Dfyjkt

I have encountered a theoretical question. I'm using pyqt5, but this is probably are very generalistic and framework independent question. I have a QMainwindow sitting around waiting for the user to do stuff. The user can show / hide dialogues (subclasses of QDockwidgets) as he chooses using the QMenu and the associated shortcuts (it's a checkable QAction for each individual dialogue). I have been struggling with showing / hiding the dialogues efficiently. Currently, I'm just initiating them all at start up, hiding those that I don't want to show up in the beginning. This makes triggering the dialogues easy, since I can just dialogue.show() /dialogue.hide() depending on the dialogues current visibility.
But I cannot believe that this is best practice and very efficient.

I have tried (I currently do not have my pyqt environment set up on this computer, so I had to strip down my actual code without being able to test if this runs):

from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *


class InfoPanel(QDockWidget):
 def __init__(self, title='Tool Box'):
 QDockWidget.__init__(self, title)
 self.setFeatures(QDockWidget.DockWidgetFloatable | QDockWidget.DockWidgetMovable | QDockWidget.DockWidgetClosable)
 self.setAllowedAreas(Qt.LeftDockWidgetArea | Qt.RightDockWidgetArea)

 frame = QFrame()
 layout = QGridLayout()
 self.canvas = QGraphicsView()
 self.canvas.setBackgroundBrush(QtGui.QBrush(QtGui.QColor(40, 40, 40)))
 layout.addWidget(self.canvas)
 frame.setLayout(layout)
 self.setWidget(frame)


class MainWindow(QMainWindow):
 def __init__(self):
 super().__init__()
 self.showpanelAct = QAction("&Show Panel", self, enabled=True,checkable=True, shortcut="F10")
 self.showpanelAct.triggered.connect(lambda: self.showPanel(0))
 self.viewMenu = QMenu("&View", self)
 self.viewMenu.addAction(self.showpanelAct)
 self.setDockOptions(QMainWindow.AnimatedDocks)

 def showPanel(self,i:int = 0): # this is not so smart - should construct and deconstuct to save memory!?
 if i == 0: #infopanel
 dialogueExists = True
 try: self.infoPanel
 #except NameError: #does not catch the error
 except:
 dialogueExists = False
 if dialogueExists:
 print('destroy')
 self.infoPanel.destroy()
 else:
 print('create')
 self.infoPanel = InfoPanel() #init
 self.infoPanel.show()

if __name__ == '__main__':
 import sys
 app = QApplication(sys.argv)
 window = MainWindow()
 window.show()
 app.exec_()

Which works the first time, but after that, it only seems to trigger the destruction of the dialogue (which, surprisingly, does not crash anything it just keeps on going).
Why is that and is there a standard way to approach the showing hiding of dialogues?

Cheers, Sacha

I took the exposed MCVE of OP and tried to make it running in my cygwin64 on Windows 10.

At first I had to apply little fixes. (OP stated that he was not able to test it at the time of publishing.)

First, I inserted a “hut” at first line for convenient start in bash:

#!/usr/bin/python3

Second, the self.viewMenu didn't appear. Hence, I inserted a line after

 self.viewMenu = QMenu("&View", self)
 self.viewMenu.addAction(self.showpanelAct)

to add the viewMenu to main menu bar:

 self.menuBar().addMenu(self.viewMenu)

which fixed it.

Third, when clicking the menu item I got:

Traceback (most recent call last):
 File "./testQDockPanelShowHide.py", line 27, in <lambda>
 self.showpanelAct.triggered.connect(lambda: self.showPanel(0))
 File "./testQDockPanelShowHide.py", line 45, in showPanel
 self.infoPanel = InfoPanel() #init
 File "./testQDockPanelShowHide.py", line 17, in __init__
 self.canvas.setBackgroundBrush(QtGui.QBrush(QtGui.QColor(40, 40, 40)))
NameError: name 'QtGui' is not defined
Aborted (core dumped)

I must admit that my Python knowledge is very limited. (I'm the guy who writes the Python bindings in C++ for the colleagues. So, my colleagues are the actual experts. At most, I play a little bit in Python when I test whether new implemented bindings do what's expected.) However, I modified

 self.canvas.setBackgroundBrush(QtGui.QBrush(QtGui.QColor(40, 40, 40)))

to:

 self.canvas.setBackgroundBrush(QBrush(QColor(40, 40, 40)))

which fixed this issue.

After this, I got the behavior described by OP and did a closer look where I (and OP) suspected the error:

 def showPanel(self,i:int = 0): # this is not so smart - should construct and deconstuct to save memory!?
 if i == 0: #infopanel
 dialogueExists = True
 try: self.infoPanel
 #except NameError: #does not catch the error
 except:
 dialogueExists = False
 if dialogueExists:
 print('destroy')
 self.infoPanel.destroy()
 else:
 print('create')
 self.infoPanel = InfoPanel() #init
 self.infoPanel.show()

I strongly believe that try: self.infoPanel doesn't do what OP thinks it would.

It tries to access self.infoPanel which isn't existing until the first call of this method. (Please, be aware, the member variable self.infoPanel isn't existing.) So, the except: branch is executed and sets dialogueExists = False which a few lines later causes self.infoPanel = InfoPanel() #init. Now, the member variable self.infoPanel is existing, and the try: self.infoPanel will never fail again until destruction of this MainWindow.

Out of curiosity, I had a look at QWidget.destroy() (to be sure not to tell something wrong):

QWidget.destroy (self, bool destroyWindow = True, bool destroySubWindows = True)

Frees up window system resources. Destroys the widget window if destroyWindow is true.

destroy() calls itself recursively for all the child widgets, passing destroySubWindows for the destroyWindow parameter. To have more control over destruction of subwidgets, destroy subwidgets selectively first.

This function is usually called from the QWidget destructor.

It definitely doesn't destroy the member variable self.infoPanel.

After having understood this, a fix was easy and obvious:

 def showPanel(self,i:int = 0): # this is not so smart - should construct and deconstuct to save memory!?
 if i == 0: #infopanel
 try: self.infoPanel
 #except NameError: #does not catch the error
 except:
 print('create')
 self.infoPanel = InfoPanel() #init
 if self.infoPanel.isVisible():
 self.infoPanel.hide()
 else:
 self.infoPanel.show()

Btw. I replaced destroy() by hide() which makes a re-creation of the InfoPanel() obsolete.

I tested this by toggling the menu item multiple times – it works as expected now (at least, it looks like).

The complete sample finally:

#!/usr/bin/python3

from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *


class InfoPanel(QDockWidget):
 def __init__(self, title='Tool Box'):
 QDockWidget.__init__(self, title)
 self.setFeatures(QDockWidget.DockWidgetFloatable | QDockWidget.DockWidgetMovable | QDockWidget.DockWidgetClosable)
 self.setAllowedAreas(Qt.LeftDockWidgetArea | Qt.RightDockWidgetArea)

 frame = QFrame()
 layout = QGridLayout()
 self.canvas = QGraphicsView()
# self.canvas.setBackgroundBrush(QtGui.QBrush(QtGui.QColor(40, 40, 40)))
 self.canvas.setBackgroundBrush(QBrush(QColor(40, 40, 40)))
 layout.addWidget(self.canvas)
 frame.setLayout(layout)
 self.setWidget(frame)


class MainWindow(QMainWindow):
 def __init__(self):
 super().__init__()
 self.showpanelAct = QAction("&Show Panel", self, enabled=True,checkable=True, shortcut="F10")
 self.showpanelAct.triggered.connect(lambda: self.showPanel(0))
 self.viewMenu = QMenu("&View", self)
 self.viewMenu.addAction(self.showpanelAct)
 self.menuBar().addMenu(self.viewMenu)
 self.setDockOptions(QMainWindow.AnimatedDocks)

 def showPanel(self,i:int = 0): # this is not so smart - should construct and deconstuct to save memory!?
 if i == 0: #infopanel
 try: self.infoPanel
 #except NameError: #does not catch the error
 except:
 print('create')
 self.infoPanel = InfoPanel() #init
 if self.infoPanel.isVisible():
 self.infoPanel.hide()
 else:
 self.infoPanel.show()

if __name__ == '__main__':
 import sys
 app = QApplication(sys.argv)
 window = MainWindow()
 window.show()
 app.exec_()

After taking a break from coding the solution to my problem was very obvious. Going back to my original code (which did not produce the expected output of creating / destroying the dialogue self.infoPanel on demand):

dialogueExists = True
try: self.infoPanel
#except NameError: #does not catch the error
except:
 dialogueExists = False
if dialogueExists:
 print('destroy')
 self.infoPanel.destroy()
else:
 print('create')
 self.infoPanel = InfoPanel() #init
 self.infoPanel.show()

My main problem was that I confused two separate things. Qt destroyed the widget contained in the object self.infoPanel when I called self.infoPanel.destroy(). But that doesn't mean the object self.infoPanel does not exist (that's exactly what I use try: ... for, to see if the object exists). The simple and obvious way to create and destroy dialogues on demand obviously involves deleting the object from the environment (del self.infoPanel).
The working code is:

dialogueExists = True
try:
 self.infoPanel.destroy() #not sure this is needed, but I guess it doesn't hurt
 del self.infoPanel #this is the deletion of the actual object
except:
 dialogueExists = False
if not dialogueExists : 
 self.infoPanel = InfoPanel()

Cheers and many thanks for the helpful advice on deciding whether to show / hide dialogues or to create / destroy them!