Socket Programming in Python

Socket programming is a way of connecting two nodes on a network to communicate with each other. One socket(node) listens on a particular port at an IP, while other socket reaches out to the other to form a connection. Server forms the listener socket while client reaches out to the server.They are the real backbones behind web browsing. In simpler terms there is a server and a client.

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

Here we made a socket instance and passed it two parameters. The first parameter is AF_INET and the second one is SOCK_STREAM. AF_INET refers to the address family ipv4. The SOCK_STREAM means connection oriented TCP protocol.

Now we can connect to a server using this socket.

Connecting to a server:
Note that if any error occurs during the creation of a socket then a socket.error is thrown and we can only connect to a server by knowing it’s ip. You can find the ip of the server by using this :

$ ping www.google.com
import socket 

ip = socket.gethostbyname('www.google.com')
print ip

#Example of a SCRIPT

# An example script to connect to Google using socket

# programming in Python

import socket # for socket

import sys 

 

try:

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

    print "Socket successfully created"

except socket.error as err:

    print "socket creation failed with error %s" %(err)

 

# default port for socket

port = 80

 

try:

    host_ip = socket.gethostbyname('www.google.com')

except socket.gaierror:

 

    # this means could not resolve the host

    print "there was an error resolving the host"

    sys.exit()

 

# connecting to the server

s.connect((host_ip, port))

 

print "the socket has successfully connected to google \

on port == %s" %(host_ip)







Output :
Socket successfully created
the socket has successfully connected to google 
on port == 173.194.40.19


First of all we made a socket.
Then we resolved google’s ip and lastly we connected to google.
Now we need to know how can we send some data through a socket.
For sending data the socket library has a sendall function. This function allows you to send data to a server to which the socket is connected and server can also send data to the client using this function.


A simple server-client program :

Server :
A server has a bind() method which binds it to a specific ip and port so that it can listen to incoming requests on that ip and port.A server has a listen() method which puts the server into listen mode. This allows the server to listen to incoming connections. And last a server has an accept() and close() method. The accept method initiates a connection with the client and the close method closes the connection with the client.





# first of all import the socket library

import socket               

 

# next create a socket object

s = socket.socket()         

print "Socket successfully created"

 

# reserve a port on your computer in our

# case it is 12345 but it can be anything

port = 12345               

 

# Next bind to the port

# we have not typed any ip in the ip field

# instead we have inputted an empty string

# this makes the server listen to requests 

# coming from other computers on the network

s.bind(('', port))        

print "socket binded to %s" %(port)

 

# put the socket into listening mode

s.listen(5)     

print "socket is listening"           

 

# a forever loop until we interrupt it or 

# an error occurs

while True:

 

   # Establish connection with client.

   c, addr = s.accept()     

   print 'Got connection from', addr

 

   # send a thank you message to the client. 

   c.send('Thank you for connecting')

 

   # Close the connection with the client

   c.close()





Reference : Python Socket Programming

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