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Handling QDialog's events safely in PyQT

I'm having troubles with a Qt Application (managed through PyQt5) which used to be reliable, until a bunch of updates (where I ended up with PyQt6). PyQt6 is not the culprit here, as I began to have those problems with later PyQt5 versions.
I'm suspecting my problems are instead linked to an abuse of exec() methods call. The full traceback is not available (I've only a RuntimeError with a cryptic message telling me the Dialog is not available anymore); what (I think) I'm seeing are the parent windows randomly disappearing. This behaviour is indeed mentionned in the Qt documentation:
Note: Avoid using this function; instead, use open(). Unlike exec(), open() is asynchronous, and does not spin an additional event loop. This prevents a series of dangerous bugs from happening (e.g. deleting the dialog's parent while the dialog is open via exec()). When using open() you can connect to the finished() signal of QDialog to be notified when the dialog is closed.
Initially, my app was constructed this way:
QMainWindow A > QDialog B > QDialog C > ...
Each QDialog was launched using .exec() method.
The pseudo code could be summarized this way [edit] :
class A(QMainWindow):
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
self.ui = Some_Ui_From_QtDesigner()
self.ui.setupUi(self)
self.ui.some_menu.triggered.connect(self.load_popup)
def load_popup():
B = QDialog(A)
if B.exec() == 1:
# Do some things (extract data from B and update A)
if C.exec() == 1:
# Do some things (extract data from C and update B)
qApp = QtWidgets.QApplication.instance()
a = A()
a.show()
qApp.setQuitOnLastWindowClosed(True)
qApp.exec()
I've tried to use .open() methods instead of .exec() and came up with something like this (using QLoopEvent's and events handling):
def process_C_qdialog(parent):
C = QDialog(parent)
# Construct your QDialog some more ...
loop = QtCore.QEventLoop()
my_result = None
def _accept():
my_result = "blah"
loop.exit(0)
def _reject():
loop.exit(-1)
f.accepted.connect(_accept)
f.rejected.connect(_reject)
f.open()
loop.exec()
return my_result
My troubles are far from over as yet. I suspect that recreating manually a QEventLoop causes the same problems than using .exec() in the first place.
So some questions:
Am I right about the QEventLoop?
Would it be safer/enough to leave the parent out of the QDialog generator?
Any advice on how to handle this the right way (keeping a modal dialog if possible)?
More generally, are they any safe ways to reproduce the static methods of Qt through a custom python function? (ie, a function which would load a customized popup, wait for the user's input and returning it)
Note: as this behaviour happens erratically (and without the full traceback available), I'm not 100% sure of this; but I suspect that even a simple QMessageBox().exec() has the same effects, even if this is not stated in the doc.

Maya Python: Button always at the center of the window

I'm starting experimenting with Maya python, and I'm trying to do some UI.
I came across to a really strange problem, I can't get a button to stay in the center of the windows.
I've tried different things but nothing seems to work, here is the code:
import maya.cmds as cmds
cmds.window( width=200 )
WS = mc.workspaceControl("dockName", retain = False, floating = True,mw=80)
submit_widget = cmds.rowLayout(numberOfColumns=1, p=WS)
cmds.button( label='Submit Job',width=130,align='center', p=submit_widget)
cmds.showWindow()
this is a simple version but still, I can't get it to work.
can someone help me?

I honestly don't know the answer as anytime I have to dig into Maya's native UI stuff it makes me question my own life.
So I know it's not exactly what you're asking for, but I'll opt with this: Use PySide instead. At first glance it might make you go "woah, that's way too hard", but it's also a million times better (and actually easier). It's much more powerful, flexible, has great documentation, and also used outside of Maya (so actually useful to learn). Maya's own interface uses the same framework, so you can even edit it with PySide once you're more comfortable with it.
Here's a bare-bones example to create a centered button in a window:
# Import PySide libraries.
from PySide2 import QtCore
from PySide2 import QtWidgets
class MyWindow(QtWidgets.QWidget): # Create a class for our window, which inherits from `QWidget`
def __init__(self, parent=None): # The class's constructor.
super(MyWindow, self).__init__(parent) # Initialize its `QWidget` constructor method.
self.my_button = QtWidgets.QPushButton("My button!") # Create a button!
self.my_layout = QtWidgets.QVBoxLayout() # Create a vertical layout!
self.my_layout.setAlignment(QtCore.Qt.AlignCenter) # Center the horizontal alignment.
self.my_layout.addWidget(self.my_button) # Add the button to the layout.
self.setLayout(self.my_layout) # Make the window use this layout.
self.resize(300, 300) # Resize the window so it's not tiny.
my_window_instance = MyWindow() # Create an instance of our window class.
my_window_instance.show() # Show it!
Not too bad, right?

Trouble Understanding Signal Mapper PyQt

So I am generating a menu of options based on some files on my system. I have a list list of objects I need to dynamically generate an option in the menu for and need to be able to let the function that is doing the creation know which object to use. After some research I found the post below. I could not comment as my rep is not high yet: How to pass arguments to callback functions in PyQt
When I run this the signal mapper is not working right. It is not even calling the handleButton correctly. Any ideas as to how to use the signal mapper correctly?
from PyQt4 import QtGui, QtCore
class Window(QtGui.QMainWindow):
def __init__(self):
QtGui.QMainWindow.__init__(self)
self.mapper = QtCore.QSignalMapper(self)
self.toolbar = self.addToolBar('Foo')
self.toolbar.setToolButtonStyle(QtCore.Qt.ToolButtonTextOnly)
for text in 'One Two Three'.split():
action = QtGui.QAction(text, self)
self.mapper.setMapping(action, text)
action.triggered.connect(self.mapper.map)
self.toolbar.addAction(action)
self.mapper.mapped['QString'].connect(self.handleButton)
self.edit = QtGui.QLineEdit(self)
self.setCentralWidget(self.edit)
def handleButton(self, identifier):
print 'run'
if identifier == 'One':
text = 'Do This'
print 'Do One'
elif identifier == 'Two':
text = 'Do That'
print 'Do Two'
elif identifier == 'Three':
print 'Do Three'
text = 'Do Other'
self.edit.setText(text)
if __name__ == '__main__':
import sys
app = QtGui.QApplication(sys.argv)
window = Window()
window.resize(300, 60)
window.show()
sys.exit(app.exec_())
EDIT:
I've found that by using old-style signal/slot connections this is fixed:
#action.triggered.connect(self.mapper.map)
self.connect(action, QtCore.SIGNAL("triggered()"), self.mapper, QtCore.SLOT("map()"))
and
#self.mapper.mapped['QString'].connect(self.handleButton)
self.connect(self.mapper, QtCore.SIGNAL("mapped(const QString &)"), self.handleButton)
Am I using the new-style connections incorrectly?
Based on this post as well as the original link I posted, I thought I was doing things correctly.

The original example code (which I wrote), works perfectly fine for me using either Python2 or Python3 with several different recent versions of PyQt4. However, if I use a really old version of PyQt4 (4.7), the handler no longer gets called.
The reason (and solution) for this is given in the response to the mailing list post you linked to:
It's actually a problem with QSignalMapper.map() being called from a
proxy rather than new-style connections.
The workaround is to explicitly specify a signal that is compatible
with map()...
self.b1.clicked[()].connect(self.mapper.map)
Tonight's PyQt snapshot will be smarter about finding a usable Qt slot
before deciding that it needs to use a proxy so that the workaround
won't be necessary.
There are some signals (like clicked and triggered) which always send a default value unless you explicitly request otherwise. With the old-style signals, you can specify the no default overload it with SIGNAL("triggered()"), but with new-style signals, you have to do it like this:
action.triggered[()].connect(self.mapper.map)
But that is only necessary with very old versions of PyQt4 - the underlying issue was fixed back in 2010 (don't know the exact version, but 4.8 should be okay).

Is deleteLater() necessary in PyQt/PySide?

Since there is already a Garbage Collector in Python, is deleteLater() necessary in PyQt/PySide?

It depends what you mean by "necessary".
An application could potentially consume a lot of memory if (for example) care is not taken when closing widgets. The QObject-based classes are designed to be (optionally) linked together in a hierarchy. When a top-level object is deleted, Qt will automatically delete all its child objects as well. However, when closing widgets (which are sub-classes of QObject), automatic deletion will only happen if the Qt.WA_DeleteOnClose attribute is set (which, by default, it usually isn't).
To illustrate, try repeatedly opening and closing the dialog in this demo script, and watch how the global list of objects grows:
import sys
from PyQt5 import QtCore, QtWidgets
class Window(QtWidgets.QWidget):
def __init__(self):
super().__init__()
self.checkbox = QtWidgets.QCheckBox('Delete')
self.button = QtWidgets.QPushButton('Open', self)
self.button.clicked.connect(self.openDialog)
layout = QtWidgets.QHBoxLayout(self)
layout.addWidget(self.checkbox)
layout.addWidget(self.button)
def openDialog(self):
widget = QtWidgets.QDialog(self)
if (self.checkbox.isChecked() and
not widget.testAttribute(QtCore.Qt.WA_DeleteOnClose)):
widget.setAttribute(QtCore.Qt.WA_DeleteOnClose)
for child in self.findChildren(QtWidgets.QDialog):
if child is not widget:
child.deleteLater()
label = QtWidgets.QLabel(widget)
button = QtWidgets.QPushButton('Close', widget)
button.clicked.connect(widget.close)
layout = QtWidgets.QVBoxLayout(widget)
layout.addWidget(label)
layout.addWidget(button)
objects = self.findChildren(QtCore.QObject)
label.setText('Objects = %d' % len(objects))
print(objects)
widget.show()
if __name__ == '__main__':
app = QtWidgets.QApplication(sys.argv)
window = Window()
window.setGeometry(500, 300, 100, 50)
window.show()
sys.exit(app.exec_())
With PyQt/PySide, there are two aspects to object ownership: the Python part, and the Qt part. Often, removing the last Python reference to an object won't be enough to fully clean up, because there could still be a reference held on the Qt side.
In general, Qt tends not to implicity delete objects. So if your application creates and removes lots of QObjects (or opens and closes lots of QWidgets), you may need to take steps to delete them explicitly if memory usage is a concern.
UPDATE:
Just to add to the points above on object ownership. Sometimes, it is possible to hold a Python reference to an object, whilst the Qt part gets deleted. When this happens, you will see an error like this:
RuntimeError: underlying C/C++ object has been deleted
Usually, the Qt documentation will give some hints about when this might happen. For instance, QAbstractItemView.setModel gives this warning:
The view does not take ownership of the model unless it is the model's parent object...
This is telling you that you must either keep a Python reference to the object, or pass a suitable parent object to the object's constructor, because Qt will not always automatically reparent it.

One application of deleteLater can be cleaning up of yourself, i.e. scheduling a delete of an QObject (for example in threading) to free resources from within the object itself.
Here for example someone is using it in connection with the signal thread.finished. It might be restricted to cases with heavy signalling though.

Error in model view implemention of GUI in pyqt

the following sample code is crashing with this error when I close the application:
QBasicTimer::start: QBasicTimer can only be used with threads started with QThread
here is my code :
import sys
from PyQt4 import QtGui ,QtCore
app = QtGui.QApplication(sys.argv)
data=[]
data.append("one")
model=QtGui.QStringListModel(data)
combobox=QtGui.QComboBox()
combobox.show()
combobox.setModel(model)
sys.exit(app.exec_())
I found out that's about using model but I don't know how to fix it.
edited:
os : win 7 64bit
pyqt4

The program is not "crashing": it is merely printing an error message during the normal shutdown process.
The reason the message is being shown is a side-effect of garbage-collection. When python shuts down, the order in which objects get deleted can be unpredictable. This may result in objects on the C++ side being deleted in the "wrong" order, and so Qt will sometimes complain when this happens.
One way to "fix" the sample code would be to simply rename some of the PyQt objects. If I change the name combobox to combo, for instance, the error message goes away. There's nothing mysterious about this - it just changes the order in which the objects are deleted.
But another, much more robust, way to fix the problem would be to make sure the QStringListModel has a parent, since it's possible that Qt doesn't take ownership of it when it's passed to the combo-box. Qt should always handle the deletion of child objects correctly when they're linked together in this way. So the code example would become:
import sys
from PyQt4 import QtGui, QtCore
app = QtGui.QApplication(sys.argv)
combobox = QtGui.QComboBox()
data = []
data.append("one")
model = QtGui.QStringListModel(data, combobox)
combobox.setModel(model)
combobox.show()
sys.exit(app.exec_())