Running the GRASS algorithm in standalone Python code using the PyQGIS processing module

Question

What is the right way to import GRASS algorithms for a standalone Python script?

I tried the solutions (solution1, solution2), but they didn't help me.

Here's the code that doesn't import GRASS algorithms, but imports native QGIS 3.22.2 algorithms well:

import sys
import qgis.core
OSGeo_folder = r'C:\OSGeo4W'
sys.path.append(fr'{OSGeo_folder}\apps\qgis\python\plugins')
import processing
from grassprovider.Grass7AlgorithmProvider import Grass7AlgorithmProvider
from grassprovider.Grass7Utils import Grass7Utils
print(f'GRASS version={Grass7Utils.installedVersion(True)}')
processing.core.Processing.Processing.initialize()
qgis.core.QgsApplication.processingRegistry().addProvider(qgis.analysis.QgsNativeAlgorithms())
qgis.core.QgsApplication.processingRegistry().addProvider(Grass7AlgorithmProvider())
if __name__ == '__main__':
 processing.algorithmHelp('grass7:r.clump')
 print('=' * 40)
 processing.algorithmHelp("native:reclassifybytable")

The output is:

GRASS version=7.8.6
Algorithm "grass7:r.clump" not found.
========================================
Reclassify by table (native:reclassifybytable)
This algorithm reclassifies a raster band by assigning new class values based on the ranges specified in a fixed table.
...truncated...

• Comrade Che: @BenW Thanks for your help. Trying to solve the problem, I installed the latest version of QGIS 3.22.2-Białowieża. Apparently, the problem is not in the bat file (I tried your bat file without success), but probably in the python code. Have you tried running the code from the question above? What was the result?
• Andrea Giudiceandrea: Is this the same as the issue github.com/qgis/QGIS/issues/45935 reported in the QGIS GitHub repository?
• Comrade Che: @AndreaGiudiceandrea Yes, it is exactly the same. But is is closed as fixed issue.
• Andrea Giudiceandrea: The issue report github.com/qgis/QGIS/issues/45935 was reopened (see github.com/qgis/QGIS/issues/45935#issuecomment-1012085166 and github.com/qgis/QGIS/issues/45935#issuecomment-1013639912) and then hopefully fixed with the merging of the PR github.com/qgis/QGIS/pull/47114.

Answer 1

Here is how GRASS should be imported (because of a bug it does not work, see more here, thanks Andrea Giudiceandrea):

import sys
import qgis.core
OSGeo_folder = r'C:\OSGeo4W'
sys.path.append(fr'{OSGeo_folder}\apps\qgis\python\plugins')
import processing
from grassprovider.Grass7AlgorithmProvider import Grass7AlgorithmProvider
processing.core.Processing.Processing.initialize()
print(f'Number of registered algorithms = {len(qgis.core.QgsApplication.processingRegistry().algorithms())}')
qgis.core.QgsApplication.processingRegistry().addProvider(Grass7AlgorithmProvider())
print(f'Number of registered algorithms = {len(qgis.core.QgsApplication.processingRegistry().algorithms())}')
processing.algorithmHelp('grass7:r.clump') # fail if "Algorithm "grass7:r.clump" not found."

• Andrea Giudiceandrea: The issue report github.com/qgis/QGIS/issues/45935 was reopened (see github.com/qgis/QGIS/issues/45935#issuecomment-1012085166 and github.com/qgis/QGIS/issues/45935#issuecomment-1013639912) and then hopefully fixed with the merging of the PR github.com/qgis/QGIS/pull/47114.

Answer 2

I do not have 3.22 installed yet, so not tested in that version. I have tested this with 3.20.3 installed with the new OSGeo4W installer. If it doesn't work for you I will delete this answer.

Tested workflow is:

Save the following batch file (e.g. pyqgis_grass78.bat):

@echo off
SET OSGEO4W_ROOT=C:\OSGeo4W
call "%OSGEO4W_ROOT%"\bin\o4w_env.bat
call "%OSGEO4W_ROOT%\apps\grass\grass78\etc\env.bat"
@echo off
path %PATH%;%OSGEO4W_ROOT%\apps\qgis\bin
path %PATH%;C:\OSGeo4W\apps\Qt5\bin
path %PATH%;C:\OSGeo4W\apps\Python39\Scripts
set QGIS_PREFIX_PATH=%OSGEO4W_ROOT:\=/%/apps/qgis
set GDAL_FILENAME_IS_UTF8=YES
rem Set VSI cache to be used as buffer, see #6448
set VSI_CACHE=TRUE
set VSI_CACHE_SIZE=1000000
set PYTHONPATH=%PYTHONPATH%;%OSGEO4W_ROOT%\apps\qgis\python
set PYTHONHOME=%OSGEO4W_ROOT%\apps\Python39
set QT_PLUGIN_PATH=%OSGEO4W_ROOT%\apps\qgis\qtplugins;%OSGEO4W_ROOT%\apps\qt5\plugins
cmd.exe

Save the following script as a .py file (e.g. r_clump_test.py):

import sys
from qgis.core import QgsApplication
QgsApplication.setPrefixPath('C:/OSGeo4W/apps/qgis', True)
qgs = QgsApplication([], False)
qgs.initQgis()
sys.path.append('C:\\OSGeo4W\\apps\\qgis\\python\\plugins')
import processing
from processing.core.Processing import Processing
Processing.initialize()
processing.algorithmHelp('grass7:r.clump')

Place the .bat and .py files in the same directory, double click the batch file, then at the command prompt type: "python r_clump_test.py" (without the quotes) and hit enter.

The output (for me) was:

r.clump (grass7:r.clump)
Recategorizes data in a raster map by grouping cells that form physically discrete areas into unique categories.
---------------- Input parameters
----------------
input: Input layer
 Parameter type: QgsProcessingParameterRasterLayer
 Accepted data types:
 - str: layer ID
 - str: layer name
 - str: layer source
 - QgsProperty
 - QgsRasterLayer
title: Title for output raster map
 Parameter type: QgsProcessingParameterString
 Accepted data types:
 - str
 - QgsProperty
-d: Clump also diagonal cells
 Parameter type: QgsProcessingParameterBoolean
 Accepted data types:
 - bool
 - int
 - str
 - QgsProperty
output: Clumps
 Parameter type: QgsProcessingParameterRasterDestination
 Accepted data types:
 - str
 - QgsProperty
 - QgsProcessingOutputLayerDefinition
threshold: Threshold to identify similar cells
 Parameter type: QgsProcessingParameterNumber
 Accepted data types:
 - int
 - float
 - QgsProperty
GRASS_REGION_PARAMETER: GRASS GIS 7 region extent
 Parameter type: QgsProcessingParameterExtent
 Accepted data types:
 - str: as comma delimited list of x min, x max, y min, y max. E.g. '4,10,101,105'
 - str: layer ID. Extent of layer is used.
 - str: layer name. Extent of layer is used.
 - str: layer source. Extent of layer is used.
 - QgsMapLayer: Extent of layer is used
 - QgsProcessingFeatureSourceDefinition: Extent of source is used
 - QgsProperty
 - QgsRectangle
 - QgsReferencedRectangle
 - QgsGeometry: bounding box of geometry is used
GRASS_REGION_CELLSIZE_PARAMETER: GRASS GIS 7 region cellsize (leave 0 for default)
 Parameter type: QgsProcessingParameterNumber
 Accepted data types:
 - int
 - float
 - QgsProperty
GRASS_RASTER_FORMAT_OPT: Output Rasters format options (createopt)
 Parameter type: QgsProcessingParameterString
 Accepted data types:
 - str
 - QgsProperty
GRASS_RASTER_FORMAT_META: Output Rasters format metadata options (metaopt)
 Parameter type: QgsProcessingParameterString
 Accepted data types:
 - str
 - QgsProperty
---------------- Outputs
----------------
output: <QgsProcessingOutputRasterLayer>
 Clumps

• Comrade Che: The output for me is as follows: "Algorithm "grass7:r.clump" is not found". Could you please update QGIS to version 3.22.2 and test again? I'm afraid that the programmers have changed something, because I haven't encountered this problem in older versions.
• Ben W: @Comrade Che, sorry mate, I can't right now. My batch file is based on python-qgis.bat in C:\OSGeo4W\bin. You can check that file in your installation- maybe some paths have changed. Also check the path to the grass env.bat. Otherwise I'm not sure.
• Comrade Che: In QGIS version 3.22, non-native providers (including GRASS) have been moved to plugins, which may affect the way it is imported. See the change log: qgis.org/en/site/forusers/visualchangelog322/…
• Ben W: Yes, probably! I will delete this answer for now.
• Comrade Che: Please don't delete your answer until we solve the problem.