Using custom Processing algorithm from standalone PyQGIS scripts (outside of GUI)

Question

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.

Answer 1

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

1. An algorithm provider (example_algorithm_provider.py, see sample provider), which helps you register your algorithm, and
2. 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:

1. After initializing Processing, we add our own algorithm provider using QgsApplication.processingRegistry().addProvider(provider).
2. 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