Introduction to
Networking in Python

Modified:

Overview

Nearly any language can be used for client/server programming; all that is required are a few simple functions for the communications services.

Python is a simple but powerful language that comes with many of the fundamental tools (TCP for connection-oriented and UDP for datagram services) needed for quickly programming networked applications. It is one of the simpler languages useful for course assignments.

Network programming

TCP

Because TCP is a connection-oriented protocol, one process acts as a server accepting connections, another process acts as the client requesting the connection. Below are basic client and server scripts; the client connects to the server on port 8888, sends 'Hello world'; the server accepts the connection, receives and prints 'Hello world':

Client

import socket

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

s.connect(('localhost', 8888))
s.send('Hello world')

s.close()

Server

import socket

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind(('', 8888))

s.listen(1)
conn, addr = s.accept()

data = conn.recv(1024)
print "Received: ", data

conn.close()

Try

Copy, paste and save the above files as:

client.py

server.py

Open two Command Prompt windows to location of the files.

Enter:

python server.py

python client.py

import socket - the TCP/IP socket library containing all the necessary function definitions.

s=socket.socket(socket.AF_INET, socket.SOCK_STREAM) - Constructs a socket and assigns to variable s.

s.connect(('localhost', 8888)) - Client requests a connection with host 'localhost' on port 8888.

s.bind(('', 8888)) - Server binds the script on host '' to port 8888.

s.listen(1) - Server listens to accept 1 connection at a time.

conn, addr = s.accept() - Server accepts connection request from a client; conn is the socket used for communication with client and addr the client's address.

s.send('Hello world') - Send a string through the socket s.

data = s.recv(1024) - Receive data through socket s, assign to variable data; a maximum of 1024 characters received at a time.

s.close() - Close the connection on socket s.

Example: Echo client and server

The following server echoes the input typed on the client.

# echoclient.py

import socket, sys

HOST = sys.argv[1]      # The remote host
PORT = 8888                 # Server port
s = socket.socket(socket.AF_INET, 
                          socket.SOCK_STREAM)
s.connect((HOST, PORT))

while True :
   message = raw_input('Send:')
   if not message : break
   s.send(message)
   data = s.recv(1024)
   print 'Received', `data`
s.close()

# echoserver.py 

import socket

HOST = ''          # Symbolic name
PORT = 8888     # port
s = socket.socket(socket.AF_INET, 
                          socket.SOCK_STREAM)
s.bind((HOST, PORT))
s.listen(1)
conn, addr = s.accept()
print 'Connected by', addr

while True:
    data = conn.recv(1024)
    if not data: break
    conn.send(data)
conn.close()

Try

Copy, paste and save the above files as:

echoclient.py

echoserver.py

Open two Command Prompt windows to location of the files.

Enter:

python echoserver.py

python echoclient.py localhost

A blank line to echoclient terminates.

Example: Echo client and server

The same echoclient and echoserver as functions.

# echoclient.py

import socket, sys

def client( host, port) :
   s = socket.socket(socket.AF_INET, 
            socket.SOCK_STREAM)
   s.connect((host, port))

   while True :
      message = raw_input('Send:')
      if not message : return
      s.send(message)
      data = s.recv(1024)
      print 'Received', `data`
   s.close()

client(sys.argv[1], 8888)

# echoserver.py 

import socket

def server( s ) :
   conn, addr = s.accept()
   print 'Connected by', addr

   while True:
       data = conn.recv(1024)
       if not data: break
       conn.send(data)
   conn.close()

s = socket.socket(socket.AF_INET, 
         socket.SOCK_STREAM)
s.bind(('', 8888))
s.listen(1)
server( s )

Exceptions

Exceptions allow a formal means of handling non-standard situations. For example, the following handles division by 0.

print '1'
try:
   print '2'
   x=9/0
   print '3'
   print x
except Exception:
   print 'division by 0'
   print '4'
print '5'

try - Encloses the statements to be tried. No statements in the try are executed after an exception is raised.

except - Encloses the statements executed only if an exception (in this case, any Exception) occurs in the statements immediately tried. After except statements are completed, execution continues with statements following the except.

The output is: 1, 2, division by 0, 4, 5

Introduction to Networking in Python

Overview

Network programming

TCP

Client

Server

Introduction to
Networking in Python