Python Cookbook

Writing a TCP Server

Credit: Luther Blissett

Problem

You want to write a server that waits for clients to connect over the network to a particular port.

Solution

Assuming you’re using the Internet to communicate:

import socket

# Create a socket
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

# Ensure that you can restart your server quickly when it terminates
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

# Set the client socket's TCP "well-known port" number
well_known_port = 8881
sock.bind(('', well_known_port))

# Set the number of clients waiting for connection that can be queued
sock.listen(5)

# loop waiting for connections (terminate with Ctrl-C)
try:
    while 1:
        newSocket, address = sock.accept(  )
        print "Connected from", address
        # loop serving the new client
        while 1:
            receivedData = newSocket.recv(1024)
            if not receivedData: break
            # Echo back the same data you just received
            newSocket.send(receivedData)
        newSocket.close(  )
        print "Disconnected from", address
finally:
    sock.close(  )

Discussion

Setting up a server takes a bit more work than setting up a client. We need to bind to a well-known port that clients will use to connect to us. Optionally, as we do in this recipe, we can set SO_REUSEADDR so we can restart the server when it terminates without waiting for a few minutes, which is quite nice during development and testing. We can also optionally call listen to control the number of clients waiting for connections that can be queued.

After this preparation, we just ...