Question

I have written this two .py code to communicate between each outher .
A.py listens to port 8888 and sends data to 7777
B.py listens to port 7777 and sends data to 8888
Both of these client part stuck in an infinite loop after starting their server.
where is the problem ??
If I use only server in A.py and client in B.py (and vice versa ) without any threading they works fine.

A.py:

import socket    
import threading
import thread
import time   


class server(threading.Thread):
    s = ''
    host = 0
    port = 0
    def __init__(self):
        threading.Thread.__init__(self)
        global s,host,port
        s = socket.socket()        
        host = socket.gethostname() 
        port = 8888

    def run(self):
        global s,host,port
        print 'Server started!'
        print 'Waiting for clients...'

        s.bind((host, port))       
        s.listen(5)                 
        c, addr = s.accept()     
        print 'Got connection from', addr
        while True:
            time.sleep(2)
            msg = c.recv(1024)
            if len(msg)==0 :  break
            print addr, ' >> ', msg




class client(threading.Thread):
    s = ''
    host = 0
    port = 0

    def __init__(self):
        threading.Thread.__init__(self)
        global s,host,port
        s = socket.socket()         
        host = socket.gethostname() 
        port = 7777 

    def run(self):

        try:
            time.sleep(5)
            global s,host,port
            print 'Connecting to ', host, port
            s.connect((host, port))
            print "Connectd"
            while True:
                time.sleep(2)
                msg = raw_input('CLIENT >> ')
                if len(msg)==0:break
                s.send(msg)
        except:
            print "Waiting"
            self.run()



thread1 = server()
thread2 = client();

thread1.start()
thread2.start()

thread1.join()
thread2.join();

B.py:

import socket    
import threading
import thread
import time   


class server(threading.Thread):
    s = ''
    host = 0
    port = 0
    def __init__(self):
        threading.Thread.__init__(self)
        global s,host,port
        s = socket.socket()        
        host = socket.gethostname() 
        port = 7777

    def run(self):
        global s,host,port
        print 'Server started!'
        print 'Waiting for clients...'

        s.bind((host, port))       
        s.listen(5)                 
        c, addr = s.accept()     
        print 'Got connection from', addr
        while True:
            time.sleep(2)
            msg = c.recv(1024)
            if len(msg)==0 :  break
            print addr, ' >> ', msg



class client(threading.Thread):
    s = ''
    host = 0
    port = 0

    def __init__(self):
        threading.Thread.__init__(self)
        global s,host,port
        s = socket.socket()         
        host = socket.gethostname() 
        port = 8888

    def run(self):
        try:
            time.sleep(5)
            global s,host,port
            print 'Connecting to ', host, port
            s.connect((host, port))
            print "connected"
            while True:
                time.sleep(2)
                msg = raw_input('CLIENT >> ')
                if len(msg)==0:break
                s.send(msg)
        except:
            print "waiting"
            self.run();



thread1 = server()
thread2 = client();

thread1.start()
thread2.start()

thread1.join()
thread2.join();

Answer 1

• Using global s, host, port is the cause of the problem. In A.py,
  for instance, the server and client classes are both changing the
  same variables s, host and port. By changing the port to be the same value, you are either messing up either the server or the client (whichever runs first).

  Never use global if you don’t have to, and you very rarely have to.
  In this case, your problem is fixed by using instance attributes.

• Also, I suggest writing the client.run method without recursive
  calls to self.run(). Python has a limit to how many recursive calls
  you can make, and if the client has to wait too long, a recursive
  call here could cause your program to fail. Instead, you could use a
  while loop. (See below).

---

import argparse
import socket    
import threading
import thread
import time   

class server(threading.Thread):
    def __init__(self, port):
        threading.Thread.__init__(self)
        self.s = socket.socket()        
        self.host = socket.gethostname() 
        self.port = port

    def run(self):
        print 'Server started!'
        print 'Waiting for clients...'

        self.s.bind((self.host, self.port))       
        self.s.listen(5)                 
        c, addr = self.s.accept()     
        print 'Got connection from', addr
        while True:
            time.sleep(2)
            msg = c.recv(1024)
            if len(msg) == 0 :  break
            print addr, ' >> ', msg

class client(threading.Thread):
    def __init__(self, port):
        threading.Thread.__init__(self)
        self.s = socket.socket()         
        self.host = socket.gethostname() 
        self.port = port

    def run(self):
        while True:
            time.sleep(5)
            print 'Connecting to ', self.host, self.port
            try:
                self.s.connect((self.host, self.port))
                break
            except Exception as err:
                print "Waiting", err
        print "Connectd"
        while True:
            time.sleep(2)
            msg = raw_input('CLIENT >> ')
            if len(msg) == 0:break
            self.s.send(msg)

if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument('--server_port', type = int, default = 8888)
    parser.add_argument('--client_port', type = int, default = 7777)
    args = parser.parse_args()

    thread1 = server(args.server_port)
    thread2 = client(args.client_port)

    thread1.start()
    thread2.start()

    thread1.join()
    thread2.join()

---

Run it with

test.py --server 8888 --client 7777
test.py --server 7777 --client 8888