[GIS] Clean way to cancel QGIS processing scripts from gui

pyqgisqgisqgis-processing

I've written a QGIS processing script that carries out a closest facility analysis. Taking the complete input data set (20k adresses as start points, ~100 target points, OSM road network with ~13k linestrings) the script runs about 3 hours, and therefore makes use of various progress indicators, to make shure to the user that it's still alive (log output, progress bar, time remaining, rubber bands indication the way to closest facililty just processed, cp. screenshot below):

Now I would like to cancel the running script and as far as I can see, this can only be done with [X] top right in the processing script gui window.

But doing so, the script remains processing as indicated by the blue rubber bands as well as the processing time and the progress bar…

… and the system resources in the task manager:

However, couple of minutes (~14min) later on the script stops processing and the system is not consuming anymore resources:

Since the time span between closing the window and stopping processing seems fairly unpredictable to me, my question is: Is this a recommended way to cancel QGIS processing scripts, or are there other ways to cleanly cancelling such scripts from the gui?

Best Answer

From command shell, "taskkill /f /im qgis-ltr-bin.exe /t" would terminate the process immediately. I don't think there's a gui solution, based on what you're going for though. In QGIS, if you wanted to end the process cleanly, you'd have to wait for the program when ending the process. But via command shell this would allow you to move forward right away.

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