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TCP client/server with Python

Welcome to the next pikoTutorial !

The minimal TCP server

A TCP server listens for incoming connections on a specified port and communicates with clients over that connection.

Python
from socket import socket, AF_INET, SOCK_STREAM
from argparse import ArgumentParser
# define command line interface
parser = ArgumentParser()
parser.add_argument('-ip', help='IP')
parser.add_argument('-server_port', type=int, help='Server port')
# parse command line options
args = parser.parse_args()
# create a TCP socket
with socket(AF_INET, SOCK_STREAM) as server_socket:
    # bind server with a specific network address
    # which consists of IP and port number
    server_socket.bind((args.ip, args.server_port))
    # start listening for requests
    server_socket.listen()
    print(f'TCP server up and listening on {args.ip}:{args.server_port}')
    # accept an incoming connection
    client_socket, client_address = server_socket.accept()
    print(f'Connection established with {client_address[0]}:{client_address[1]}')
    # receive data from the client
    data: bytes = client_socket.recv(1024)
    print(f'Received message: {data.decode()}')
    # send response to client
    client_socket.send('Message received'.encode())

The minimal TCP client

A TCP client establishes a connection with a server, sends data, and receives responses.

Python
from socket import socket, AF_INET, SOCK_STREAM
from argparse import ArgumentParser
# define command line interface
parser = ArgumentParser()
parser.add_argument('-ip', help='IP')
parser.add_argument('-server_port', type=int, help='Server port')
# parse command line options
args = parser.parse_args()
# create a TCP socket
with socket(AF_INET, SOCK_STREAM) as client_socket:
    # connect to the server
    client_socket.connect((args.ip, args.server_port))
    # send message to the server
    print(f'Sending message to {args.ip}:{args.server_port}')
    client_socket.send('Hello from TCP client!'.encode())
    # receive response from server
    data: bytes = client_socket.recv(1024)
    print(f'Received response from server: {data.decode()}')
Read also on dev.to: Communication via UDP in Python

Bidirectional communication

You can now create the following setup:

UDP/TCP client server connection

Note for beginners: on the image above, I didn’t put any specific number of the client’s port because in the first example that number is assigned by operating system at random. If you want to know how to specify the client’s port as well, you’ll find it in the second example.

Run server with the command below:

Bash
python3 server.py -ip 127.0.0.1 -server_port 12345

And the client:

Bash
python3 client.py -ip 127.0.0.1 -server_port 12345

You’ll see the following output in the server console:

TCP server up and listening on 127.0.0.1:12345
Connection established with 127.0.0.1:36486
Received message: Hello from TCP client!

And on the client side, you’ll get:

Sending message to 127.0.0.1:12345
Received response from server: Message received

Specifying client’s port

In the previous example, although we defined the server port, the client’s port was randomly assigned by the operating system. If you need to specify the client’s port, you can bind it manually before connecting to the server:

Python
from socket import socket, AF_INET, SOCK_STREAM
from argparse import ArgumentParser
# define command line interface
parser = ArgumentParser()
parser.add_argument('-ip', help='IP')
parser.add_argument('-server_port', type=int, help='Server port')
parser.add_argument('-client_port', type=int, help='Client port')
# parse command line options
args = parser.parse_args()
# create a TCP socket
with socket(AF_INET, SOCK_STREAM) as client_socket:
    # bind client to the specific address
    client_socket.bind((args.ip, args.client_port))
    # connect to the server
    client_socket.connect((args.ip, args.server_port))
    # send message to the server
    print(f'Sending message to {args.ip}:{args.server_port}')
    client_socket.send('Hello from TCP client!'.encode())
    # receive response from server
    data: bytes = client_socket.recv(1024)
    print(f'Received response from server: {data.decode()}')

If you run the above script with the following command:

Bash
python3 client.py -ip 127.0.0.1 -server_port 12345 -client_port 56789

You’ll get the connection as presented on the image below:

Specify UDP/TCP client port

You’ll also see the client’s port number in the server’s logs:

TCP server up and listening on 127.0.0.1:12345
Connection established with 127.0.0.1:56789
Received message: Hello from TCP client!

Read also: