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

[Matt Brauer]

Thanks! That did work. I had tried modifying QT_QPA_PLATFORM_PLUGIN_PATH
earlier, but it didn't work so I didn't look back. I think at that time I
had tried it with the standalone QGIS install. That may have had a
detrimental effect, although I don't know why that would be the case.

Modifying the QT_QPA_PLATFORM_PLUGIN_PATH to point to ~\apps\Qt5\plugins
directory did the trick.

MB

On Thu, Jan 10, 2019 at 2:59 PM DT <dmofot at gmail.com> wrote:

> Matt, I don’t know if this was specific to OSX or of it might be relevant
> to your issue on Windows, but I had to set another Qt path.  See this gist:
> https://gist.github.com/dmofot/94345126db10660053ccfb0d76a1e32a
>
> DT
>
> On Thu, Jan 10, 2019 at 4:44 PM Matt Brauer <jbrauer90 at gmail.com> wrote:
>
>> 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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