PyQt: timers cannot be started from another thread

Question

I am making a Qt GUI with python and i am getting the error: QObject::startTimer: timers cannot be started from another thread. It occurs when I run the readModemSnap method. I've been working on this for almost a week trying many different design patterns for threading in Qt that i've found on the web but nothign works.

class ModemScopeWindow(QMainWindow, Ui_ModemScope):
def __init__(self, parent=None):
    super(ModemScopeWindow, self).__init__(parent)

    # Set up the user interface from Designer.
    self.setupUi(self)

    self.thread = MainThread()

    """
    signal connections
    """

    self.thread.newSnap.connect(self.updateScene)       
    self.thread.updateStatus.connect(self.setStatus) 


    self.thread.connectionLock.lock()
    self.thread.runLock.lock()

    self.connect(self.runButton, SIGNAL("clicked()"), self.thread.runLock.unlock, Qt.QueuedConnection)

    self.connect(self.connectButton, SIGNAL("clicked()"), self.thread.connectionLock.unlock, Qt.QueuedConnection)


class MainThread(QThread):

newSnap = pyqtSignal(QGraphicsScene)
updateStatus = pyqtSignal(str)
initConnect = pyqtSignal()

def __init__(self, parent = None):
    super(MainThread, self).__init__(parent)

    self.samples = []

    self.connectionLock = QMutex()
    self.runLock = QMutex()        
    self.cliMute = QMutex()

    self._displayCrosshairs = True
    self._displayGrid = True
    self.persistantMode = False
    self.sampleDepth = 1

    self._currentHaam = "4"

    color = QColor(10,255,71)
    self.plotPen = QPen(color)


    self._leftXscene = -VIEW_SIZE/2
    self._topYscene = -VIEW_SIZE/2
    self._rightXscene = VIEW_SIZE/2
    self._bottomYscene = VIEW_SIZE/2
    self._leftXworld = -10.0
    self._topYworld = 10.0
    self._rightXworld = 10.0
    self._bottomYworld = -10.0
    self._scene = QGraphicsScene(self._leftXscene, self._topYscene, VIEW_SIZE, VIEW_SIZE, self)

    self.start(QThread.HighestPriority)

def run(self):

    self.updateStatus.emit("Enter target IP address and press Connect")

    self.connectionLock.lock()
    self.connectModem()

    while(1):
        self.runLock.lock() 
        #compile scene

        self.readModemSnap()
        self.newSnap.emit(self._scene)
        self.runLock.unlock()

def readModemSnap(self):
    self.updateStatus.emit("Reading Modem Snap...")

    print len(self.samples)
    if len(self.samples) >= self.sampleDepth:# and not self.persistantMode:
        self.samples.pop(0)

    self.cliMute.lock()
    temp = cli.getModemSnap()
    self.cliMute.unlock()
    self.samples.append(temp)


    self.cliMute.lock()
    modType = cli.modemRead(80)
    self.cliMute.unlock()

    if((modType | 0x0FFFFFFF) == 0x0FFFFFFF):
        modType = "0";

    else:
        modType = "%x"%modType
        modType = str(modType)


    modType = "0"
    self.updateStatus.emit("Done") 

    self.refresh()

    self._currentHaam = modType[0]
    if self._displayGrid:
        self.plotModulation(self._currentHaam)

    self.handleSnapshotResponse()

    self.updateStatus.emit("Ready to Run")
def refresh(self):

    #delete scene
    items = self._scene.items()

    for x in items:
        self._scene.removeItem(x)

    #repaint the crosshairs
    if self._displayCrosshairs:
        self.plotLine(-VIEW_SIZE,0,+VIEW_SIZE,0, self.plotPen)
        self.plotLine(0, -VIEW_SIZE,0, +VIEW_SIZE, self.plotPen)
        self.plotScaleTicks()

    #repaint grid
    if self._displayGrid:
        self.plotModulation(self._currentHaam)

    self.newSnap.emit(self._scene)

def handleSnapshotResponse(self):

    for x in range(len(self.samples)):
        for sample in self.samples[x]:
            upper = (sample >> 16) & 0xffff;
            lower = sample & 0xffff
            if (upper & 0x8000):
                upper -= 0x10000
            if (lower & 0x8000):
                lower -= 0x10000
            upper = float(upper)/128.0
            lower = float(lower)/128.0
            self.plot(upper, lower)

as you can see Im not starting any thread from another thread. i use the main to start the UI which creates a MainThread that starts itself upon construction. When i commented lines out to localize the problem I found that its when i call self.refresh() and self.handleSnapshotResponse() in the readModemSnap method. Can anyone point me in the direction of what im doing wrong? or any tutorials on QThreading? thanks in advance

Answer 1

This is the rule: you cannot call any GUI functions from any thread other than the main thread running the Qt event loop. When you see errors about QTimer, it's probably because something in the GUI uses a timer internally and it is being triggered from another thread.

The most likely culprit in your case is that you are operating on a QGraphicsScene from the worker thread. I would try rearranging such that the code in MainThread.reload is called in response to the newSnap signal, rather than before it.