How to run a background procedure while constantly checking for input - threading?

concurrency, multithreading, python, python-multithreading, sockets

Solution

Since you want the server process to be able to handle the client while in the same time receiving input from the server's `stdin`, you can just put the whole current server code in a `Thread`, then wait input from `stdin`.

import socket
from time import sleep
import threading

def process():
    sock = socket.socket()
    sock.bind(("127.0.0.1",12346))
    sock.listen(3)
    print "Waiting on connection"
    conn = sock.accept()
    print "Client connected"

    while True:
        m = conn[0].recv(4096)
        conn[0].send(m[::-1])

    sock.shutdown(socket.SHUT_RDWR)
    sock.close()

thread = threading.Thread(target=process)
thread.daemon = True
thread.start()
while True:
    exit_signal = raw_input('Type "exit" anytime to stop server\n')
    if exit_signal == 'exit':
        break

and you can remove the "exit" checking in client.

In this code, the server will be doing nothing after the client disconnect, though, it will just wait for "exit" to be typed in the `stdin`. You might want to extend the code to make the server able to accept new clients, since you don't want the client to have the ability to close the server. In that case, you can put another `while` loop from `conn = sock.accept()` to `sock.close()`.

And as @usmcs suggested, if you don't have any other command to be sent to the server, it will be better if you use CTRL-C (`KeyboardInterrupt`) instead, so you don't need the thread, while it can still end the server gracefully (meaning no error due to the CTRL-C is reported) with this code:

import socket
from time import sleep
import threading

sock = socket.socket()
sock.bind(("127.0.0.1",12346))
sock.listen(3)
print "Waiting on connection"
conn = sock.accept()
print "Client connected"

while True:
    try:
        m = conn[0].recv(4096)
        conn[0].send(m[::-1])
    except KeyboardInterrupt:
        break

sock.close()

Problem

Original source