the application-specific jobs of the server. Table 1 lists the classes and modules that this section covers.

Table 1. Python classes and modules
Class/module Description
Socket Low-level networking interface (per the BSD API)
SocketServer Provides classes that simplify the development of network servers

Let’s look at each of these modules to understand what they can do for you.

The Socket module

The Socket module provides the basic networking services with which UNIX® programmers are most familiar (otherwise known as the BSD API). This module has everything you need to build socket servers and clients.

The difference between this API and the standard C API is in its object orientation. In C, a socket descriptor is retrieved from the socket call then used as a parameter to the BSD API functions. In Python, the socket method returns a socket object to which the socket methods can be applied. Table 2 provides some of the class methods and Table 3 shows a subset of the instance methods.

Table 2. Class methods for the Socket module
Class method Description
Socket Low-level networking interface (per the BSD API)
socket.socket(family, type) Create and return a new socket object
socket.getfqdn(name) Convert a string quad dotted IP address to a fully qualified domain name
socket.gethostbyname(hostname) Resolve a hostname to a string quad dotted IP address
socket.fromfd(fd, family, type) Create a socket object from an existing file descriptor

Table 3. Instance methods for the Socket module
Instance method Description
sock.bind( (adrs, port) ) Bind the socket to the address and port
sock.accept() Return a client socket (with peer address information)
sock.listen(backlog) Place the socket into the listening state, able to pend backlog outstanding connection requests
sock.connect( (adrs, port) ) Connect the socket to the defined host and port
sock.recv( buflen[, flags] ) Receive data from the socket, up to buflen bytes
sock.recvfrom( buflen[, flags] ) Receive data from the socket, up to buflen bytes, returning also the remote host and port from which the data came
sock.send( data[, flags] ) Send the data through the socket
sock.sendto( data[, flags], addr ) Send the data through the socket
sock.close() Close the socket
sock.getsockopt( lvl, optname ) Get the value for the specified socket option
sock.setsockopt( lvl, optname, val ) Set the value for the specified socket option

The difference between a class method and an instance method is that instance methods require a socket instance to be performed (returned from socket), where class methods do not.

The SocketServer module

The SocketServer module is an interesting module that simplifies the development of socket servers. A discussion of its use is far beyond the scope of this tutorial, but I’ll demonstrate its basic use and then refer you to the Resources section for links to a more detailed discussion.

Consider the simple case shown in Listing 2. Here, I implement a simple “Hello World” server which emits the message when a client connects. I first create a request handler that inherits the SocketServer.StreamRequestHandler class. I define a method called handle that handles the requests for the server. Everything the server does must be handled within the context of this function (at the end, the socket is closed). This process works in simple cases, but you can implement even simple HTTP servers with this class. In the handle method, I emit my salutation and then exit.

Now that the connection handler is ready, all that’s left is to create the socket server. I use the SocketServer.TCPServer class, providing the address and port number (to which the server will be bound) and my request-handler method. The result is a TCPServer object. Calling the serve_forever method starts the server and makes it available for connections.

Listing 2. Implementing a simple server with the SocketServer module
import SocketServer

class hwRequestHandler( SocketServer.StreamRequestHandler ):
  def handle( self ):
    self.wfile.write("Hello World!\n")

server = SocketServer.TCPServer( ("", 2525), hwRequestHandler )

That’s it! Python permits a number of variations on this theme, including UDPServers and forking and threading servers.

Sockets programming in Python

In languages with sockets, the socket is universally the same — it’s a conduit between the two applications that can communicate with one another.


Whether you’re writing a sockets application in Python, Perl, Ruby, Scheme, or any other useful language (and by useful I mean languages that have a sockets interface), the socket is universally the same. It’s a conduit between the two applications that can communicate with one another (either locally on a single machine or between two machines in separate locations).

The difference with sockets programming in a language like Python is in the helper classes and methods that can simplify sockets programming. In this section I’ll demonstrate the Python socket API. You can execute the Python interpreter with a script or, if you execute Python by itself, you can interact with it one line at a time. In this way, you can see the result of each method invoked.

The following example illustrates interacting with the Python interpreter. Here, I use the socket class method gethostbyname to resolve a fully qualified domain name ( to a string quad-dotted IP address (‘’):

Listing 3. Using the socket API from the interpreter command line
[camus]$ python
Python 2.4 (#1, Feb 20 2005, 11:25:45)
[GCC 3.2.2 20030222 (Red Hat Linux 3.2.2‑5)] on linux2
Type "help", "copyright", "credits" or "license" for more
>>> import socket
>>> socket.gethostbyname('')

After the socket module is imported, I invoke the gethostbyname class method to resolve the domain name to an IP address.

Now, I’ll discuss the basic socket methods and communicating through sockets. Feel free to follow along with your Python interpreter.

Creating and destroying sockets

To create a new socket, you use the socket method of the socket class. This is a class method because you don’t yet have a socket object from which to apply the methods. The socket method is similar to the BSD API, as demonstrated in the creation of a stream (TCP) and datagram (UDP) socket:

Listing 4. Creating stream and datagram sockets
streamSock = socket.socket( socket.AF_INET, socket.SOCK_STREAM )

dgramSock = socket.socket( socket.AF_INET, socket.SOCK_DGRAM )

In each case, a socket object is returned. The AF_INET symbol — argument one — indicates that you’re requesting an Internet Protocol (IP) socket, specifically IPv4. The second argument is the transport protocol type (SOCK_STREAM for TCP sockets and SOCK_DGRAM for UDP sockets). If your underlying operating system supports IPv6, you can also specify socket.AF_INET6 to create an IPv6 socket.

To close a connected socket, you use the close method:


Finally, you can delete a socket with the del statement:

del streamSock

This statement permanently removes the socket object. Attempting to reference the socket thereafter produces an error.

Socket addresses

An endpoint address for a socket is a tuple consisting of an interface address and a port number. Because Python can represent tuples easily, the address and port are represented as such. This illustrates an endpoint for interface address and port 80:

( '', 80 )

You can also use a fully qualified domain name here, such as:

( '', 25 )

This example is simple and certainly beats the sockaddr_in manipulation that’s necessary in C. The following discussion provides examples of addresses in Python.

Server sockets

Server sockets are typically those that expose a service on a network. Because server and client sockets are created in different ways, I discuss them independently.

After you create the socket, you use the bind method to bind an address to it, the listen method to place it in the listening state, and finally the accept method to accept a new client connection. This is demonstrated below:

Listing 5. Using server sockets
sock = socket.socket( socket.AF_INET, socket.SOCK_STREAM )
sock.bind( ('', 2525) )
sock.listen( 5 )
newsock, (remhost, remport) = sock.accept()

For this server, the address ('', 2525) is used which means that the wildcard is used for the interface address (''), allowing incoming connections from any interface on the host. You also bind to port number 2525.

Note here that the accept method returns not only the new socket object that represents the client connection (newsock) but also an address tuple (the remote address and port number of the peer end of the socket). Python’s SocketServer module can simplify this process even further, as demonstrated above.

You can also create datagram servers, but they are connectionless and therefore have no associated accept method. The following example creates a datagram server socket:

Listing 6. Creating a datagram server socket
sock = socket.socket( socket.AF_INET, socket.SOCK_DGRAM )
sock.bind( ('', 2525) )

The upcoming discussion of sockets I/O shows how I/O works for both stream and datagram sockets.

Now, let’s explore how a client creates a socket and connects it to a server.

Client sockets

The mechanisms for creating and connecting client sockets are similar to the setup of server sockets. Upon creating a socket, an address is needed — not to locally bind the socket (as is the case with a server) but rather to identify where the socket should attach. Say there’s a server on a host with an interface IP address of ‘’ and port 2525. The following code creates a new socket and connects it to the defined server:

Listing 7. Creating a stream socket and connecting to the server
sock = socket.socket( socket.AF_INET, socket.SOCK_STREAM )
sock.connect( ('', 2525) )

For datagram sockets, the process is a bit different. Recall that datagram sockets are by nature disconnected. One way to the think about it is as follows: Whereas stream sockets are pipes between two endpoints, datagram sockets are message-based, able to communicate with multiple peers at the same time. Here’s an example of a datagram client.

Listing 8. Creating a datagram socket and connecting to the server
sock = socket.socket( socket.AF_INET, sock.sock_DGRAM )
sock.connect( ('', 2525) )

What’s different here is that even though I’ve used the connect method, there’s no real connection between the client and server. The connect here is a simplification for later I/O. Typically in datagram sockets, you must provide the destination information with the data that you want to send. By using connect, I’ve cached this information with the client and send methods can occur much like stream socket versions (no destination address is necessary). You can call connect again to re-specify the target of the datagram client’s messages.

Stream sockets I/O

Sending or receiving data through stream sockets is simple in Python. Several methods exist to move data through a stream socket (such as send, recv, read, and write).

This first example demonstrates a server and client for stream sockets. In this demonstration, the server echoes whatever it receives from the client.

The echo stream server is presented in Listing 9. Upon creating a new stream socket, an address is bound to it (accept connections from any interface and port 45000) and then the listen method is invoked to enable incoming connections. The echo server then goes into a loop for client connections. The accept method is called and blocks (that is, does not return) until a new client connects, at which point the new client socket is returned along with address information for the remote client. With this new client socket, I call recv to get a string from the peer, then write this string back out to the socket. I then immediately close the socket.

Listing 9. Simple Python stream echo server
import socket

srvsock = socket.socket( socket.AF_INET, socket.SOCK_STREAM )
srvsock.bind( ('', 23000) )
srvsock.listen( 5 )

while 1:

  clisock, (remhost, remport) = srvsock.accept()
  str = clisock.recv(100)
  clisock.send( str )

Listing 10 shows the echo client that corresponds with the server in Listing 9. Upon creating a new stream socket, the connect method is used to attach this socket to the server. When connected (when the connect method returns), the client emits a simple text message with the send method, then awaits the echo with the recv method. The print statement is used to emit what’s read. When this is done, the close method is performed to close the socket.

Listing 10. Simple Python stream echo server
import socket

clisock = socket.socket( socket.AF_INET, socket.SOCK_STREAM )

clisock.connect( ('', 23000) )

clisock.send("Hello World\n")
print clisock.recv(100)


Datagram sockets I/O

Datagram sockets are disconnected by nature which means that communication requires that a destination address be provided. Similarly, when a message is received through a socket, the source of the data must be returned. The recvfrom and sendto methods support the additional address information as you can see in the datagram echo server and client implementations.

Listing 11 shows the datagram echo server. A socket is first created and then bound to an address using the bind method. An infinite loop is then entered for serving client requests. The recvfrom method receives a message from the datagram socket and returns not only the message but also the address of the source of the message. This information is then turned around with the sendto method to return the message to the source.

Listing 11. Simple Python datagram echo server
import socket

dgramSock = socket.socket( socket.AF_INET, socket.SOCK_DGRAM )
dgramSock.bind( ('', 23000) )

while 1:

  msg, (addr, port) = dgramSock.recvfrom( 100 )
  dgramSock.sendto( msg, (addr, port) )

The datagram client is even simpler. After creating a datagram socket, I use the sendto method to send a message to a specific address. (Remember: Datagrams have no connection.) After sendto finishes, I await the echo response with recv, then print it. Note that I don’t use recvfrom here because I’m not interested in the peer address information.

Listing 12. Simple Python datagram echo client
import socket

dgramSock = socket.socket( socket.AF_INET, socket.SOCK_DGRAM )

dgramSock.sendto( "Hello World\n", ('', 23000) )
print dgramSock.recv( 100 )

Socket options

Sockets default to a set of standard behaviors, but it’s possible to alter the behavior of a socket using options. You manipulate socket options with the setsockopt method and capture them with the getsockopt method.

Using socket options is simple in Python, as demonstrated in Listing 13. In the first example, I read the size of the socket send buffer. In the second example, I get the value of the SO_REUSEADDR option (reuse the address within the TIME_WAIT period) and then enable it.

Listing 13. Using socket options
sock = socket.socket( socket.AF_INET, socket.SOCK_STREAM )

#Get the size of the socket's send buffer
bufsize = sock.getsockopt( socket.SOL_SOCKET, socket.SO_SNDBUF )

#Get the state of the SO_REUSEADDR option
state = sock.getsockopt( socket.SOL_SOCKET, socket.SO_REUSEADDR )

#Enable the SO_REUSEADDR option
sock.setsockopt( socket.SOL_SOCKET, socket.SO_REUSEADDR, 1 )

The SO_REUSEADDR option is most often used in socket server development. You can increase the socket send and receive buffers for greater performance, but given that you’re operating here in an interpreted scripting language, it may not provide you with much benefit.

Asynchronous I/O

Python offers asynchronous I/O as part of the select module. This feature is similar to the C select mechanism but has some simplifications. I’ll first introduce select and then show you how to use it in Python.

The select method allows you to multiplex events for several sockets and for several different events. For example, you can instruct select to notify you when a socket has data available, when it’s possible to write data through a socket, and when an error occurs on a socket; and you can perform these actions for many sockets at the same time.

Where C works with bitmaps, Python uses lists to represent the descriptors to monitor and also the return descriptors whose constraints are satisfied. Consider the following example in which you await some input from standard input:

Listing 14. Awaiting input from stdin
rlist, wlist, elist = [sys.stdin], [], [] )


The arguments passed to select are lists representing read events, write events, and error events. The select method returns three lists containing the objects whose events were satisfied (read, write, exception). In this example, upon return rlist should be [sys.stdin], indicating that data are available to read on stdin. This data are then read with the read method.

The select method also works on socket descriptors. In the following example (see Listing 15), two client sockets are created and connected to a remote peer. The select method is then used to identify which socket has data available for reading. The data are then read and emitted to stdout.

Listing 15. Demonstrating the select method with multiple sockets
import socket
import select

sock1 = socket.socket( socket.AF_INET, socket.SOCK_STREAM )
sock2 = socket.socket( socket.AF_INET, socket.SOCK_STREAM )

sock1.connect( ('', 25) )
sock2.connect( ('', 25) )

while 1:

  #Await a read event
  rlist, wlist, elist = [sock1, sock2], [], [], 5 )

  #Test for timeout
  if [rlist, wlist, elist] == [ [], [], [] ]:

    print "Five seconds elapsed.\n"


    #Loop through each socket in rlist, read and print the available data
    for sock in rlist:

      print sock.recv( 100 )

Building a Python chat server

A simple chat server

You’ve explored the basic networking APIs for Python; now you can put this knowledge to use in a simple application. In this section, you’ll build a simple chat server. Using Telnet, clients can connect to your Python chat server and globally communicate with one another. Messages submitted to the chat server are viewed by others (in addition to management information, such as clients joining or leaving the chat server). This model is shown graphically in Figure 1.

Figure 1. The chat server uses the select method to support an arbitrary number of clients
The chat server uses the select method to support an arbitrary number of clients

An important requirement to place on your chat server is that it must be scalable. The server must be able to support an arbitrary number of stream (TCP) clients.

To support an arbitrary number of clients, you use the select method to asynchronously manage your client list. But you also use a feature of select for your server socket. The read event of select determines when a client has data available for reading, but it also can be used to determine when a server socket has a new client trying to connect. You exploit this behavior to simplify the development of the server.

Next, I’ll explore the source of the Python chat server and identify the ways in which Python helps simplify its implementation.

The ChatServer class

Let’s start by looking at the Python chat server class and the __init__ method — the constructor that’s invoked when a new instance is created.

The class is made up of four methods. The run method is invoked to start the server and permit client connections. The broadcast_string and accept_new_connection methods are used internally in the class and will be discussed shortly.

The __init__ method is a special method that’s invoked when a new instance of the class is created. Note that all methods take the self argument, a reference to the class instance itself (much like the this parameter in C++). You’ll see the self parameter, part of all instance methods, used here to access instance variables.

The __init__ method creates three instance variables. The port is the port number for the server (passed in the constructor). The srvsock is the socket object for this instance, and descriptors is a list that contains each socket object for the class. You use this list within the select method to identify the read event list.

Finally, Listing 16 shows the code for the __init__ method. After creating a stream socket, the SO_REUSEADDR socket option is enabled so that the server can be quickly restarted, if necessary. The wildcard address is bound with the defined port number. Then the listen method is invoked to permit incoming connections. The server socket is added to the descriptors list (the only element at present), but all client sockets will be added as they arrive (see accept_new_connection). A salutation is provided to stdout indicating that the server has started.

Listing 16. The ChatServer class with the init method
import socket
import select

class ChatServer:

  def init( self, port ):
    self.port = port;

    self.srvsock = socket.socket( socket.AF_INET, socket.SOCK_STREAM )
    self.srvsock.setsockopt( socket.SOL_SOCKET, socket.SO_REUSEADDR, 1 )
    self.srvsock.bind( ("", port) )
    self.srvsock.listen( 5 )

    self.descriptors = self.srvsock    print 'ChatServer started on port %s' % port

  def run( self ):

  def broadcast_string( self, str, omit_sock ):

  def accept_new_connection( self ):

The run method

The run method is the server loop for your chat server (see Listing 17). When called, it enters an infinite loop, providing communication between connected clients.

The core of the server is the select method. I pass the descriptor list (which contains all the server’s sockets) as the read event list to select (and null lists for write and exception). When a read event is detected, it’s returned as sread. (I ignore the swrite and sexc lists.) The sread list contains the socket objects that will be serviced. I iterate through the sread list, checking each socket object found.

The first check in the iterator loop is if the socket object is the server. If it is, a new client is trying to connect and the accept_new_connection method is called. Otherwise, the client socket is read. If a null is returned from recv, the peer socket closed.

In this case, I construct a message and send it to all connected clients, close the peer socket, and remove the corresponding object from the descriptor list. If the recv return is not null, a message is available and stored in str. This message is distributed to all other clients using broadcast_string.

Listing 17. The chat server run method is the core of the chat server
def run( self ):

  while 1:

    #Await an event on a readable socket descriptor
    (sread, swrite, sexc) = self.descriptors, [], [] )

    #Iterate through the tagged read descriptors
    for sock in sread:

      #Received a connect to the server (listening) socket
      if sock == self.srvsock:

        #Received something on a client socket
        str = sock.recv(100)

        #Check to see if the peer socket closed
        if str == '':
          host,port = sock.getpeername()
          str = 'Client left %s:%s\r\n' % (host, port)
          self.broadcast_string( str, sock )
          host,port = sock.getpeername()
          newstr = '[%s:%s] %s' % (host, port, str)
          self.broadcast_string( newstr, sock )

Helper methods

The two helper methods in the chat server class provide methods for accepting new client connections and broadcasting messages to the connected clients.

The accept_new_connection method (see Listing 18) is called when a new client is detected on the incoming connection queue. The accept method is used to accept the connection, which returns the new socket object and remote address information. I immediately add the new socket to the descriptors list, then send a salutation to the new client welcoming the client to the chat. I create a string identifying that the client has connected and broadcast this information to the group using broadcast_string (see Listing 19).

Note that in addition to the string being broadcast, a socket object is also passed. The reason is that I want to selectively omit some sockets from getting certain messages. For example, when a client sends a message to the group, the message goes to the group but not back to itself. When I generate the status message identifying a new client joining the group, it goes to the group but not the new client. This task is performed in broadcast_string with the omit_sock argument. This method walks through the descriptors list and sends the string to all sockets that are not the server socket and not omit_sock.

Listing 18. Accepting a new client connection on the chat server
def accept_new_connection( self ):

  newsock, (remhost, remport) = self.srvsock.accept()
  self.descriptors.append( newsock )

  newsock.send("You're connected to the Python chatserver\r\n")
  str = 'Client joined %s:%s\r\n' % (remhost, remport)
  self.broadcast_string( str, newsock )

Listing 19. Broadcasting a message to the chat group
def broadcast_string( self, str, omit_sock ):

  for sock in self.descriptors:
    if sock != self.srvsock and sock != omit_sock:

  print str,

Instantiating a new ChatServer

Now that you’ve seen the Python chat server (under 50 lines of code), let’s see how to instantiate a new chat server object in Python.

Start the server by creating a new ChatServer object (passing the port number to be used), then calling the run method to start the server and allow incoming connections:

Listing 20. Instantiating a new chat server
myServer = ChatServer( 2626 )

At this point, the server is running and you can connect to it from one or more clients. You can also chain methods together to simplify this process (as if it needs to be simpler):

Listing 21. Chaining methods
myServer = ChatServer( 2626 ).run()

which achieves the same result. I’ll show the ChatServer class in operation.

Demonstrating the ChatServer

Here’s the ChatServer in action. I show the output of the ChatServer (see Listing 22) and the dialog between two clients (see Listing 23 and Listing 24). The user-entered text appears in bold.

Listing 22. Output from the ChatServer
[plato]$ python
ChatServer started on port 2626
Client joined
Client joined
[] Hello, is anyone there?
[] Yes, I'm here.
[]  Client left

Listing 23. Output from Chat Client #1
[plato]$ telnet localhost 2626
Connected to localhost.localdomain (
Escape character is '^]'.
You're connected to the Python chatserver
Client joined
[] Hello, is anyone there?
Yes, I'm here.^]
telnet> close
Connection closed.

Listing 24. Output from Chat Client #2
[plato]$ telnet localhost 2626
Connected to localhost.localdomain (
Escape character is '^]'.
You're connected to the Python chatserver
Hello, is anyone there?
[] Yes, I'm here.
[] Client left

As you see in Listing 22, all dialog between all clients is emitted to stdout, including client connect and disconnect messages.

High-level networking classes

Networking modules

Python includes several specialized modules for application-layer protocols (built on the standard socket module). The available modules are wide and varied and they provide module implementations of the Hypertext Transfer Protocol (HTTP), the Simple Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP) and Post Office Protocol (POP3), the Network News Transfer Protocol (NNTP), XML-RPC (remote procedure call), FTP, and many others.

This section demonstrates the modules shown in Table 4.

Table 4. Useful application-layer protocol modules
Module Protocol implemented
httplib HTTP client
smtplib SMTP client
poplib POP3 client

The httplib (HTTP client)

The HTTP client interface can be useful when developing Web robots or other Internet scraping agents. The Web protocol is request/response in nature over stream sockets. Python makes it easy to build Web robots through a simple Web interface.

Listing 25 demonstrates the httplib module. You create a new HTTP client instance with HTTPConnection, providing the Web site to which you want to connect. With this new object (httpconn), you can request files with the request method. Within request, you specify the HTTP GET method (which requests a file from the server, compared to HEAD which simply retrieves information about the file). The getresponse method parses the HTTP response header to understand if an error was returned. If the file was successfully retrieved, the read method on the new response object returns and prints the text.

Listing 25. Building a simple (non-rendering) HTTP client with httplib
import httplib

httpconn = httplib.HTTPConnection("")

httpconn.request("GET", "/developerworks/index.html")

resp = httpconn.getresponse()

if resp.reason == "OK":

  resp_data =

  print resp_data


The smptlib (SMTP client)

SMTP allows you to send e-mail messages to a mail server which can be useful in networking systems to relay status about the operation of a device. The Python module for sending e-mail messages is simple and consists of creating an SMTP object, sending an e-mail message using the sendmail method, then closing the connection with the quit method.

The example in Listing 26 demonstrates sending a simple e-mail message. The msg string contains the body of the message (which should include the subject line).

Listing 26. Sending a short e-mail message with smtplib
import smtplib

fromAdrs = ''
toAdrs = ''
msg = 'From:\r\nTo:\r\nSubject:Hello\r\nHi!\r\n'

mailClient = smtplib.SMTP('')
mailClient.sendmail( fromAdrs, toAdrs, msg )

The poplib (POP3 client)

POP3 is another useful application-layer protocol for which a module exists within Python. The POP3 protocol allows you to connect to a mail server and download new mail, which can be useful for remote commanding — embedding commands within the body of an e-mail message. After executing the embedded command, you can use smptlib to return a response e-mail message to the source.

This demonstration in Listing 27 shows a simple application that connects to a mail server and emits the subject lines for all pending e-mail for the user.

The poplib is relatively simple but offers several methods for gathering and managing e-mail at the server. In this example, I create a new POP3 object with the POP3 method, specifying the mail server. The user and pass_ methods authenticate the application to the server; the stat method returns the number of messages waiting for the user and the total number of bytes taken up by all messages.

Next, I loop through each available message and use the retr method to grab the next e-mail message. This method returns a list of the form:

(response, ['line, ...], octets)

where response is the POP3 response for the particular message, the linelist represents the individual lines of the e-mail message, and the final element, octets, is the number of bytes for the e-mail message. The inner loop simply iterates over the second element ([1]) of the list which is the list of the e-mail message body. For each line, I test whether ‘Subject:’ is present; if so, I print this line.

After all e-mail messages have been checked, a call to the quit method ends the POP3 session.

Instead of using the retr method, you could also use the top method to extract the header for the e-mail message. This step would be faster and minimize the amount of data transferred to this client.

Listing 27. Retrieving e-mail messages from a POP3 mail server and emitting the subject line
import poplib
import re

popClient = poplib.POP3('')


numMsgs, mboxSize = popClient.stat()

print "Number of messages ", numMsgs
print "Mailbox size", mboxSize

for id in range (numMsgs):
  for mail in popClient.retr(id+1)[1]:
    if 'Subject:', mail ):
      print mail




This tutorial reviewed the basics of the sockets API and showed how to build networking applications in Python. The standard sockets module was introduced as a way to build both client and server applications, as well as the SocketServer module which simplifies the construction of simple socket servers. I presented a simple chat server implemented in Python that offered support for a scalable number of clients using the select method. In closing, I previewed some of Python’s high-level networking classes that simplify the development of applications requiring application-layer networking protocols.

Python is an interesting and useful language that is well worth your time to learn.