Question

Is there a Python version of the C++ class QtSingleApplication from Qt Solutions?

QtSingleApplication is used to make sure that there can never be more than one instance of an application running at the same time.

Answer 1

Here is my own implementation.
It has been tested with Python 2.7 and PySide 1.1.

It has essentially the same interface as the C++ version of QtSingleApplication. The main difference is that you must supply an application unique id to the constructor. (The C++ version by default uses the path to the executable as a unique id; that would not work here because the executable will most likely be python.exe.)

from PySide.QtCore import *
from PySide.QtGui import *
from PySide.QtNetwork import *

class QtSingleApplication(QApplication):

    messageReceived = Signal(unicode)

    def __init__(self, id, *argv):

        super(QtSingleApplication, self).__init__(*argv)
        self._id = id
        self._activationWindow = None
        self._activateOnMessage = False

        # Is there another instance running?
        self._outSocket = QLocalSocket()
        self._outSocket.connectToServer(self._id)
        self._isRunning = self._outSocket.waitForConnected()

        if self._isRunning:
            # Yes, there is.
            self._outStream = QTextStream(self._outSocket)
            self._outStream.setCodec('UTF-8')
        else:
            # No, there isn't.
            self._outSocket = None
            self._outStream = None
            self._inSocket = None
            self._inStream = None
            self._server = QLocalServer()
            self._server.listen(self._id)
            self._server.newConnection.connect(self._onNewConnection)

    def isRunning(self):
        return self._isRunning

    def id(self):
        return self._id

    def activationWindow(self):
        return self._activationWindow

    def setActivationWindow(self, activationWindow, activateOnMessage = True):
        self._activationWindow = activationWindow
        self._activateOnMessage = activateOnMessage

    def activateWindow(self):
        if not self._activationWindow:
            return
        self._activationWindow.setWindowState(
            self._activationWindow.windowState() & ~Qt.WindowMinimized)
        self._activationWindow.raise_()
        self._activationWindow.activateWindow()

    def sendMessage(self, msg):
        if not self._outStream:
            return False
        self._outStream << msg << '\n'
        self._outStream.flush()
        return self._outSocket.waitForBytesWritten()

    def _onNewConnection(self):
        if self._inSocket:
            self._inSocket.readyRead.disconnect(self._onReadyRead)
        self._inSocket = self._server.nextPendingConnection()
        if not self._inSocket:
            return
        self._inStream = QTextStream(self._inSocket)
        self._inStream.setCodec('UTF-8')
        self._inSocket.readyRead.connect(self._onReadyRead)
        if self._activateOnMessage:
            self.activateWindow()

    def _onReadyRead(self):
        while True:
            msg = self._inStream.readLine()
            if not msg: break
            self.messageReceived.emit(msg)

Here is a simple test program:

import sys
from PySide.QtGui import *
from QtSingleApplication import QtSingleApplication

appGuid = 'F3FF80BA-BA05-4277-8063-82A6DB9245A2'
app = QtSingleApplication(appGuid, sys.argv)
if app.isRunning(): sys.exit(0)

w = QWidget()
w.show()
app.setActivationWindow(w)
sys.exit(app.exec_())