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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.

Getting, Storing, Setting and Modifying Transform Attributes through PyMel

I'm working on something that gets and stores the transforms of an object moved by the user and then allows the user to click a button to return to the values set by the user.
So far, I have figured out how to get the attribute, and set it. However, I can only get and set once. Is there a way to do this multiple times within the script running once? Or do I have to keep rerunning the script? This is a vital question for me get crystal clear.
basically:
btn1 = button(label="Get x Shape", parent = layout, command ='GetPressed()')
btn2 = button(label="Set x Shape", parent = layout, command ='SetPressed()')
def GetPressed():
print gx #to see value
gx = PyNode( 'object').tx.get() #to get the attr
def SetPressed():
PyNode('object').tx.set(gx) #set the attr???
I'm not 100% on how to do this correctly, or if I'm going the right way?
Thanks

You aren't passing the variable gx so SetPressed() will fail if you run it as written(it might work sporadically if you tried executing the gx = ... line directly in the listener before running the whole thing -- but it's going to be erratic). You'll need to provide a value in your SetPressed() function so the set operation has something to work with.
As an aside, using string names to invoke your button functions isn't a good way to go -- you code will work when executed from the listener but will not work if bundled into a function: when you use a string name for the functions Maya will only find them if they live the the global namespace -- that's where your listener commands go but it's hard to reach from other functions.
Here's a minimal example of how to do this by keeping all of the functions and variables inside another function:
import maya.cmds as cmds
import pymel.core as pm
def example_window():
# make the UI
with pm.window(title = 'example') as w:
with pm.rowLayout(nc =3 ) as cs:
field = pm.floatFieldGrp(label = 'value', nf=3)
get_button = pm.button('get')
set_button = pm.button('set')
# define these after the UI is made, so they inherit the names
# of the UI elements
def get_command(_):
sel = pm.ls(sl=True)
if not sel:
cmds.warning("nothing selected")
return
value = sel[0].t.get() + [0]
pm.floatFieldGrp(field, e=True, v1= value[0], v2 = value[1], v3 = value[2])
def set_command(_):
sel = pm.ls(sl=True)
if not sel:
cmds.warning("nothing selected")
return
value = pm.floatFieldGrp(field, q=True, v=True)
sel[0].t.set(value[:3])
# edit the existing UI to attech the commands. They'll remember the UI pieces they
# are connected to
pm.button(get_button, e=True, command = get_command)
pm.button(set_button, e=True, command = set_command)
w.show()
#open the window
example_window()
In general, it's this kind of thing that is the trickiest bit in doing Maya GUI -- you need to make sure that all the functions and handlers etc see each other and can share information. In this example the function shares the info by defining the handlers after the UI exists, so they can inherit the names of the UI pieces and know what to work on. There are other ways to do this (Classes are the most sophisticated and complex) but this is the minimalist way to do it. There's a deeper dive on how to do this here

pyqt: A correct way to connect multiple signals to the same function in pyqt (QSignalMapper not applicable)

I've ready many posts on how to connect multiple signals to the same event handler in python and pyqt. For example, connecting several buttons or comboboxes to the same function.
Many examples show how to do this with QSignalMapper, but it is not applicable when the signal carries a parameter, as with combobox.currentIndexChanged
Many people suggest it can be made with lambda. It is a clean and pretty solution, I agree, but nobody mentions that lambda creates a closure, which holds a reference - thus the referenced object can not be deleted. Hello memory leak!
Proof:
from PyQt4 import QtGui, QtCore
class Widget(QtGui.QWidget):
def __init__(self):
super(Widget, self).__init__()
# create and set the layout
lay_main = QtGui.QHBoxLayout()
self.setLayout(lay_main)
# create two comboboxes and connect them to a single handler with lambda
combobox = QtGui.QComboBox()
combobox.addItems('Nol Adyn Dwa Tri'.split())
combobox.currentIndexChanged.connect(lambda ind: self.on_selected('1', ind))
lay_main.addWidget(combobox)
combobox = QtGui.QComboBox()
combobox.addItems('Nol Adyn Dwa Tri'.split())
combobox.currentIndexChanged.connect(lambda ind: self.on_selected('2', ind))
lay_main.addWidget(combobox)
# let the handler show which combobox was selected with which value
def on_selected(self, cb, index):
print '! combobox ', cb, ' index ', index
def __del__(self):
print 'deleted'
if __name__ == '__main__':
import sys
app = QtGui.QApplication(sys.argv)
wdg = Widget()
wdg.show()
wdg = None
sys.exit(app.exec_())
The widget is NOT deleted though we clear the reference. Remove the connection to lambda - it gets deleted properly.
So, the question is: which is the proper way to connect several signals with parameters to a single handler without leaking memory?

It is simply untrue that an object cannot be deleted because a signal connection holds a reference in a closure. Qt will automatically remove all signal connections when it deletes an object, which will in turn remove the reference to the lambda on the python side.
But this implies that you cannot always rely on Python alone to delete objects. There are two parts to every PyQt object: the Qt C++ part, and the Python wrapper part. Both parts must be deleted - and sometimes in a specific order (depending on whether Qt or Python currently has ownership of the object). In addition to that, there's also the vagaries of the Python garbage-collector to factor in (especially during the short period when the interpreter is shutting down).
Anyway, in your specific example, the easy fix is to simply do:
# wdg = None
wdg.deleteLater()
This schedules the object for deletion, so a running event-loop is required for it have any effect. In your example, this will also automatically quit the application (because the object is the last window closed).
To more clearly see what's happening, you can also try this:
#wdg = None
wdg.deleteLater()
app.exec_()
# Python part is still alive here...
print(wdg)
# but the Qt part has already gone
print(wdg.objectName())
Output:
<__main__.Widget object at 0x7fa953688510>
Traceback (most recent call last):
File "test.py", line 45, in <module>
print(wdg.objectName())
RuntimeError: wrapped C/C++ object of type Widget has been deleted
deleted
EDIT:
Here's another debugging example that hopefully makes it even clearer:
wdg = Widget()
wdg.show()
wdg.deleteLater()
print 'wdg.deleteLater called'
del wdg
print 'del widget executed'
wd2 = Widget()
wd2.show()
print 'starting event-loop'
app.exec_()
Output:
$ python2 test.py
wdg.deleteLater called
del widget executed
starting event-loop
deleted

in many cases the parameter carried by signal can be catched in another way, e.g. if an objectName is set for the sending object, so QSignalMapper can be used:
self.signalMapper = QtCore.QSignalMapper(self)
self.signalMapper.mapped[str].connect(myFunction)
self.combo.currentIndexChanged.connect(self.signalMapper.map)
self.signalMapper.setMapping(self.combo, self.combo.objectName())
def myFunction(self, identifier):
combo = self.findChild(QtGui.QComboBox,identifier)
index = combo.currentIndex()
text = combo.currentText()
data = combo.currentData()

Lock a choice in PyQt4 QComboBox using a QCheckBox

I am beginning to write a GUI using PyQt4. This is my first experience with GUIs (and also oo-programming is somewhat new to me). Part of that GUI will be like 4 to 5 instances of QComboBox. As many choices are to be made, I want the user to be able to lock a choice, such that is not being changed unintenionally later. For one QComboBox I can solve the problem with this code that I wrote:
import sys
from PyQt4 import QtGui, QtCore
class MyGui(QtGui.QWidget):
def __init__(self):
QtGui.QMainWindow.__init__(self)
self.resize(250, 50)
# vertical layout for widgets
self.vbox = QtGui.QVBoxLayout()
self.setLayout(self.vbox)
# Create a combo box with some choices
self.combo_color = QtGui.QComboBox()
self.vbox.addWidget(self.combo_color)
items = 'Red Yellow Purple'.split()
self.combo_color.addItems(items)
self.connect(self.combo_color, QtCore.SIGNAL('activated(QString)'), self.use_choice)
# add a checkbox next to the combobox which (un-)locks the the combo-choice
self.checkbox_color = QtGui.QCheckBox('Lock Choice', self)
self.vbox.addWidget(self.checkbox_color)
self.connect(self.checkbox_color, QtCore.SIGNAL('stateChanged(int)'), self.lock_choice)
def use_choice(self, text):
# do something very useful with the choice
print 'The current choice is: {choice}'.format(choice=text)
def lock_choice(self):
if self.checkbox_color.isChecked():
self.combo_color.setEnabled(False)
print 'Choice {choice} locked'.format(choice=self.combo_color.currentText())
else:
self.combo_color.setEnabled(True)
print 'Choice unlocked'
if __name__ == "__main__":
app = QtGui.QApplication(sys.argv)
mygui = MyGui()
mygui.show()
app.exec_()
This code does what it should, but I am very unhappy with its design, because the method lock_choice is hard-coded to only lock the choice of the QComboBox combo_color. What if I now want to do the same thing for another QComboBox (say combo_name) and a second QCheckBox (say checkbox_name) which could be realized by appending the following code to the classes __init__(self) code block:
# create second combo box with some other choices
self.combo_name = QtGui.QComboBox()
self.vbox.addWidget(self.combo_name)
items = 'Bob Peter'.split()
self.combo_name.addItems(items)
self.connect(self.combo_name, QtCore.SIGNAL('activated(QString)'), self.use_choice)
# add a checkbox next to the combobox which (un-)locks the the combo-choice
self.checkbox_name = QtGui.QCheckBox('Lock Choice', self)
self.vbox.addWidget(self.checkbox_name)
self.connect(self.checkbox_name, QtCore.SIGNAL('stateChanged(int)'), self.lock_choice) # <-- obviously wrong, as it (un-)locks color choice at the moment
Both QComboBoxes can share the method use_choice() right now, but they cannot share the method lock_choice(), as both checkboxes lock the color-choice. I want the checkbox checkbox_name to lock the name-choice, without copy and pasting the current lock_choice()-method and switching the hardcoded combobox.
I am sure there is an easy way, like passing the target-combobox to the method, which i just don't know yet. Help would be appreciated!

Try partial functions-
from functools import partial
with the connect signal, pass the QWidget name:
self.connect(self.checkbox_color, QtCore.SIGNAL('stateChanged(int)'), partial(self.lock_choice, self.combo_color))
And in your method, you may add an argument.
We'll add an argument in the method like below, to handle the QWidget(QComboBox) that we'll pass through the partial function above.
def lock_choice(self, combos):
if combos.isEnabled(True):
combos.setEnabled(False)
print 'Choice {choice} locked'.format(choice=combos.currentText())

The simplest solution would be to use the toggled signal with the setDisabled slot:
self.checkbox_color.toggled.connect(self.combo_color.setDisabled)
(And note how much cleaner the new-style sytax is when making signal connections).
It's also worth pointing out that you can also use a lambda for making inline signal connections, like this:
self.checkbox_color.toggled.connect(
lambda checked: self.combo_color.setDisabled(checked))
This is probably the most idiomatic solution when there are no convenient signal/slot pairings (and of course partial functions can achieve more or less the same thing in different way).

PyQt: updating GUI from a callback

Using Python3 and PyQt4 I have a function (run) that takes as an input a callable to provide status updates.
class Windows(QtGui.QWidget):
# Creates a widget containing:
# - a QLineEdit (status_widget)
# - a button, connected to on_run_clicked
def on_run_clicked(self):
def update(text):
self.widget.setText(text)
threading.Thread(target=run, args=(update, )).start()
This works ok (i.e. the text updates are displayed properly in the widget). However, when I replace QLineEdit by QTextEdit and use the append method to add text, I get:
QObject::connect: Cannot queue arguments of type 'QTextCursor'
(Make sure 'QTextCursor' is registered using qRegisterMetaType().)
It still works but point to the fact that I am doing something wrong, and I am not sure that I will keep working when more threads are active. Usually, I do this type of updates using signals and slots, but the run function is not PyQt specific. The questions are:
Why does it work without a warning for QLineEdit and not for
QTextEdit?
What is the right way to deal with a situation like this?

I don't know the specific reason why one class works and the other doesn't - nor do I really know the difference between using Python threading vs. Qt's threading...however, I can tell you that it is very tempremental if you don't set it up properly. Namely, you cannot (or at the very least, should not) modify GUI objects from a thread. Again, not sure the difference of a python vs. a Qt thread on that. But, the safe way to modify your interface from a GUI is by sending signals to your window...easiest way I know to do this is via the Qt threading.
class MyThread(QtCore.QThread):
updated = QtCore.pyqtSignal(str)
def run( self ):
# do some functionality
for i in range(10000):
self.updated.emit(str(i))
class Windows(QtGui.QWidget):
def __init__( self, parent = None ):
super(Windows, self).__init__(parent)
self._thread = MyThread(self)
self._thread.updated.connect(self.updateText)
# create a line edit and a button
self._button.clicked.connect(self._thread.start)
def updateText( self, text ):
self.widget.setText(text)