Programming
Python

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.

Getting Started

Download Python for Windows
Download Python Documentation - Includes a useful tutorial.
Look at the Python tutorial
Start | Programs | Python 2.3 | IDLE (Python GUI)

Interactive mode

Python can be entered one statement at a time in IDLE shell window; useful for learning the language and debugging. Enter:

2+2
a='He'
a
a=a+'llo'
a

Editing and running programs

The editor is used to edit files and run Python programs.

def factorial(n) :
if n <= 1 : return 1
return n*factorial(n-1)

b = 16

Try creating a new file

Open Python.
In IDLE shell click: File | New Window
Copy and Paste the above program.
Click: File | Save | test.py
Click:

Run | Run Module
b
factorial( 5 )

Debugging in interactive mode

Interactive mode makes debugging programs much simpler (a dynamic debugger is part of the IDLE Shell). After running the module (i.e. Run Module) global variables can be examined and functions called.

Restarting - Restart a Python program by closing the Shell and running the module again.

Examining variables - Examine the global b variable by entering:

b

Calling functions - Call the factorial function with actual parameter of 5 by:

factorial( 5 )

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:

Client

import socket
s=socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('localhost', 8888))
s.send('Hello world')
data = s.recv(1024)
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)
conn.send(data)
conn.close()

Try

Open Python in two windows: Start | Programs | Python | IDLE
In each window:

File | New

Copy, paste and save the above files as:

client.py
server.py

In the server window run by pressing F5
In the client window run by pressing F5

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

# echoclient.py

import socket, sys

HOST = sys.argv[1]        # The remote host
PORT = 8888                 # Port used by the server
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     # Arbitrary non-privileged 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

Open Python in two windows: Start | Programs | Python | IDLE
In each window:

File | New

Copy, paste and save the above files as:

echoclient.py
echoserver.py

In the echoserver window run by pressing F5
In the echoclient window run by pressing F5

Programming

Data types

Python is provides very flexible data types useful for network programming:

Try

Lists - A universal data type that can be heterogeneous; for example:

L = [12, 'A', ['a nested list'], 3.14 ]
L[2]
L.append('another')

Tuples - Similar to a C struc; used to group related data; represents heterogeneous data; example:

T = ('A', [1,2,3])
T[1]
percept = (('A','Clean'), ('B','Dirty'))
(x, y) = (('A','Clean'), ('B','Dirty'))
x
z = x + y
z
( (a, b), (c, d) ) = (('A','Clean'), ('B','Dirty'))
a
d

Dictionary - An associative array; consists of keys and value pairs; example:

D = { 'B438' : 'networking', 'a list':[1,2,3], ('A', 'Clean'):5 }
D['B438']
D['a list']
D[('A', 'Clean')]

Variables (dynamically typed)

Variables are dynamically typed by the value to which it is bound. In the program:

A='A' binds variable A to a string; A can be used as a string; printing, concatenation, ...
percepts = [] binds to an empty list; percepts can be used as a list; taking the head, inserting ...
table is bound to a dictionary; allows look-ups on an index and other dictionary operations

Try

Use the string concatenation operator on A, a string:

A='abc'
A=A+'1234'
A

Bind L to a list and perform some list operations:

L = [4, 'Hi', ('A', 'Clean')]

L

L[2]

L.append('cow')

L

L.append(['a list', ['a nested list']])

L

Attempt to use string concatenation on lists:

A = A + L

Indentation

Used instead of { } to specify scope.

Control statements

Similar to C++.

x = 4
if x<5 :
    print 'x<5'
    x+=1
else :
    print 'x>=y'

Scope (static)

Any identifier defined outside a function is global; inside local.

Functions

Similar to C++. The factorial function; notice the indentation:

def factorial(n) :
    if n <= 1 :
        return 1
    return factorial(n-1)*n

Try

Write the power function for xⁿ where n is a positive integer.

Click File | New Window
Click File | Save | power.py
Type the function.
Click Run | Run Shell | OK to save
In Shell enter: power(4,3)

Example: 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 )

Try

Open Python in two windows: Start | Programs | Python | IDLE
In each window:
- File | New
Copy, paste and save the above files as:
- echoclient.py
- echoserver.py
In the echoserver window run by pressing F5
In the echoclient window run by pressing F5

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

ProgrammingPython

Network programming

TCP

Client

Server

Example: Echo client and server

Programming
Python