QGIS Processing algorithm output

Question

I am developing a plugin in QGIS using Python language. This plugin uses algorithms from Processing Toolbox and I save some results in the temporary folder assigning QgsProcessing.TEMPORARY_OUTPUT in the output field of each algorithm. However, my plugin is just working well when I return the temporary folder directory using QMessageBox. If I do not use this, my plugin does not work, so nothing is saved in the temporary folder. Why this happens?

Example of code working:

 clip_ndvi = Processing.runAlgorithm("saga:cliprasterwithpolygon", {
 'INPUT': str(self.dlg.lineEdit.text()),
 'POLYGONS': vl,
 'OUTPUT': QgsProcessing.TEMPORARY_OUTPUT})
 out_ndvi = clip_ndvi['OUTPUT']
 QMessageBox.about(self.dlg,'teste', str(out_ndvi))

Example of code not working:

clip_ndvi = Processing.runAlgorithm("saga:cliprasterwithpolygon", {
 'INPUT': str(self.dlg.lineEdit.text()),
 'POLYGONS': vl,
 'OUTPUT': QgsProcessing.TEMPORARY_OUTPUT})
 out_ndvi = clip_ndvi['OUTPUT']

The difference is in the QMessageBox.

• lejedi76: It's very strange. Did you have the same thing with processing.run instead of Processing.runAlgorithm ?
• Bárbara Duarte: The processing.run is only used in the Python console in QGIS and that is ok. But if I need to use in a Python script I need to use Processing.runAlgorithm.... I think that the problem is in the output be saved in temporary folder. Because sometimes it does not create the output raster... But I do not understand why. And also if there is another way to save in temporary folder.
• lejedi76: Try to force with QgsProcessingUtils.generateTempFilename('example.gpkg') instead of QgsProcessing.TEMPORARY_OUTPUT
• Bárbara Duarte: It works. At least for now :) Thank you. If you want answer my question and I will vote.

Answer 1

Strange issue. I don't understand the why or how but consider to replace QgsProcessing.TEMPORARY_OUTPUT by :

QgsProcessingUtils.generateTempFilename('example.gpkg')

Answer 2

If you run the saga:cliprasterwithpolygon algorithm in the QGIS GUI, and try to set a temporary output, the processing tool will fail.

This is because both GRASS and SAGA processing tools don't seem to work with temporary outputs (despite their GUI's suggesting that they will accept a temporary output).

Needless to say, once you set the output in both your code or in the GUI, the algorithm will work.