Error: Algorithm not found using grassprovider in PyQGIS standalone script

Question

With QGIS 3.22 the GRASS algorithms got its own provider (grassprovider.Grass7AlgorithmProvider) and are no more part of processing.algs. Knowing this, in a standalone pyqgis-script I am trying to add the GRASS algs to the processingRegistry with QgsApplication.processingRegistry().addProvider(Grass7AlgorithmProvider()).

If I am now trying to run a GRASS alg, e.g. processing.run("grass7:v.dissolve", ...) I get

_core.QgsProcessingException: Error: Algorithm grass7:v.dissolve not found

When I then list all the registered algs

for alg in QgsApplication.processingRegistry().algorithms():
    print("{}:{} --> {}".format(alg.provider().name(), alg.name(), alg.displayName()))

I get (after the entries of GDAL algs) a list of blank entries:

: -->
: -->
: -->
: -->
...

Obviously the GRASS algs are registered, but with no provider name and with no alg name resulting that I can't call the algs.

Is there anything I am doing wrong or I s it a bug?

Answer 1

My current setup for the Windows command line:

call "C:\Program Files\QGIS 3.36.1\OSGeo4W.bat"
call "C:\Program Files\QGIS 3.36.1\bin\python-grass83.bat" -c ""
call "C:\Program Files\QGIS 3.36.1\bin\python-qgis.bat" -c ""
python file.py

Content of file.py:

import os
import sys

qgis_root_path = r"C:\Program Files\QGIS 3.36.1"
sys.path.append(os.path.join(qgis_root_path, "apps\qgis\python\plugins"))

from qgis.core import *

QgsApplication.setPrefixPath(qgis_root_path, True)

qgs = QgsApplication([], False)
qgs.initQgis()

import processing
from processing.core.Processing import Processing

Processing.initialize()

for alg in QgsApplication.processingRegistry().algorithms():
    if (alg.id().startswith("grass")):
        print(alg.id(), "->", alg.displayName())

# processing.run("grass:<algorithm.name>", {})

qgs.exitQgis()

Output:

grass:g.extension.list -> g.extension.list
grass:g.extension.manage -> g.extension.manage
grass:g.version -> g.version
...