38

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.

3 Answers 3

59

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)

5
  • @GermánCarrillo Somehow a very similar code dosen't work. See more: gis.stackexchange.com/questions/286281/… Jun 14, 2018 at 5:35
  • 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, 2018 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, 2019 at 19:36
  • 1
    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 2, 2020 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, 2020 at 14:41
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)

0

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

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