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QtCreator's UI is not applied to the window, it's always empty (Qt for Python)

I have a fresh QtCreator installation, and I set it up to run using a fresh install of Python3.8 on which I pip-installed both pyside2 and pyside6.
When I create a new Qt for Python - Window (UI file) application, whatever I do to the UI file the window always shows up empty and with the default size when I run the app.
I've tried with a QDialog, QMainApplication, using Pyside2 or Pyside6, I've checked that it was correctly loading the UI (and the right one) - no dice. It just won't update, and appears not to have any reason not to.
Default code for completeness:
# This Python file uses the following encoding: utf-8
import os
from pathlib import Path
import sys
from PySide2.QtWidgets import QApplication, QDialog
from PySide2.QtCore import QFile
from PySide2.QtUiTools import QUiLoader
class Dialog(QDialog):
def __init__(self):
super(Dialog, self).__init__()
self.load_ui()
def load_ui(self):
loader = QUiLoader()
path = os.fspath(Path(__file__).resolve().parent / "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 = Dialog()
widget.show()
sys.exit(app.exec_())
(In the UI I just drag-dropped a button right in the middle and saved the file)
Am I forgetting something fundamental? I'm only used to programming in C++ using QtCreator.

I was expecting this to work right out of the box, but it's not.
This only dynamically load the UI as a new widget, with this custom class as a parent.
If I want signals and slots to work, the only thing I've found was to add a custom build step:
Command: <path to pyside6 install, use pip show to know where>\uic.exe
Arguments: <filename of the .ui file> -o <the python translation .py
which will serve as baseclass> -g python
Working directory: [your project dir]
Then signals and slots need to be connected manually, so QtCreator really only enables you to draw the user interface but all the logic still needs to be done by hand. Component variables are normally named after their UI name, but you can see for yourself in the baseclass file. This is a big step back from how QtCreator is used in C++ ("go to slot" will not work).
Code to use:
import sys
from PySide6.QtWidgets import QApplication, QMainWindow
from PySide6.QtCore import QFile
from PySide6.QtUiTools import QUiLoader
from YourGenPyFileName import Ui_YourUIName
class MainWindow(QMainWindow, Ui_YourUIName):
def __init__(self):
super(MainWindow, self).__init__()
self.setupUi(self)
if __name__ == "__main__":
app = QApplication([])
widget = MainWindow()
widget.show()
sys.exit(app.exec_())

converting ui file into py using pyside6

I just try pyside6 to convert ui files into py files.
When I was using pyside2, I was writing this commande line to convert file:
pyside2-uic MainWindow.ui -o ui_mainwindow.py -x
But with pyside6, it is not working anymore: the "-x" doesn't look necessary. So you have to write:
pyside6-uic MainWindow.ui -o ui_mainwindow.py
BUT, when I run the new file generated, nothing happend. I had a look at the end of file, and it is missing a part there was with pyside2. This part of code is not here anymore:
if __name__ == "__main__":
import sys
app = QtWidgets.QApplication(sys.argv)
w = MainWindow()
w.show()
sys.exit(app.exec_())
What I am doing wrong ?

You're not doing anything wrong, that part of the code is added to create an "executable" version of the pyuic file, and its purpose is mainly to test it.
I don't know if the support for -x has been dropped, but that makes sense, and it won't matter a lot anyway: pyuic generated files are not intended to be executed, and should not ever be manually modified, since they should only be used as imports as explained in the official guidelines about using Designer.

The way I do it is import the class the uic created:
from ui_mainwindow import Ui_MainWindow
Then I create a QMainWindow class and instantiate the uic class.
You need to call the setupUi method on the instance.
class UI(QtWidgets.QMainWindow):
def __init__(self):
super().__init__()
self.ui = Ui_MainWindow()
self.ui.setupUi(self)
...
if __name__ == "__main__":
app = QtWidgets.QApplication(sys.argv)
win = UI()
win.show()
app.exec()
Now, you can access all the widgets via self.ui:
self.ui.myPushButton.clicked.connect(self.doSomething)

Best way to handle multiple windows Qt4 python

I have got two main-windows which will run all the time and open other windows. Every window has to communicate with each other. All windows are created with qt designer and as I read in another question I should not modify these files, so I tried to make a new class (communicator) to handle all the jobs.
What is the best way to structure a project like this. So how can i communicate best between these windows now, e.g. if I press a button in 'self.mainWindows_images' I want to do something in 'self.mainWindows_classifier'
from classifier import Form as Form_classifier
from cnn_avg import Form as Form_cnn_avg
from images import Form as Form_images
from PyQt4.QtGui import *
from PyQt4.QtCore import *
import sys
class Communicator():
def __init__(self, parent=None):
self.mainWindow_images = Form_images()
self.mainWindow_images.show()
self.mainWindow_classifier = Form_classifier()
self.mainWindow_classifier.show()
def main():
app = QApplication(sys.argv)
app.setStyle('cleanlooks')
comm = Communicator()
sys.exit(app.exec_())
if __name__ == '__main__':
main()
One of the window files, generated by Qt Designer, looks like with the additional -w flag:
class Ui_Form(object):
def setupUi(self, Form):
Form.setObjectName(_fromUtf8("Form"))
Form.resize(1145, 654)
...
class Form(QtGui.QWidget, Ui_Form):
def __init__(self, parent=None, f=QtCore.Qt.WindowFlags()):
QtGui.QWidget.__init__(self, parent, f)
self.setupUi(self)
I want to do something like this and add the setImage-method to the mainWindow_classifier, becaus as I mentioned I should not modify the generated qt python file:
self.mainWindow_classifier.pushButton.clicked.connect(self.setImage)
def setImage(self):
# change QLabel in self.mainWindow_images

Running python files

I am working on pyqt4 and python26 application.I created forms using qt designer (.ui files).
I converted them to .py and .pyc files.But when i try to run .py file ,python command line comes and goes within a second,the form (corresponding .ui file) cannot be seen...what can be the problem??
this is my code:(.py file)
from DlgAbout_ui import Ui_DlgAbout
from PyQt4.QtCore import *
from PyQt4.QtGui import *
import resources
class DlgAbout(QDialog, Ui_DlgAbout):
def __init__(self, parent=None):
QDialog.__init__(self, parent)
self.setupUi(self)
self.logo.setPixmap( QPixmap( ":/icons/faunalia_logo.png" ) )
text = self.txt.toHtml()
text = text.replace( "$PLUGIN_NAME$", "RT Sql Layer" )
self.txt.setHtml(text)

First, don't use:
from PyQt4.QtCore import *
from PyQt4.QtGui import *
Instead:
from PyQt4 import QtCore, QtGui
And reference the modules explicitly.
class DlgAbout(QtGui.QDialog, Ui_DlgAbout):
etc.
In your code, all you've done is defined a dialog box. You haven't defined any main application to run, or any way to show the dialog box.
For an example, here's a basic main application to run:
from PyQt4 import QtGui
import sys
class MyMainWindow(QtGui.QMainWindow):
def __init__(self, parent=None):
super(MyMainWindow, self).__init__(parent)
self.form_widget = FormWidget(self)
self.setCentralWidget(self.form_widget)
class FormWidget(QtGui.QWidget):
def __init__(self, parent):
super(FormWidget, self).__init__(parent)
self.layout = QtGui.QVBoxLayout(self)
self.button = QtGui.QPushButton("Button!")
self.layout.addWidget(self.button)
if __name__ == "__main__":
app = QtGui.QApplication([])
foo = MyMainWindow()
foo.show()
sys.exit(app.exec_())
This defines a main window, and a form (Which MyMainWindow sets up, as you can see).
I then check if this is the main file being run (if __name__ == "__main__":), and I start the application (The app = QtGui.QApplication([]), create the main window, and show the main window.
In your case, you could define a main application like I did, and make it alert your QDialog.

Your python code just imports some modules and then defines a new class. It doesn't do anything with the class it has defined, though. In other words, once Python is done creating the new class, it is finished, and it exits.
I don't know PyQT at all, but most likely you need to start the GUI's main loop, and also instantiate an instance of your new class and pass it to PyQT.

Loading QtDesigner's .ui files in PySide

I am looking for a simple example of how to directly load a QtDesigner generated .ui file into a Python application.
I simply would like to avoid using pyuic4.

For the complete noobs at PySide and .ui files, here is a complete example:
from PySide import QtCore, QtGui, QtUiTools
def loadUiWidget(uifilename, parent=None):
loader = QtUiTools.QUiLoader()
uifile = QtCore.QFile(uifilename)
uifile.open(QtCore.QFile.ReadOnly)
ui = loader.load(uifile, parent)
uifile.close()
return ui
if __name__ == "__main__":
import sys
app = QtGui.QApplication(sys.argv)
MainWindow = loadUiWidget(":/forms/myform.ui")
MainWindow.show()
sys.exit(app.exec_())

PySide, unlike PyQt, has implemented the QUiLoader class to directly read in .ui files.
From the linked documentation,
loader = QUiLoader()
file = QFile(":/forms/myform.ui")
file.open(QFile.ReadOnly)
myWidget = loader.load(file, self)
file.close()

Another variant, based on a shorter load directive, found on https://askubuntu.com/questions/140740/should-i-use-pyqt-or-pyside-for-a-new-qt-project#comment248297_141641. (Basically, you can avoid all that file opening and closing.)
import sys
from PySide import QtUiTools
from PySide.QtGui import *
app = QApplication(sys.argv)
window = QtUiTools.QUiLoader().load("filename.ui")
window.show()
sys.exit(app.exec_())
Notes:
filename.ui should be in the same folder as your .py file.
You may want to use if __name__ == "__main__": as outlined in BarryPye's answer

Here is some example for PySide6 and Windows. (For linux you need use /, not \\)
from PySide6.QtUiTools import QUiLoader
from PySide6.QtCore import QFile
from PySide6.QtWidgets import QApplication
import sys
if __name__ == "__main__":
app = QApplication(sys.argv)
loader = QUiLoader()
file = QFile("gui.ui")
file.open(QFile.ReadOnly)
ui = loader.load(file)
file.close()
ui.show()
sys.exit(app.exec_())

For people coming from PyQt5/6 who are thoroughly confused by this:
PySide does not have the same functionality that we're used to, which is to load the ui file at the top of the window/widget subclass like so:
class MainWindow(QtWidgets.QMainWindow):
def __init__(self, *args, **kwargs):
super(MainWindow, self).__init__(*args, **kwargs)
'''Load ui'''
uic.loadUi("mainwindow.ui", self)
There is nothing very similar to this in PySide.
Instead, the best thing to do is embrace the ui-file compilation that you've avoided because loading the ui file is so easy in PyQt. This has a couple of advantages
There is a performance advantage - the ui file does not need to be compiled at run time
You get type hints for all your widgets without needing to manually add them
The disadvantage is that you have to use pyside6-uic to compile the *.ui files each time you edit them, but this can be made less painful by using scripts to automate the process - either setting it up in VSCode, a batch file or a powershell script. After you've done this, the code is nice:
#ui_mainwindow is the ui_mainwindow.py file
#Ui_MainWindow is the class that was produced within that .py file
from ui_mainwindow import Ui_MainWindow
class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
def __init__(self):
super(MainWindow, self).__init__()
'''Load the ui'''
self.setupUi(self)