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I am writing a script which must work outside of QGIS GUI. I call some API functions from qgis.core but I would like to use the processing plugin.

I am able to import processing with sys.path.append() but I cannot run any process. Moreover, all "native" algs are missing in QgsApplication.processingRegistry().algorithms()

Is it possible to run processing that way?

import os, sys
from qgis.core import *

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


sys.path.append('/usr/share/qgis/python/plugins')
from processing.core.Processing import Processing
Processing.initialize()
import processing


layer1 = QgsVectorLayer('data/ROUTE_PRIMAIRE.SHP')
layer2 = QgsVectorLayer('data/ROUTE_SECONDAIRE.SHP')


processing.run('qgis:union', layer1, layer2, 'test.shp') # returns nothing

I am using QGIS 3.0.1 on Debian 9.

PolyGeo
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vidlb
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3 Answers3

63

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)

Germán Carrillo
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  • @GermánCarrillo Somehow a very similar code dosen't work. See more: https://gis.stackexchange.com/questions/286281/why-qgis-3-0-3-nativeextractvertices-algorithm-dosent-work-properly-it-stand – Comrade Che Jun 14 '18 at 05:35
  • I try running a QGIS algorithm in a QGIS Plugin, does this work the same way? Because I can't make it run – gHupf Nov 12 '18 at 19:03
  • yes the code work should work with gui but you need a lot of other things to make it run! you should take a look at the Plugin Builder extension – vidlb Jan 18 '19 at 19:36
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    I was able to just copy import processing then the processing.run(. . . ) snippet into my plugin's if result: clause and it ran great :) Thank you! – mr.adam Apr 02 '20 at 22:08
  • @GermánCarrillo Great stuff, still when I run the script I get na error qt.qpa.plugin: Could not find the Qt platform plugin "windows" in "C:\OSGEO4~1\apps\Qt5\plugins\platforms , I've read stackoverflow on this but no anwser seems to work, I'm using PyCharm 2020.1 / QGIS 3.12, appreciate help! – cincin21 May 22 '20 at 14:41
  • I don't know why this has been accepted as the correct answer (maybe it's just outdated) but for those interested in using the qgis processing module, here's how you import it from qgis import processing. As for from processing.core.Processing import Processing; Processing.initialize() it's no longer required. – WhyNotTryCalmer Oct 23 '23 at 14:31
3

I had an error:

NameError: name 'QgsNativeAlgorithms' is not defined

when I tried to use a native algorithm as above, in an OSGeo4W install that included QGIS 3.4.4.

The solution turned out to be a missing import:

from qgis.analysis import QgsNativeAlgorithms

(from QGIS 3.4/3.6 standalone script)

Taras
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Andrew_S
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2

For standalone QGIS processing, you can now use qgis_process command line (available since QGIS 3.14, doc at https://docs.qgis.org/3.16/en/docs/user_manual/processing/standalone). Because of this, it's now possible to the following

qgis_process run native:union --INPUT=/path/to/geodata/lines_1.shp --OVERLAY=/path/to/geodata/lines_2.shp --OUTPUT="/tmp/output_layer.gpkg

You will suffer from one limitation at the moment compared to Python standalone approach: you can't specify the layer name output e.g https://github.com/qgis/QGIS/issues/38272

ThomasG77
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