Why does python input function return with empty string instead of waiting for user to input?

Question

The code below is a simple chat client. I start a local tcp server with netcat -l 8000 then connect with it by the running the script below. Depending on whether i start t1 or t2 first. I can either read from the server or not. I also notice, with the print statement, that i seem to be getting empty message on reading from terminal using both sys.readline and input functions.

I have also tried with threads instead of processes and the result is the same.

Edit: based on the response given by @paulsm4 i used ncat instead of netcat for as my server. this resulted in something crazier happening. Instead of netcat/nc if I use ncat -k -l -m 100 8000 and replace Process with Thread. The code works as it should. But when I use Process the code breaks down.

import sys
import socket
from multiprocessing import Process
from time import sleep


def connect(host, port):
    s = socket.socket()
    s.connect((host, port))
    return s

def send_message(host, port):
    s = connect(host, port)
    while True:
        sleep(1)
        message = None
        try:
            message = input('> ')
        except EOFError:
            print("try failed")
            pass
        print('message is ', message)
        if message:
            val = s.send(message.encode('utf8'))
            print('message sent')


def receive(host, port):
    s = connect(host, port)
    while True:
        sleep(1)
        message = s.recv(1024).decode('utf8')
        print('\n', message)


def main(host, port):
    t1 = Process(target=send_message, args=(host, port))
    t2 = Process(target=receive, args=(host, port))
    t1.start()
    t2.start()
    t1.join()
    t2.join()


if __name__ == '__main__':
    host, port = sys.argv[1], int(sys.argv[2])
    main(host, port)

paulsm4 · Accepted Answer · 2017-05-26 20:00:33Z

1

https://docs.python.org/2/howto/sockets.html

When a recv returns 0 bytes, it means the other side has closed (or is in the process of closing) the connection. You will not receive any more data on this connection. Ever. You may be able to send data successfully; I’ll talk more about this later.

In other words, you need to make sure your "server" has started, and is ready to accept connections BEFORE (and WHILE) your client attempts to talk to it.

The above link has a great example program. Here's another:

Python socket – network programming tutorial

answered May 26, 2017 at 20:00

paulsm4

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1 Comment

Prasanjit Prakash Over a year ago

ah yes!! stupid question. netcat and nc by default listen to only one connection. thanks for pointing me in the right direction!

cfrancklin · Accepted Answer · 2017-05-26 20:09:04Z

0

You should add s.bind((host,port)) to connect the socket to the IP and port specified, s.listen(1) so the server waits for incoming packages and finally conn, addr = s.accept() to establish a new connection. On the client side you would add s.connect((host,port)) to connect your client to the server.

answered May 26, 2017 at 20:09

cfrancklin

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