PyQGIS – Using Custom Processing Algorithm from Standalone Scripts Outside QGIS GUI

pyqgisqgis-3qgis-processingstandalone

How can I run a custom algorithm using processing.run ?

I've extended a QgsProcessingAlgorithm following qgis docs.

I've managed to run a native algorithm in a standalone PyQGIS script following
Using QGIS3 Processing algorithms from standalone PyQGIS scripts (outside of GUI)

QgsApplication.processingRegistry().algorithms() does not show my algorithm.

Best Answer

Based on Using QGIS3 Processing algorithms from standalone PyQGIS scripts (outside of GUI), you need at least 2 new files:

An algorithm provider (example_algorithm_provider.py, see sample provider), which helps you register your algorithm, and
Your Processing algorithm (example_processing_algorithm.py, see sample algorithm), where you define the algorithm logic.

The PyQGIS standalone script would be like this:

import sys
from qgis.core import (
     QgsApplication, 
     QgsVectorLayer
)

# See https://gis.stackexchange.com/a/155852/4972 for details about the prefix 
QgsApplication.setPrefixPath('/docs/dev/qgis/core/QGIS/build_master/output', True)
qgs = QgsApplication([], False)
qgs.initQgis()

# Append the path where processing plugin can be found
sys.path.append('/docs/dev/qgis/core/QGIS/build_master/output/python/plugins')

import processing
from processing.core.Processing import Processing
Processing.initialize()

# Add our own algorithm provider
from example_algorithm_provider import ExampleAlgorithmProvider
provider = ExampleAlgorithmProvider()
QgsApplication.processingRegistry().addProvider(provider)

# Run our custom algorithm
layer = QgsVectorLayer("/docs/geodata/bogota/ideca/Loca.shp", "layer", "ogr")
params = {'INPUT': layer}
print("RESULT:", processing.run("my_provider:my_algorithm", params)['OUTPUT'])

Notes:

After initializing Processing, we add our own algorithm provider using QgsApplication.processingRegistry().addProvider(provider).
You can see a full example (i.e., this PyQGIS standalone script, the example_algorithm_provider.py and the example_processing_algorithm.py) at this GitHub repo: https://github.com/gacarrillor/pyqgis_scripts/tree/master/pyqgis_custom_processing_algorithm_standalone

Related Solutions

PyQGIS 3 – Using QGIS Processing Algorithms from Standalone Scripts

You can run a QGIS Processing algorithm in standalone (no GUI) mode in this way:

import sys
from qgis.core import (
     QgsApplication, 
     QgsProcessingFeedback, 
     QgsVectorLayer
)

# See https://gis.stackexchange.com/a/155852/4972 for details about the prefix 
QgsApplication.setPrefixPath('/usr', True)
qgs = QgsApplication([], False)
qgs.initQgis()

# Append the path where processing plugin can be found
sys.path.append('/docs/dev/qgis/build/output/python/plugins')

import processing
from processing.core.Processing import Processing
Processing.initialize()

layer1 = QgsVectorLayer('/path/to/geodata/lines_1.shp', 'layer 1', 'ogr')
layer2 = QgsVectorLayer('/path/to/geodata/lines_2.shp', 'layer 2', 'ogr')

# You can see what parameters are needed by the algorithm  
# using: processing.algorithmHelp("qgis:union")
params = { 
    'INPUT' : layer1,
    'OVERLAY' : layer2, 
    'OUTPUT' : '/path/to/output_layer.gpkg|layername=output'
}
feedback = QgsProcessingFeedback()

# See https://gis.stackexchange.com/a/276979/4972 for a list of algorithms
res = processing.run('qgis:union', params, feedback=feedback)
res['OUTPUT'] # Access your output layer

Note for QGIS versions LESS THAN 3.16.5

If you want to use a native algorithm (i.e., an algorithm from the native provider, whose algorithms are written in C++), you need to add the provider after initializing Processing:

import sys
from qgis.core import (
     QgsApplication, 
     QgsProcessingFeedback, 
     QgsVectorLayer
)
from qgis.analysis import QgsNativeAlgorithms

# See https://gis.stackexchange.com/a/155852/4972 for details about the prefix 
QgsApplication.setPrefixPath('/usr', True)
qgs = QgsApplication([], False)
qgs.initQgis()

# Append the path where processing plugin can be found
sys.path.append('/docs/dev/qgis/build/output/python/plugins')

import processing
from processing.core.Processing import Processing
Processing.initialize()
QgsApplication.processingRegistry().addProvider(QgsNativeAlgorithms())

layer = QgsVectorLayer('/path/to/geodata/lines.shp', 'my layer', 'ogr')

# You can see what parameters are needed by the algorithm  
# using: processing.algorithmHelp("native:extractvertices")
params = {
    'INPUT': layer,
    'OUTPUT': 'memory:'
}
feedback = QgsProcessingFeedback()

# See https://gis.stackexchange.com/a/276979/4972 for a list of algorithms
res = processing.run("native:extractvertices", params, feedback=feedback)
res['OUTPUT'] # Access your output layer

Note: If you actually wrote your own Processing algorithm and would like to run it in a standalone PyQGIS script, see: Using custom Processing algorithm from standalone PyQGIS scripts (outside of GUI)

QGIS PyQGIS – Running PyQGIS Scripts Outside QGIS GUI Using GRASS Provider

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
Grass7Utils.checkGrassIsInstalled()

I hope someone else finds this useful.

Best Answer

Related Solutions

PyQGIS 3 – Using QGIS Processing Algorithms from Standalone Scripts

Note for QGIS versions LESS THAN 3.16.5

QGIS PyQGIS – Running PyQGIS Scripts Outside QGIS GUI Using GRASS Provider

Related Question