Python provides two levels of access to the network services. At a low level, you can access the basic socket support in the underlying operating system, which allows you to implement clients and servers for both connection-oriented and connectionless protocols.
Python also has libraries that provide higher-level access to specific application-level network protocols, such as FTP, HTTP, and so on.
| Protocol | Common function | Port No | Python module |
|---|---|---|---|
| HTTP | Web pages | 80 | httplib, urllib, xmlrpclib |
| NNTP | Usenet news | 119 | nntplib |
| FTP | File transfers | 20 | ftplib, urllib |
| SMTP | Sending email | 25 | smtplib |
| POP3 | Fetching email | 110 | poplib |
| IMAP4 | Fetching email | 143 | imaplib |
| Telnet | Command lines | 23 | telnetlib |
| Gopher | Document transfers | 70 | gopherlib, urllib |
Socket programming is a technique in which we communicate between two nodes connected in a network where the server node listens to the incoming requests from the client nodes.
In Python, the socket module is used for socket programming. The socket module in the standard library included functionality required for communication between server and client at hardware level.
This module provides access to the BSD socket interface. It is available on all operating systems such as Linux, Windows, MacOS.
Sockets are the endpoints of a bidirectional communications channel. Sockets may communicate within a process, between processes on the same machine, or between processes on different continents.
A socket is identified by the combination of IP address and the port number. It should be properly configured at both ends to begin communication.
Sockets may be implemented over a number of different channel types: Unix domain sockets, TCP, UDP, and so on. The socket library provides specific classes for handling the common transports as well as a generic interface for handling the rest.
The term socket programming implies programmatically setting up sockets to be able to send and receive data.
There are two types of communication protocols −
TCP or Transmission Control Protocol is a connection-oriented protocol. The data is transmitted in packets by the server, and assembled in the same order of transmission by the receiver. Since the sockets at either end of the communication need to be set before starting, this method is more reliable.
UDP or User Datagram Protocol is connectionless. The method is not reliable because the sockets does not require establishing any connection and termination process for transferring the data.
The socket module is used for creating and managing socket programming for the connected nodes in a network. The socket module provides a socket class. You need to create a socket using the socket.socket() constructor.
An object of the socket class represents the pair of host name and the port numbers.
The following is the syntax of socket.socket() constructor –
socket.socket (socket_family, socket_type, protocol=0)
family − AF_INET by default. Other values - AF_INET6 (eight groups of four hexadecimal digits), AF_UNIX, AF_CAN (Controller Area Network) or AF_RDS (Reliable Datagram Sockets).socket_type − should be SOCK_STREAM (the default), SOCK_DGRAM, SOCK_RAW or perhaps one of the other SOCK_ constants.protocol − number is usually zero and may be omitted.This method returns a socket object.
Once you have the socket object, then you can use the required methods to create your client or server program.
The socket instantiated on server is called server socket. Following methods are available to the socket object on the server −
bind() method − This method binds the socket to specified IP address and port number.listen() method − This method starts server and runs into a listen loop looking for connection request from client.accept() method − When connection request is intercepted by server, this method accepts it and identifies the client socket with its address.To create a socket on a server, use the following snippet −
import socket
server = socket.socket()
server.bind(('localhost',12345))
server.listen()
client, addr = server.accept()
print ("connection request from: " + str(addr))
By default, the server is bound to local machine’s IP address ‘localhost’ listening at arbitrary empty port number.
Similar socket is set up on the client end. It mainly sends connection request to server socket listening at its IP address and port number
This method takes a two-item tuple object as argument. The two items are IP address and port number of the server.
obj=socket.socket()
obj.connect((host,port))
Once the connection is accepted by the server, both the socket objects can send and/or receive data.
The server sends data to client by using the address it has intercepted.
client.send(bytes)
Client socket sends data to socket it has established connection with.
similar to send(). However, unlike send(),this method continues to send data from bytes until either all data has been sent or an error occurs. None is returned on success.
This method is to be used in case of UDP protocol only.
This method is used to retrieve data sent to the client. In case of server, it uses the remote socket whose request has been accepted.
client.recv(bytes)
This method is used in case of UDP protocol.
To write Internet servers, we use the socket function available in socket module to create a socket object. A socket object is then used to call other functions to setup a socket server.
Now call the bind(hostname, port) function to specify a port for your service on the given host.
Next, call the accept method of the returned object. This method waits until a client connects to the port you specified, and then returns a connection object that represents the connection to that client.
import socket
host = "127.0.0.1"
port = 5001
server = socket.socket()
server.bind((host,port))
server.listen()
conn, addr = server.accept()
print ("Connection from: " + str(addr))
while True:
data = conn.recv(1024).decode()
if not data:
break
data = str(data).upper()
print (" from client: " + str(data))
data = input("type message: ")
conn.send(data.encode())
conn.close()
Let us write a very simple client program, which opens a connection to a given port 5001 and a given localhost. It is very simple to create a socket client using the Python’s socket module function.
The socket.connect(hosname, port) opens a TCP connection to hostname on the port. Once you have a socket open, you can read from it like any IO object. When done, remember to close it, as you would close a file.
The following code is a very simple client that connects to a given host and port, reads any available data from the socket, and then exits when ‘q’ is entered.
import socket
host = '127.0.0.1'
port = 5001
obj = socket.socket()
obj.connect((host,port))
message = input("type message: ")
while message != 'q':
obj.send(message.encode())
data = obj.recv(1024).decode()
print ('Received from server: ' + data)
message = input("type message: ")
obj.close()
The following program demonstrates how socket communication can be used to transfer a file from server to the client
The code for establishing connection is same as before. After the connection request is accepted, a file on server is opened in binary mode for reading, and bytes are successively read and sent to the client stream till end of file is reached.
import socket
host = "127.0.0.1"
port = 5001
server = socket.socket()
server.bind((host, port))
server.listen()
conn, addr = server.accept()
data = conn.recv(1024).decode()
filename='test.txt'
f = open(filename,'rb')
while True:
l = f.read(1024)
if not l:
break
conn.send(l)
print('Sent ',repr(l))
f.close()
print('File transferred')
conn.close()
On the client side, a new file is opened in wb mode. The stream of data received from server is written to the file. As the stream ends, the output file is closed. A new file will be created on the client machine.
import socket
s = socket.socket()
host = "127.0.0.1"
port = 5001
s.connect((host, port))
s.send("Hello server!".encode())
with open('recv.txt', 'wb') as f:
while True:
print('receiving data...')
data = s.recv(1024)
if not data:
break
f.write(data)
f.close()
print('Successfully received')
s.close()
print('connection closed')
The socketserver module in Python’s standard library is a framework for simplifying task of writing network servers. There are following classes in module, which represent synchronous servers −
| BaseServer | o | o |
|---|---|---|
| TCPServer | –> | Unix Stream Server |
| UDPServer | –> | Unix Datagram Server |
These classes work with corresponding RequestHandler classes for implementing the service. BaseServer is the superclass of all Server objects in the module.
TCPServer class uses the internet TCP protocol, to provide continuous streams of data between the client and server. The constructor automatically attempts to invoke server_bind() and server_activate(). The other parameters are passed to the BaseServer base class.
You must also create a subclass of StreamRequestHandler class. IT provides self.rfile and self.wfile attributes to read or write to get the request data or return data to the client.
UDPServer and DatagramRequestHandler − These classes are meant to be used for UDP protocol.DatagramRequestHandler and UnixDatagramServer − These classes use Unix domain sockets; they’re not available on non-Unix platforms.You must write a RequestHandler class. It is instantiated once per connection to the server, and must override the handle() method to implement communication to the client.
import socketserver
class MyTCPHandler(socketserver.BaseRequestHandler):
def handle(self):
self.data = self.request.recv(1024).strip()
host,port=self.client_address
print("{}:{} wrote:".format(host,port))
print(self.data.decode())
msg=input("enter text .. ")
self.request.sendall(msg.encode())
On the server’s assigned port number, an object of TCPServer class calls the forever() method to put the server in the listening mode and accepts incoming requests from clients.
if __name__ == "__main__":
HOST, PORT = "localhost", 9999
with socketserver.TCPServer((HOST, PORT), MyTCPHandler) as server:
server.serve_forever()
When working with socketserver, the client code is more or less similar with the socket client application.
import socket
import sys
HOST, PORT = "localhost", 9999
while True:
with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
# Connect to server and send data
sock.connect((HOST, PORT))
data = input("enter text .. .")
sock.sendall(bytes(data + "\n", "utf-8"))
# Receive data from the server and shut down
received = str(sock.recv(1024), "utf-8")
print("Sent: {}".format(data))
print("Received: {}".format(received))
Run the server code in one command prompt terminal. Open multiple terminals for client instances.