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...

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."

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