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 the other socket reaches out to the other to form a connection. The server forms the listener socket while the client reaches out to the server. 

They are the real backbones behind web browsing. In simpler terms, there is a server and a client. 
Socket programming is started by importing the socket library and making a simple socket. 

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 its IP. You can find the IP of the server by using this : 

$ ping www.google.com

You can also find the IP using python: 

import socket 

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

Here is an example of a script for connecting to Google.

Python3

# 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")

Output : 

Socket successfully created
there was an error resolving the host

Here when we will be successfully connected the output will be:

Socket successfully created
the socket has successfully connected to google

• 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 the 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 listening 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. 

Python3

# 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. encoding to send byte type.
  c.send('Thank you for connecting'.encode())

  # Close the connection with the client
  c.close()

  # Breaking once connection closed
  break

• First of all, we import socket which is necessary.
• Then we made a socket object and reserved a port on our pc.
• After that, we bound our server to the specified port. Passing an empty string means that the server can listen to incoming connections from other computers as well. If we would have passed 127.0.0.1 then it would have listened to only those calls made within the local computer.
• After that we put the server into listening mode.5 here means that 5 connections are kept waiting if the server is busy and if a 6th socket tries to connect then the connection is refused.
• At last, we make a while loop and start to accept all incoming connections and close those connections after a thank you message to all connected sockets.

Client :
Now we need something with which a server can interact. We could telnet to the server like this just to know that our server is working. Type these commands in the terminal: 

# start the server
$ python server.py

# keep the above terminal open 
# now open another terminal and type: 
 
$ telnet localhost 12345

If ‘telnet’ is not recognized. On windows search windows features and turn on the “telnet client” feature.

Output : 

# in the server.py terminal you will see
# this output:
Socket successfully created
socket binded to 12345
socket is listening
Got connection from ('127.0.0.1', 52617)

# In the telnet terminal you will get this:
Trying ::1...
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
Thank you for connectingConnection closed by foreign host.

This output shows that our server is working.
Now for the client-side: 

Python3

# Import socket module
import socket

# Create a socket object
s = socket.socket()

# Define the port on which you want to connect
port = 12345

# connect to the server on local computer
s.connect(('127.0.0.1', port))

# receive data from the server and decoding to get the string.
print (s.recv(1024).decode())
# close the connection
s.close()

• First of all, we make a socket object.
• Then we connect to localhost on port 12345 (the port on which our server runs) and lastly, we receive data from the server and close the connection.
• Now save this file as client.py and run it from the terminal after starting the server script.

# start the server:
$ python server.py
Socket successfully created
socket binded to 12345
socket is listening
Got connection from ('127.0.0.1', 52617)

# start the client:
$ python client.py
Thank you for connecting

Reference: Python Socket Programming