Qt Creator and Pyside2 - Blank Window

JonB

@Marcus-Adamski
PySide 2 QUiLoader().load() returns a widget you need to show().
https://stackoverflow.com/questions/50128293/pyside2-quiloader-returns-an-empty-window

Marcus Adamski

Thanks very much - I'll try tomorrow morning. The code I supplied was directly generated by Qt Creator ... strange it doesn't work?

I'll reply back once I've tested your suggestion.

JonB

@Marcus-Adamski
It's only a suggestion :) If it was generated by Creator I agree it would be odd. Compare your generated code to what they are saying in that post. Now that I look at it, maybe you/it are doing the show(), have a play with your loader.load(ui_file, self) line.

Marcus Adamski

OK - this is the currently situation. Using the latest QT Creator and just PySide2 installed,

when creating a new project, and selecting "Qt for Python - Window (UI) file and "QMainWindow" as the base class, the following code is autogenerated (I've not modified it):

# This Python file uses the following encoding: utf-8
import sys
import os


from PySide2.QtWidgets import QApplication, QMainWindow
from PySide2.QtCore import QFile
from PySide2.QtUiTools import QUiLoader


class main_window(QMainWindow):
    def __init__(self):
        super(main_window, self).__init__()
        self.load_ui()

    def load_ui(self):
        loader = QUiLoader()
        path = os.path.join(os.path.dirname(__file__), "form.ui")
        ui_file = QFile(path)
        ui_file.open(QFile.ReadOnly)
        loader.load(ui_file, self)
        ui_file.close()

if __name__ == "__main__":
    app = QApplication([])
    widget = main_window()
    widget.show()
    sys.exit(app.exec_())

When run, I only get a blank window. To me, it appears the UI file is not being loaded and associated to QMainWindow. Experimenting, in the designer, I've changed the window title and added one button, but when run, neither change is shown on the resulting blank window.

However, instead, if I select "QWidget" as my base class, apart from the imports, the same code is geneated, but this works. (Again this code has not be altered):

# This Python file uses the following encoding: utf-8
import sys
import os


from PySide2.QtWidgets import QApplication, QWidget
from PySide2.QtCore import QFile
from PySide2.QtUiTools import QUiLoader


class main_window(QWidget):
    def __init__(self):
        super(main_window, self).__init__()
        self.load_ui()

    def load_ui(self):
        loader = QUiLoader()
        path = os.path.join(os.path.dirname(__file__), "form.ui")
        ui_file = QFile(path)
        ui_file.open(QFile.ReadOnly)
        loader.load(ui_file, self)
        ui_file.close()

if __name__ == "__main__":
    app = QApplication([])
    widget = main_window()
    widget.show()
    sys.exit(app.exec_())

Could someone please explain why the QMainWindow based autogenerated code doesn't work, but the QWidget code does?

Many thanks

SGaist

Hi,

Can you share the UI code as well ?

Marcus Adamski

Sure, the following is the UI for QWidget.

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>main_window</class>
 <widget class="QWidget" name="main_window">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>800</width>
    <height>600</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>main_window</string>
  </property>
  <widget class="QPushButton" name="pushButton">
   <property name="geometry">
    <rect>
     <x>40</x>
     <y>40</y>
     <width>75</width>
     <height>23</height>
    </rect>
   </property>
   <property name="text">
    <string>PushButton</string>
   </property>
  </widget>
 </widget>
 <resources/>
 <connections/>
</ui>

... and for the QMainApplication:

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>main_window</class>
 <widget class="QMainWindow" name="main_window">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>800</width>
    <height>600</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>main_window</string>
  </property>
  <widget class="QWidget" name="centralwidget">
   <widget class="QPushButton" name="pushButton">
    <property name="geometry">
     <rect>
      <x>70</x>
      <y>80</y>
      <width>75</width>
      <height>23</height>
     </rect>
    </property>
    <property name="text">
     <string>PushButton</string>
    </property>
   </widget>
  </widget>
  <widget class="QMenuBar" name="menubar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>800</width>
     <height>21</height>
    </rect>
   </property>
  </widget>
  <widget class="QStatusBar" name="statusbar"/>
 </widget>
 <resources/>
 <connections/>
</ui>

On both projects, in the above UIs the only change is the addition of a single pushbutton to identify the window

Marcus Adamski

Actually, for QMainWindow, I don't think PySide2 allows the UI file to be loaded from within the constructor.

Marcus Adamski

Thanks @Denni-0 for the valuable input. It's really helpful to have a solid template from which to explore the rest of Qt. I planned to use Qt Designer to get the feel of how the Qt framework behaves, then start coding the UI by hand. Right - next step, become more familiar with the Qt layouts

Thanks again

Marcus Adamski

Just ran your code - initially got the error "'MainWindow' object has no attribute 'SetStatusBar'".
However, after commenting out the following lines:

     #self.StatBar = self.statusBar()
     #self.SetStatusBar('Ready')

and running the code, unfortunately a single window is displayed with the title "Da Pusher" .... but there's no button, just a blank window.

Have to pop out now - will take another look at the code this evening.

Just out of interested, why does QWidget check:

if self.Parent == None:

is this so the QWidget can also run as an individual window if needed?

Thanks

JonB

@Marcus-Adamski
In the code you have been given there are a couple of mistakes.

Apart from the self.SetStatusBar you noted, the problem you see stems from:

        VBox = QVBoxLayout()
        HBox.addStretch(1)
        HBox.addLayout(HBox)
        HBox.addStretch(1)

This gives a warning "QLayout: Cannot add layout QHBoxLayout/ to itself", and under Linux dumps core on exit from the app.

If you correct this to:

        VBox = QVBoxLayout()
        VBox.addStretch(1)
        VBox.addLayout(HBox)
        VBox.addStretch(1)

the problems go away, and you will see the push button.

JonB

@Marcus-Adamski
Now going all the way back to your original Designer UI files, and why your code works with a QWidget but not with a QMainWindow.

In your QMainWindow .ui file you have:

  <widget class="QWidget" name="centralwidget">
   <widget class="QPushButton" name="pushButton">

This is not right, and Designer will be showing you a warning "no-entry" marker on centralwidget in the pane where it shows your widget hierarchy. You ignore this at your peril! Layout will "go wrong" if you make a QWidget a direct child on another (non-container) QWidget such as QMainWindow.centralWidget without you putting in a layout. The same would happen if you did this in your own code, though you wouldn't get any warning.

Go back into Designer and give your central widget a layout: in the widget hierarchy pane, right-click on main_window, pick Lay out, pick Lay Out Horizontally.

Now, to make it work in the QMainWindow case: I originally told you:

@Marcus-Adamski
PySide 2 QUiLoader().load() returns a widget you need to show().
https://stackoverflow.com/questions/50128293/pyside2-quiloader-returns-an-empty-window

but you did not act on this. You need to change two lines:

loader.load(ui_file, self).show()
# widget.show()

So, I have made the return result from loader.load(ui_file, self) be the widget to call show() on, and got rid of your show() on the object constructed via main_window(). Once you see that works you can tidy the code to do this in a better way.

You should get the push button visible as you do in the plain QWidget-non-QMainWindow case. You can then continue to use the Designer for your application if you wish to, as many do.

Marcus Adamski

Thanks to both @JonB and @Denni-0 , I now have a solid template (and better understanding) to explore QT further. Much appreciated.

Erriez

I had the same problem with PySide6 on Ubuntu 22.04 and Windows 10: Creating a new project with QMainWindow does not work. See my question on StackOverflow.

Complete fixed example:

# This Python file uses the following encoding: utf-8
import os
from pathlib import Path
import sys

from PySide6.QtWidgets import QApplication, QMainWindow
from PySide6.QtCore import QFile
from PySide6.QtUiTools import QUiLoader


class MainWindow(QMainWindow):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.load_ui()

    def load_ui(self):
        loader = QUiLoader()
        path = Path(__file__).resolve().parent / "form.ui"
        ui_file = QFile(path)
        ui_file.open(QFile.ReadOnly)

-        loader.load(ui_file, self)
+        widget = loader.load(ui_file, self)

        ui_file.close()

+        widget.show()


if __name__ == "__main__":
    app = QApplication(sys.argv)
    widget = MainWindow()
-    # widget.show()
    sys.exit(app.exec())

As beginner I spent a lot of time to figure out why. Can someone please make the generated code functional in Qt Creator when creating a new project?

JonB

@Erriez said in Qt Creator and Pyside2 - Blank Window:

Can you please fix this issue when creating a new project?

Hi. What does this mean? If you mean can somebody here fix what Python code Designer generates you would have to report an issue to Qt developers, we are just a forum of users like yourself!

Erriez

@JonB Yes, my question is for the Qt developers to fix the generated code in Qt Creator. I had a bad start when creating a new project which does not work out of the box.
You're right, https://bugreports.qt.io is used to report bugs and not this forum. I'll follow-up here. Thanks!