[Qgis-user] How to fix 'Could not find the Qt platform plugin “Windows” in “”' when implementing qgis in custom Python3 application?

[Matt Brauer]

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
locationQgsApplication.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 at 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.
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