Question

My application written in PyQt4 doesn’t seem to get any data.
readyRead event isn’t even fired.
Use of the applications is following:

python server.py -s -p 50000 #(server on port 50000)
python server.py -c -p 50000 #(client sending data to port 50000)

//

import sys
import time

from PyQt4 import QtNetwork, QtCore
from optparse import OptionParser

class Server(object):

    def __init__(self, port):

        self.port = port

        try:
            self.socket = QtNetwork.QUdpSocket()
            self.socket.bind(QtNetwork.QHostAddress.Broadcast, int(self.port), QtNetwork.QUdpSocket.ShareAddress)

            self.socket.readyRead.connect(self.receiver)

        except QtNetwork.QUdpSocket.NetworkError:
            print "EXCEPTION DURING INITIALIZING SERVER'S SOCKET"
            sys.exit(1)

    def receiver(self):
        print "DEBUG: RECEIVE"
        while(self.socket.hasPendingDatagrams()):
            try:
                size = self.socket.pendingDatagramSize()
                msg, host, port = self.socket.readDatagram(size)
            except:
                print "EXCEPTION DURING RECEIVEING AND READING DATAGRAM"
            else:
                print "HOST %s:%s MSG: %s" % (str(host), str(port), str(msg))

    def __del__(self):
        print "DESTRUCTOR"
        self.socket.close()

class Client(object):

    def __init__(self, port):

        self.port = port

        try:
            self.socket = QtNetwork.QUdpSocket()
        except:
            print "EXCEPTION DURING INITIALIZING CLIENT'S SOCKET"

        self.main_loop()

    def main_loop(self):
        for i in range(20):
            self.debug_msg()
            time.sleep(0.5)
        print "EXITING"
        self.socket.close()


    def debug_msg(self):
        msg = "DEBUG"
        self.socket.writeDatagram(msg, QtNetwork.QHostAddress.Broadcast, int(self.port))


if __name__ == "__main__":

    parser = OptionParser()
    parser.add_option("-p", "", action="store", type="string", dest="port")
    parser.add_option("-c", "", action="store_true", dest="client")
    parser.add_option("-s", "", action="store_true", dest="server")

    options, args = parser.parse_args()

    if not (options.server or options.client):
        print "Client/Server not specified. Could not continue..."
        sys.exit(1)

    elif not options.port:
        print "Server's port not specified. Could not continue..."
        sys.exit(1)

    else:

        if options.server:
            serv = Server(options.port)

            App = QtCore.QCoreApplication(sys.argv)
            sys.exit(App.exec_())
        else:
            client = Client(options.port)

Answer 1

In your client, you are not starting Qt’s event loop. You should allocate an application object and either call exec_ on it and handle the debug_msg calls via timers or you should pump the event loop using QCoreApplication.processEvents().

Another option is to use flush():

http://doc.qt.nokia.com/latest/qabstractsocket.html#flush

bool QAbstractSocket::flush ()

This function writes as much as possible from the internal write
buffer to the underlying network socket, without blocking. If any data
was written, this function returns true; otherwise false is returned.

Call this function if you need QAbstractSocket to start sending
buffered data immediately. The number of bytes successfully written
depends on the operating system. In most cases, you do not need to
call this function, because QAbstractSocket will start sending data
automatically once control goes back to the event loop. In the absence
of an event loop, call waitForBytesWritten() instead.