I'm writing a script that I would like to work outside of QGIS GUI. The script uses the processing algorithm standalone just fine for the native algorithms, however I am experiencing errors when trying to run grass algorithms. The code I am currently using to initialise the environment come from the following post: Using QGIS3 Processing algorithms from standalone PyQGIS scripts (outside of GUI)

import sys
from qgis.core import QgsProcessingRegistry
from qgis.analysis import QgsNativeAlgorithms
from qgis.core import (

QgsApplication.setPrefixPath('/usr', True)
qgs = QgsApplication([], False)

# Append the path where processing plugin can be found

from processing.core.Processing import Processing
import processing

This code works to initialise the environment and allows me to run QGIS native processing operations in standalone scripts. The issue arises when i try to run the following code:

# Grass Process
parameters = {
    '-f': False,
    'areas': 'C:/TEMP/areas.tif',
    'direction': 'C:/TEMP/direction.tif',
    'format': 0,
    'input': 'C:/TEMP/DEM.tif',
    'output': 'C:/TEMP/DEM_Filled.tif'

The process runs successfully from within QGIS running from the python console, however when I run the script externally I get the following error from processing:

File "C:/OSGeo4W64/apps/qgis/python/plugins\processing\tools\general.py", line 96, in run
    return Processing.runAlgorithm(algOrName, parameters, onFinish, feedback, context)
File "C:/OSGeo4W64/apps/qgis/python/plugins\processing\core\Processing.py", line 181, in runAlgorithm
    raise QgsProcessingException(msg)
_core.QgsProcessingException: There were errors executing the algorithm.

I am running the scripts in the python environment initialised by: C:\OSGeo4W64\bin\python-qgis.bat

  • Have you added exception and printed it to see the error – lat long Sep 26 '18 at 11:27
  • The processing class already has an exception handler. The last line in the second code block is the exception message: – Dan Knott Sep 27 '18 at 23:16
  • after initializing Processing framework , also import processing and use that to run algo.. from processing.core.Processing import Processing Processing.initialize() import processing – lat long Sep 28 '18 at 7:11
  • I had missed the line import processing in the above code, now edited. This has not fixed the issue with running the grass7 algorithm. – Dan Knott Oct 2 '18 at 7:03

I've done some more digging in the QGIS code and have found a solution. In the definition for processing.run() I found the method to get the QgsProcessingAlgorithm object from the string name. This class has a method canExecute(). This method returns true for me with no error message and subsequently running the processing algorithm works:

alg = QgsApplication.processingRegistry().createAlgorithmById('grass7:r.fill.dir')
canExecute, errorMessage = alg.canExecute()
print(str(canExecute) + str(errorMessage))
processing.run('grass7:r.fill.dir', parameters)

I traced the canExecute() method through the QGIS code and found the same method within the processing definition for Grass7Algorithm.py. Here I discovered the solution could be simplified to the following two lines:

from processing.algs.grass7.Grass7Utils import Grass7Utils

I hope someone else finds this useful.

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