In PyQGIS 2.X I could handle the output of processings algorithm as follows:

import processing
# None is the OUTPUT parameter here
buffer = processing.runalg('qgis:fixeddistancebuffer', network_from_nv_tmp, 0.1, 10, True, None)
clip = processing.runalg('qgis:clip', self.network_layer, buffer['OUTPUT'], None)

key approach here is to set the output parameter to None for running the algorithm and access it via result['OUTPUT'] in subsequent algorithms.

Now I try the same (or comparitivly the same) in PyQGIS 3:

import processing
buffer = processing.run('qgis:buffer', {'INPUT':network_from_nv_tmp, 'DISTANCE':0.1, 'SEGMENTS':10, 'END_CAP_STYLE':0, 'JOIN_STYLE':0, 'MITER_LIMIT':2, 'DISSOLVE':True, 'OUTPUT':None})        
clip = processing.run('qgis:clip', {'INPUT':self.network_layer, 'OVERLAY':buffer['OUTPUT'], 'OUTPUT':None})

which throws the following traceback:

_core.QgsProcessingException: Unable to execute algorithm
Could not create destination layer for OUTPUT: invalid value

So my question is: What is a valid value for parameter OUTPUT here?


I just stumbled upon a code snippet in https://anitagraser.com/2018/03/24/revisiting-point-polygon-joins/ that solved my issue. This indicates that the string 'memory:' is one possible valid value for OUTPUT parameter generating a temporary memory layer.

Furthermore, any string is a valid value. If a string e.g. 'buffered' is provided the result is stored in a Geopackage called buffered.gpkg. Thus, I strongly recommend not to provide the same string in subsequent algorithms, because the second algorithm will throw an exeption that the *.gpkg cannot be created since it has allready be created by the first.

  • how to do loops and use processing tools then? using 'memory:' in outputs result in a similar error message: CRITICAL There were errors executing the algorithm. 2020-02-05T10:01:47 INFO Results: {'OUTPUT': 'Buffered_8f549d26_b615_45d4_aafa_0129d2d4ed4b'} instead of Could not create layer bufferedlayer.gpkg: Creation of data source failed (OGR error: A file system object called 'bufferedlayer.gpkg' already exists.) – Leo Feb 4 '20 at 23:25
  • @Leo Interesting... I strongly recommend to ask this as a separate question! – Jochen Schwarze Feb 6 '20 at 7:12

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