Python Socket programming essentials | Python theme month

To create/initialize a socket, we use the socket.socket() method. It is defined in the Socket module in Python. The syntax is as follows.

sock_obj = socket.socket(socket_family, socket_type, protocol=0)
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Where is it,

• Socket_family: Defines the protocol family used as the transport mechanism. It can have either of two values.

  • AF_UNIX, or
  • AF_INET (IP version 4 or IPv4).

• Socket_type: Defines the communication type between two endpoints. It can have the following values.

  • SOCK_STREAM (for connection-oriented protocols such as TCP), or
  • SOCK_DGRAM (for connectionless protocols such as UDP).

• Protocol: We usually leave this field or set this field to zero.

import socket   #for sockets

# Instantiate an AF_INET, STREAM socket (TCP)

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

print ('Socket Initialized')
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That’s how you create a socket object. But what if the above example fails to instantiate the socket. How will you resolve the error?

You need to wrap the above code in Python’s try and except blocks. With Python exception handling, you can trace the cause of the error.

Error in managing Python socket programming
 
import socket   #for sockets
import sys  #for exit
 
try:
    # Create AF_INET, STREAM socket (TCP)

    sock_obj = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
except socket.error as err_msg:
    print ('Unable to instantiate socket. Error code: ' + str(err_msg[0]) + ' , Error message : ' + err_msg[1])
    sys.exit();
 
print ('Socket Initialized')
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In the following sections, we will explain the functions available in the Socket library to create client/server programs.

Server socket method

• sock_object.bind

  • This method binds the socket to the address (host name, port number pair)

• Sock_object. Listen (backlog) :

  • This method is used to listen for connections associated with a socket.
  • The backlog parameter indicates the maximum number of queued connections.
  • The maximum can be 5, and the minimum should be at least zero.

• sock_object.accept():

  • This function returns a (conn, address) pair, where “CONN” is the new socket object used to send and receive data on the communication channel, and “address” is the IP address bound to the socket at the other end of the channel.
  • The ACCEPT() method returns a socket object that is different from the socket object created using socket.socket().
  • This new socket object is designed to manage communication with the particular client where the acceptance occurs.
  • This mechanism also helps the server stay connected to n clients simultaneously.

Generic socket methods

• sock_object.recv():

  • Use this method to receive messages at the endpoint when the value of the protocol parameter is TCP.

• sock_object.send():

  • If the protocol is TCP, this method is applied to send messages from the endpoint.

• sock_object.recvfrom():

  • If the protocol used is UDP, this method is called to receive messages at the endpoint.

• sock_object.sendto():

  • If the protocol parameter is UDP, this method is called to send messages from the endpoint.

• sock_object.gethostname():

  • This method returns the host name.

• sock_object.close():

  • This method is used to close the socket. The remote endpoint does not receive data from this end.

So far, we’ve listed all the socket tools the “Socket” library provides for Python socket programming. Next, we’ll show you the socket function call workflow that implements client-server communication. Refer to the following snapshot. It describes each socket call required to establish a channel between the client and server.

The server module will send and receive data to/from the client.

• The python-server. py file contains the code to create the Server socket that waits until a request is received from the client.

• Whenever a client connects, the server accepts the connection.

  • The client will then start delivering messages to the server.
  • The server processes the messages and sends the response back to the client.

• In the code below, we also ask the user to enter the response he wants to pass to the client.

import socket
import time

def Main() :
    host = "127.0.0.1"
    port = 5001

    mySocket = socket.socket()
    mySocket.bind((host,port))
                
    mySocket.listen(1)
    conn, addr = mySocket.accept()
    print ("Connection from: " + str(addr))

    while True:
        data = conn.recv(1024).decode()
        if not data:
            break
        print ("from connected user: " + str(data))
                                                
        data = str(data).upper()
        print ("Received from User: " + str(data))

        data = input("? ")
        conn.send(data.encode())
                                                
    conn.close()
                
if __name__ == '__main__':
    Main()
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Python-Client.py

On the client side, we create a socket and connect to the server using the host and port values provided.

• The client code has a while loop that is used to exchange messages. It keeps printing all the data it gets from the server.
• After that, theThe inputFunction to request a client response. The response is then passed to the server.
• The user can also enter “Q” at any point in time to stop communication.

import socket

def Main() :
    host = '127.0.0.1'
    port = 5001

    mySocket = socket.socket()
    mySocket.connect((host,port))

    message = input("? ")

    whilemessage ! ='q':
        mySocket.send(message.encode())
	data = mySocket.recv(1024).decode()

	print ('Received from server: ' + data)
	message = input("? ")

    mySocket.close()

if __name__ == '__main__':
    Main()
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How do I run a client-server program?

You need to run both modules from separate command Windows, or you can run them in two different IDLE instances.

First, you’ll execute the server module, then the client. We have given a complete execution summary of the client-server program.

Python 3.51. (v3. 51.:37a07cee5969, Dec  6 201501:54:25) [MSC v1900. 64 bit (AMD64)] on win32
Type "copyright"."credits" or "license()" for more information.

 RESTART: C:\Users\Techbeamers\AppData\Local\Programs\Python\Python35\Python-Server.py 
Connection from: ('127.0.0.1'.50001)
from connected  user: Hello TechBeamers
Received from User: HELLO TECHBEAMERS
 ? Hello Dear Reader
from connected  user: You posts are awesome :)
Received from User: YOU POSTS ARE AWESOME :)
 ? Thank you very much. This is what keeps us motivated.
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Python 3.51. (v3. 51.:37a07cee5969, Dec 6 201501:54:25) [MSC v1900. 64 bit (AMD64)] on win32
Type "copyright"."credits" or "license()" for more information.
 
 RESTART: C:\Users\Techbeamers\AppData\Local\Programs\Python\Python35\Python-Client.py 
 ? Hello TechBeamers

Received from server: Hello Dear Reader
 ? You posts are awesome :)

Received from server: Thank you very much. This is what keeps us motivated.
 ? q
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Check program compatibility

Note that we have tested the above client-server code using Python version 3. But you can easily convert the above code to run on Python 2.7. You need to replace the following lines of code.

data = input("? ")
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Use the following Python input functions for Python 2.7.

data = raw_input("? ")
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We’ve listed more differences in the points below.

• Some other functions in Python 2.7, such as printing, do not require closed braces.
• None of Python 2.7’s socket functions (such as send()/recv()) need to decode their return value, as Python 3 does.

Quick summary — Python Socket programming

We hope that the above tutorial has given you some new knowledge about Socket programming in Python. If you enjoyed this article and are interested in seeing more of it, follow me here (Github/Gitee) for more information. Here is a summary of all my original and work source code

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