Qt qpa plugin could not find the qt platform plugin windows in

I am implementing qgis 3.4.3 in a custom application, but when I instantiate the QgsApplication() class, I receive the error «Could not find the Qt platform plugin «Windows» in «».

I’ve attempted using various installs of qgis 3.x+ including the standalone installer and OSGeo4W web installer. I currently am sticking with the OSGeo4W web installer installation. I am using a Python 3.7 installation separate from what comes with OSGeo4W and attempting to integrate qgis functionality.

I have followed the instructions in the following QGIS help doc under the section: «Using PyQGIS in Custom Applications»
https://docs.qgis.org/2.18/en/docs/pyqgis_developer_cookbook/intro.html#run-python-code-when-qgis-starts

After attempting to run the script I realized a dll plugin was missing. After some research, I found its the qwindows.dll that qt uses. The qwindows.dll is included in the OSGeo4W installation under: C:\OSGeo4W\apps\Qt5\plugins\platforms

I changed the QT_PLUGIN_PATH variable on the local command prompt to include the directory above, but the same error prevailed. I also changed the QT_DEBUG_PLUGINS variable to 1 which printed out the locations Qt is looking for plugins. Interestingly, it wasn’t looking for plugins in the path I specified in the QT_PLUGIN_PATH variable.

Python Code:

import sys
#add qgis python libraries to instance python path in case not added 
#at the environment variable %PYTHONPATH% level
sys.path.extend(['C:\\OSGeo4W\\apps\\qgis\\python', 
'C:\\OSGeo4W\\apps\\Python37\\Lib\\site-packages'])

from qgis.core import *

# supply path to qgis install location
QgsApplication.setPrefixPath('C:\\OSGeo4W\\apps\\qgis', True)

# create a reference to the QgsApplication
# setting the second argument to True enables the GUI, which we need to do
# since this is a custom application
qgs = QgsApplication([], True)

# load providers
qgs.initQgis()

# Write your code here to load some layers, use processing algorithms, etc.

# When your script is complete, call exitQgis() to remove the provider and
# layer registries from memory
qgs.exitQgis()

Batch File Code to Start Cmd Prompt with Proper Variables:

@echo off
SET OSGEO4W_ROOT=C:\OSGeo4W
::Include environment variables set by qgis on startup
call %OSGEO4W_ROOT%\bin\o4w_env.bat
call %OSGEO4W_ROOT%\bin\qt5_env.bat
call %OSGEO4W_ROOT%\bin\py3_env.bat
@echo off
path %PATH%;%OSGEO4W_ROOT%\apps\qgis\bin
path %PATH%;C:\OSGeo4W\apps\Qt5\bin
path %PATH%;C:\OSGeo4W\apps\Python37\Scripts
path %PATH%;C:\OSGeo4W\apps\Python37

set PYTHONPATH=%PYTHONPATH%;%OSGEO4W_ROOT%\apps\qgis\python
set PYTHONHOME=%OSGEO4W_ROOT%\apps\Python37
set QT_PLUGIN_PATH=%OSGEO4W_ROOT%\apps\Qt5\plugins;%OSGEO4W_ROOT%\apps\qgis\qtplugins
set QT_DEBUG_PLUGINS=1

cmd.exe

I expect to run the python script and receive no errors since I’m pointing to the directory where the qwindows.dll missing plugin is stored. However, I am still receiving the missing windows plugin error.

Here is the actual message with QT_DEBUG_PLUGINS set to 1:
QFactoryLoader::QFactoryLoader() checking directory path «C:/OSGeo4W/apps/qgis/qtplugins/platforms» …
QFactoryLoader::QFactoryLoader() checking directory path «C:/OSGeo4W/apps/Python37/platforms» …
qt.qpa.plugin: Could not find the Qt platform plugin «windows» in «»
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Description of the issue

When packaging an app which uses PySide6 (which seems to be a recent release, December 2020), the app builds fine but upon execution it crashes with the error qt.qpa.plugin: Could not find the Qt platform plugin "windows" in "".

This can be fixed by copying over the platforms folder from [env]\Lib\site-packages\PySide6\plugins into the pyinstaller dist directory (or where the executable is, if built with --onefile)

From googling around, it seems like people had this same problem back in the PyQt5 days, and seemed to have been fixed with
this pull request. The reason this might still broken might be that all the checks in pyinstaller/PyInstaller/utils/hooks/qt.py use an hardcoded «PySide2» string, while the newer package has been renamed to PySide6.

That’s made more likely from the fact that using PySide2 works (noting that all documentation / homepage links for it redirect to PySide6, pretty confusing naming scheme)

Context information (for bug reports)

• Output of pyinstaller --version: 3.6
• Version of Python: 3.9.1
• Platform: Windows-10-10.0.19041-SP0
• Did you also try this on another platform? Does it work there? Haven’t tried, but likely not

A minimal example program which shows the error

import sys

import PySide6.QtGui
import PySide6.QtCore
from PySide6.QtWidgets import QApplication, QMainWindow

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

def main():
    a = QApplication(sys.argv)
    w = MainWindow()
    sys.exit(a.exec_())

if __name__ == "__main__":
    main()

Stacktrace / full error message

qt.qpa.plugin: Could not find the Qt platform plugin "windows" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

@HZMD said in «could not find or load the Qt platform plugin «windows» in «»,:

Do all of them use Qt? — I’m not sure, and I’m not sure how to find out.

In general, you check for the presence of Qt DLLs (Qt5Core.dll, qwindows.dll) in the program’s folders.

How did I install Krita? — It would be non-helpful if I said «in the normal way» wouldn’t it? I’m definitely not a proper programmer, though I’m «programmer adjacent» in some aspects of my work, learning enough from programmers to describe it to non-programmers in pictures. Anyway, I downloaded an executable installer from Krita.org and let it do its thing. Is that the right information?

Yes, that’s the right information Other possible ways to install Krita include «compile it from source».

Anyway, since you used the «normal way», that rules out the possibility of the error being caused by incorrect installation/compilation/deployment

Does uninstalling and reinstalling help? — On the others I have encountered this problem with, it has. I haven’t done Krita yet, because I was hoping to find the original culprit, and thought there might be clues.

From the symptoms you’ve described and the ListDLLs output you posted, I can’t see an easy way to find the root culprit, unfortunately.

The easiest thing to do might be to reinstall the affected software and move on with your life.

One other thing you could try: If you create a new user account on your PC, does that account manage to run the affected programs normally? If it does, that means something has gone wrong in your current Windows account itself. (I can’t explain how though)

qwindows.dll is missing — Hey, I think Krita uses Qt! I found qwindows.dll in C:\Program Piles\Krita (x64)\bin\platforms

Using that newfound knowledge, I looked for it in…

…I’ll come back and edit this with any others it finds, or that it finds in Program Files (x86)

That’s OK, you don’t have to give us a comprehensive list.

I asked about other Qt-based software because sometimes, problems occur when a piece of Qt-based software adds itself to the system-wide PATH (or QT_PLUGIN_PATH). This can cause other Qt-based software to load the wrong version of Qt DLLs.

However, given that you can repair individual programs by re-installing them, that probably means your problem is caused by something else.

I did not find qwindows in the folder of the pen display driver / control software I suspected of having monkeyed things up. Am I right in thinking that means that driver / control software is innocent? With no more evidence than I had before, I now suspect Logitech. I’m sure this is how witch trials get started.

It is difficult to prove that someone/thing didn’t do it.