I'm trying to modify my main layout from another thread. But the function run() is never called
and i'm having the error:
QObject::setParent: Cannot set parent, new parent is in a different
thread
Here's my code:
class FeedRetrievingThread(QtCore.QThread):
def __init__(self, parent=None):
super(FeedRetrievingThread, self).__init__(parent)
self.mainLayout = parent.mainLayout
def run(self):
# Do things with self.mainLayout
class MainWindow(QtGui.QDialog):
def __init__(self, parent=None):
super(MainWindow, self).__init__(parent)
self.mainLayout = QtGui.QGridLayout()
self.setLayout(self.mainLayout)
self.feedRetrievingThread = FeedRetrievingThread(self)
self.timer = QtCore.QTimer()
self.timer.timeout.connect(self.updateFeed)
self.timer.start(1000)
def updateFeed(self):
if not self.feedRetrievingThread.isRunning():
print 'Running thread.'
self.feedRetrievingThread.start()
if __name__ == '__main__':
app = QtGui.QApplication(sys.argv)
mainWindow = MainWindow()
mainWindow.show()
sys.exit(app.exec_())
I really don't get it, why is it so difficult to access the GUI with PyQt?
In C# you have Invoke. Is there anything of the kind in PyQt?
I tried creating the thread directly from MainWindow.__init__ (without using the timer) but it didn't work either.
In Qt you should never attempt to directly update the GUI from outside of the GUI thread.
Instead, have your threads emit signals and connect them to slots which do the necessary updating from within the GUI thread.
See the Qt documentation regarding Threads and QObjects.
Related
I'm trying to implement a webcam using PyQt5 from an example I found (here, but not really relevant).
Getting the example to work wasn't an issue, but I wanted to modify some things and I am stuck on one particular problem.
I have two classes, one QObject Capture which has a QBasicTimer that I want to start, and a QWidget MyWidget with a button that is supposed to start the timer of the Capture object, which is inside a QThread.
If I directly connect the button click to the method that starts the timer, everything works fine.
But I want to do some other things when I click the button, so I connected the button to a method of MyWidget first and call the start method of Capture from there. This, however, doesn't work: the timer doesn't start.
Here is a minimal working example:
from PyQt5 import QtCore, QtWidgets
import sys
class Capture(QtCore.QObject):
def __init__(self, parent=None):
super(Capture, self).__init__(parent)
self.m_timer = QtCore.QBasicTimer()
def start(self):
print("capture start called")
self.m_timer.start(1000, self)
def timerEvent(self, event):
print("time event")
class MyWidget(QtWidgets.QWidget):
def __init__(self, parent=None):
super(MyWidget, self).__init__(parent)
lay = QtWidgets.QVBoxLayout(self)
self.btn_start = QtWidgets.QPushButton("Start")
lay.addWidget(self.btn_start)
self.capture = Capture()
captureThread = QtCore.QThread(self)
captureThread.start()
self.capture.moveToThread(captureThread)
# self.btn_start.clicked.connect(self.capture.start) # this works
self.btn_start.clicked.connect(self.startCapture) # this doesn't
# self.capture.start() # this doesn't either
self.show()
def startCapture(self):
self.capture.start()
def run_app():
app = QtWidgets.QApplication(sys.argv)
mainWin = MyWidget()
mainWin.show()
app.exec_()
run_app()
It is some problem with the QThread, because if I don't use threading it works. I thought maybe it has something to do with the thread not being in some way available when called from a different method than the one it was created in, but calling self.capture.start() directly from the init does not work either.
I only have a very basic grasp of threads. Can someone tell me how I can properly call self.capture.start() from MyWidget and why it works without problems when directly connecting it to the button click?
If you connect the button's clicked signal to the worker's start slot, Qt will automatically detect that it's a cross-thread connection. When the signal is eventually emitted, it will be queued in the receiving thread's event-queue, which ensures the slot will be called within the worker thread.
However, if you connect the button's clicked signal to the startCapture slot, there's no cross-thread connection, because the slot belongs to MyWidget (which lives in the main thread). When the signal is emitted this time, the slot tries to create the timer from within the main thread, which is not supported. Timers must always be started within the thread that creates them (otherwise Qt will print a message like "QBasicTimer::start: Timers cannot be started from another thread").
A better approach is to connect the started and finished signals of the thread to some start and stop slots in the worker, and then call the thread's start and quit methods to control the worker. Here's a demo based on your script, which shows how to implement that:
from PyQt5 import QtCore, QtWidgets
import sys
class Capture(QtCore.QObject):
def __init__(self, parent=None):
super(Capture, self).__init__(parent)
self.m_timer = QtCore.QBasicTimer()
def start(self):
print("capture start called")
self.m_timer.start(1000, self)
def stop(self):
print("capture stop called")
self.m_timer.stop()
def timerEvent(self, event):
print("time event")
class MyWidget(QtWidgets.QWidget):
def __init__(self, parent=None):
super(MyWidget, self).__init__(parent)
lay = QtWidgets.QVBoxLayout(self)
self.btn_start = QtWidgets.QPushButton("Start")
lay.addWidget(self.btn_start)
self.capture = Capture()
self.captureThread = QtCore.QThread(self)
self.capture.moveToThread(self.captureThread)
self.captureThread.started.connect(self.capture.start)
self.captureThread.finished.connect(self.capture.stop)
self.btn_start.clicked.connect(self.startCapture)
self.show()
def startCapture(self):
if not self.captureThread.isRunning():
self.btn_start.setText('Stop')
self.captureThread.start()
else:
self.btn_start.setText('Start')
self.stopCapture()
def stopCapture(self):
self.captureThread.quit()
self.captureThread.wait()
def closeEvent(self, event):
self.stopCapture()
def run_app():
app = QtWidgets.QApplication(sys.argv)
mainWin = MyWidget()
mainWin.show()
app.exec_()
run_app()
How to receive close event in following code?
class Main(QMainWindow):
def __init__(self, parent=None):
super(Main, self).__init__(parent)
self.view = QUiLoader().load("sample.ui", self)
self.view.show()
def closeEvent(self, e):
print "close event recieved"
def main():
app = QApplication(sys.argv)
a=Main()
sys.exit(app.exec_())
if __name__ == "__main__":
main()
If I convert sample.ui to sample.py using pyside-uic and importing this into main.py then I was able to receive close event.
from sample import Ui_MainWindow
class Main(QMainWindow, Ui_MainWindow):
def __init__(self, parent=None):
super(Main, self).__init__(parent)
self.setupUi(self)
def closeEvent(self, e):
print "close event recieved"
app = QApplication(sys.argv)
a=Main()
a.show()
sys.exit(app.exec_())
The second example works because it effectively becomes a subclass of the top-level class from Qt Designer. By contrast, the first example uses composition rather than subclassing, which puts all the gui elements inside an internal namespace. The Main class is just a container that acts as the parent of the view widget, and is never actually shown (which in turn means it doesn't receive any close events).
In PyQt, the uic module has several funtions which allow you to work around these issues, but there is currently nothing like that in PySide. Instead, you have to roll your own function. See this answer for an explanation of how to do that.
Alternatively, you could change the top-level class in Qt Designer to a QWidget, and then make view the central widget of your Main class. This is a lot less flexible than the above method, though.
I'm having problem with QThreads in python. I want to change background color of label.
But My application crash while starting.
"QThread: Destroyed while thread is still running"
class MainWindow(QMainWindow):
def __init__(self):
QMainWindow.__init__(self)
self.ui = Ui_MainWindow()
self.ui.setupUi(self)
statusTh = statusThread(self)
self.connect(statusTh, SIGNAL('setStatus'), self.st, Qt.QueuedConnection)
statusTh.start()
def st(self):
if self.status == 'ON':
self.ui.label.setStyleSheet('background-color:green')
else:
self.ui.label.setStyleSheet('background-color:red')
class statusThread(QThread):
def __init__(self, mw):
super(statusThread, self).__init__()
def run(self):
while True:
time.sleep(1)
self.emit(SIGNAL('setStatus'))
if __name__ == "__main__":
app = QApplication(sys.argv)
main_window = MainWindow()
main_window.show()
sys.exit(app.exec_())
You're not storing a reference to the thread after it's been created, which means that it will be garbage collected (ie. destroyed) some time after the program leaves MainWindows __init__. You need to store it at least as long as the thread is running, for example use self.statusTh:
self.statusTh = statusThread(self)
self.connect(self.statusTh, SIGNAL('setStatus'), self.st, Qt.QueuedConnection)
self.statusTh.start()
I know it's quite necroposting but this may be useful. In the main section of your script, a first level customized widget need to be stored in variable, not only created.
For example I have a custom widget class called MainWindow that create a QThread. My main is like that:
from myPackage import MainWindow
if __name__ == "__main__":
app = QApplication([])
widget=MainWindow()
sys.exit(app.exec())
if I avoid the widged = definition and only call for MainWindow(), my script will crash with QThread: Destroyed while thread is still running
I have a QTreeWidget which needs to be populated with a large sum of information. So that I can style it and set it up the way I really wanted, I decided I'd create a QWidget which was styled and dressed all pretty-like. I would then populate the TreeWidget with generic TreeWidgetItems and then use setItemWidget to stick the custom QWidgets in the tree. This works when the QWidgets are called inside the main PyQt thread, but since there is a vast sum of information, I'd like to create and populate the QWidgets in the thread, and then emit them later on to be added in the main thread once they're all filled out. However, when I do this, the QWidgets appear not to be getting their parents set properly as they all open in their own little window. Below is some sample code recreating this issue:
from PyQt4.QtGui import *
from PyQt4.QtCore import *
class ItemWidget(QWidget):
def __init__(self, parent=None):
QWidget.__init__(self, parent)
btn = QPushButton(self)
class populateWidgets(QThread):
def __init__(self):
QThread.__init__(self)
def run(self):
widget = ItemWidget()
for x in range(5):
self.emit(SIGNAL("widget"), widget)
class MyMainWindow(QMainWindow):
def __init__(self, parent=None):
QMainWindow.__init__(self, parent)
self.tree = QTreeWidget(self)
self.tree.setColumnCount(2)
self.setCentralWidget(self.tree)
self.pop = populateWidgets()
self.connect(self.pop, SIGNAL("widget"), self.addItems)
self.pop.start()
itemWidget = QTreeWidgetItem()
itemWidget.setText(0, "This Works")
self.tree.addTopLevelItem(itemWidget)
self.tree.setItemWidget(itemWidget, 1, ItemWidget(self))
def addItems(self, widget):
itemWidget = QTreeWidgetItem()
itemWidget.setText(0, "These Do Not")
self.tree.addTopLevelItem(itemWidget)
self.tree.setItemWidget(itemWidget, 1, widget)
if __name__ == "__main__":
import sys
app = QApplication(sys.argv)
ui = MyMainWindow()
ui.show()
sys.exit(app.exec_())
As you can see, doing it inside MyMainWindow is fine, but somehow things go awry once it gets processed in the thread and returns. Is this possible to do? If so, how do I properly parent the ItemWidget class inside the QTreeWidgetItem? Thanks in advance.
AFAICT Qt doesn't support the creation of QWidgets in a thread other than the thread where the QApplication object was instantiated (i.e. usually the main() thread). Here are some posts on the subject with responses from Qt developers:
http://www.qtcentre.org/archive/index.php/t-27012.html
http://www.archivum.info/qt-interest#trolltech.com/2009-07/00506/Re-(Qt-interest)-QObject-moveToThread-Widgets-cannot-be-moved-to-a-new-thread.html
http://www.archivum.info/qt-interest#trolltech.com/2009-07/00055/Re-(Qt-interest)-QObject-moveToThread-Widgets-cannot-be-moved-to-a-new-thread.html
http://www.archivum.info/qt-interest#trolltech.com/2009-07/00712/Re-(Qt-interest)-QObject-moveToThread-Widgets-cannot-be-moved-toa-new-thread.html
(if it were possible, the way to do it would be to call moveToThread() on the QWidgets from within the main thread to move them to the main thread -- but apparently that technique doesn't work reliably, to the extent that QtCore has a check for people trying to do that prints a warning to stdout to tell them not to do it)
I am using PyQt for a project. But not all of a sudden I am getting an error:
QPixmap: It is not safe to use pixmaps outside the GUI thread in PyQt
I am not using QPixmap anywhere in my code... please help.
class itemCheckBtn(QtGui.QDialog):
qApp = None;
okCallback = None;
def __init__(self,parent=None):
itemCheckBtn.qApp=None;
QtGui.QWidget.__init__(self, None)
self.ui = Ui_merchantPriceFrom();
self.ui.setupUi(self)
QtCore.QObject.connect(self.ui.itemCheckButton, QtCore.SIGNAL("clicked()"), self.submit)
def submit(self):
print "Hi";
The main class is
class MyForm(QtGui.QMainWindow):
serverThreadObject = None;
qApp = None;
sock = None;
def __init__(self, qApp,parent=None):
MyForm.qApp=qApp;
QtGui.QWidget.__init__(self, parent)
self.ui = Ui_bluwavemerchantmain()
self.ui.setupUi(self)
self.ui.server_connection_status_label.setText("Server Offline..");
QtCore.QObject.connect(self.ui.pushButton, QtCore.SIGNAL("clicked()"), self.connectUser )
QtCore.QObject.connect(self.ui.actionStart_Server, QtCore.SIGNAL("triggered()"), self.startServer);
QtCore.QObject.connect(self.ui.actionStop_Server, QtCore.SIGNAL("triggered()"), self.stopServerFromGui);
QtCore.QObject.connect(self.ui.actionExit, QtCore.SIGNAL("triggered()"), self.closeEventFromMenu);
QtCore.QObject.connect(self, QtCore.SIGNAL("triggered()"), self.closeEvent);
i am getting the error when I try to call the class "itemCheckBtn" from the "MyForm" class.
It looks like you are using threads, and somehow you are trying to change the GUI from some thread other than the main GUI thread (this is not allowed). This could be happening somewhat indirectly--for example your server thread calls a function on MyForm, which tries to update the itemCheckBtn. Even though the code is part of MyForm, it is still being executed from the server thread. Instead, you need to use some thread-safe mechanism to inform the GUI thread that a change has occurred, and let it do the GUI work. (see http://doc.qt.nokia.com/4.6/threads-qobject.html)