Simple TELNET in Python - loop paused if no data coming

Question

I'm trying to build a very simple TELNET client in Python and I'm getting problem on the last part: sending/receiving data to/from the server.

With the code I have, if no data arrives at the very beginnig, the loop get paused and I can't even send commands.

Here the interested part of the code:

# Infinite cycle that allows user to get and send data from/to the host
while True:

    incoming_data = my_socket.recv(4096)
    if not incoming_data:
        print('Problem occurred - Connection closed')
        my_socket.close()
        sys.exit()
    else:
        # display data sent from the host trough the stdout
        sys.stdout.write(incoming_data)
        # Commands sent to the host
    command = sys.stdin.readline()
    my_socket.send(command)

(I think the program kinda of works if I try to connect to some hosts that send data at the beginning.)

The idea would be have two loops, running at the same time, getting data or sending data, but I can't get it to work. I can't use the telnet library and I don't want to use the select library (only sys and socket).

Answer 1

You want to use the threading library.

The following program runs the receiving in one thread and the sending in another:

import socket
from threading import Thread


def listen(conn):
    while True:
        received = conn.recv(1024).decode()
        print("Message received: " + received)


def send(conn):
    while True:
        to_send = input("Input message to send: ").encode()
        conn.sendall(to_send)

host = "127.0.0.1"
port = 12345    

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

Thread(target=listen, args=[sock]).start()
Thread(target=send, args=[sock]).start()

This program is for Python 3. Python 2 is very similar, except print() works differently, and you don't need to encode() and decode() everything being sent through a socket.

The listen and send functions are run in parallel, so that as soon as data arrives, it is printed, but you can also send data at any time. Practically, you would probably want to make some changes so that the data isn't just printed over the input prompt. However, this would be hard just in a command line application.

Research queues for control over data passing between threads.

Let me know if you have any more questions.