[GIS] How to run Processing algorithms in QGIS 3 Plugins

pyqgisqgis-3qgis-pluginsqgis-processing

In QGIS 3 I am trying to run Processing algorithms in a worker thread in a QGIS plugin (not a Processing plugin) using processing.run, and experience crashes.

I guess I need to provide a QgsProcessingContext when running algorithms, but have not found a way to get a proper one.

mycontext = QgsProcessingContext()    
inpbuff = processing.run("native:buffer", {'INPUT': self.inpvl, 'DISTANCE': radius, 'SEGMENTS': 10, 'END_CAP_STYLE': 0, 'JOIN_STYLE': 0, 'MITER_LIMIT': 1, 'DISSOLVE': True, 'OUTPUT': 'memory:'}, feedback=None, context=mycontext)

Both with and without the context parameter, I get many identical warnings before QGIS crashes:

Warning: QObject::setParent: Cannot set parent, new parent is in a different thread

What is the recommended way of running Processing algorithms in a worker thread in a Plugin in QGIS 3?

Best Answer

This seems to have been a problem with early versions of QGIS 3.

With version 3.4.2, I do not get these warnings and crashes when running algorithms in a separate thread in a QGIS plugin.

Related Solutions

qgis – Using Memory Layer for Processing Algorithms in QGIS 3

As Germán noted, you should use "memory:" as the output string. But you'll also need to store the algorithm results, or the memory layer will be garbage collected by python!

results = processing.run('grass7:v.net',
            {"input":inputLayer,
             ...
            "output": 'memory:'})
result_layer = results['output']

But more generally - it's a bit inefficient to use a memory layer here. Because you're calling a grass algorithm (not a native QGIS one), all the outputs and inputs are being converted to disk based formats for use by grass, and all outputs from grass are also disk-based (behind the scenes these are shapefiles in a temporary directory). So when you ask for a memory layer output, QGIS has to read over the shapefile created by grass and convert that to a QGIS memory layer. So in this case, it's actually more efficient to just give the algorithm a path to save the shapefile and then read that directly.

[GIS] QGIS 3.2 Grass processing algorithms not working

You need to add quote in "C:\Program Files\QGIS 3.2\bin\grass74.bat" file so it would be like this:

call "%OSGEO4W_ROOT%\bin\o4w_env.bat"
call "%OSGEO4W_ROOT%\apps\grass\grass-7.4.1\etc\env.bat"

Best Answer

Related Solutions

qgis – Using Memory Layer for Processing Algorithms in QGIS 3

[GIS] QGIS 3.2 Grass processing algorithms not working

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