QGIS » QGIS Application

Using the modeler in QGIS 3.2, if you give a name to the output of an algorithm, you can't use that output as input for a subsequent algorithm. Trying to do so results in a processing error (`Could not load source layer for INPUT: invalid value`). I have attached a test project with some data and a model to demonstrate the issue. Included is a folder with an equivalent project in QGIS 2.18 showing that the same model worked in previous versions.

The model runs two parallel processes which are identical (polygonize raster then extract by attribute) except the second names the output of the polygonize step before using it in the extract step. I am fairly sure that this is not an issue with the test algorithms used here, but more broadly with the mapping of input and output parameters in models. You can see in the model log below that the first 'extract by attribute' algorithm was able to find the input file (standard named `OUTPUT.shp`) but the second time the algorithm is executed it has an empty INPUT parameter. This is probably because the output of the previous step results in a file named `gdal_polygonize_2_DEM_vector.shp`, not `OUTPUT.shp` as usual.

I am not sure where the mapping of output and input names happens in the processing code (and whether it's executed in c++ or Python). If it's in Python I'd be happy to look into it if someone can tell me which class or file to start the investigation.

Processing algorithm…
    Algorithm 'named_output_used_as_input' starting…
    Input parameters:
{ 'dem' : 'C:/Users/rudi_000/Dropbox/GIS/shared/named_output_used_as_input_test/dem.tif', 'gdal:polygonize_2:DEM_vector' : 'C:/Users/rudi_000/AppData/Local/Temp/processing_37290c9f65694832953690d6bd83266d/bfa5cb1aca8a4c8b8a9b7da1b85da366/gdal_polygonize_2_DEM_vector.shp', 'native:extractbyattribute_1:Output 1' : 'memory:', 'native:extractbyattribute_2:Output 2 (named output)' : 'memory:' }

Prepare algorithm: gdal:polygonize_1
    Running Polygonize 1 [1/4]
    Input Parameters:
{ BAND: 1, EIGHT_CONNECTEDNESS: False, FIELD: 'DN', INPUT: 'C:/Users/rudi_000/Dropbox/GIS/shared/named_output_used_as_input_test/dem.tif', OUTPUT: 'C:/Users/rudi_000/AppData/Local/Temp/processing_37290c9f65694832953690d6bd83266d/e715a21a7ae64c79a5b864dc72c5a6b1/OUTPUT.shp' }
    GDAL command:
    cmd.exe /C gdal_polygonize.bat C:/Users/rudi_000/Dropbox/GIS/shared/named_output_used_as_input_test/dem.tif C:/Users/rudi_000/AppData/Local/Temp/processing_37290c9f65694832953690d6bd83266d/e715a21a7ae64c79a5b864dc72c5a6b1/OUTPUT.shp -b 1 -f "ESRI Shapefile" None DN
    GDAL command output:
    0...10...20...30...40...50...60...70...80...90...Creating output C:/Users/rudi_000/AppData/Local/Temp/processing_37290c9f65694832953690d6bd83266d/e715a21a7ae64c79a5b864dc72c5a6b1/OUTPUT.shp of format ESRI Shapefile.

100 - done.

OK. Execution took 29.186 s (1 outputs).
    Prepare algorithm: native:extractbyattribute_1
    Running Extract by attribute [2/4]
    Input Parameters:
{ FIELD: 'DN', INPUT: 'C:/Users/rudi_000/AppData/Local/Temp/processing_37290c9f65694832953690d6bd83266d/e715a21a7ae64c79a5b864dc72c5a6b1/OUTPUT.shp', OPERATOR: 0, OUTPUT: 'memory:', VALUE: '1' }
    OK. Execution took 0.116 s (1 outputs).
    Prepare algorithm: gdal:polygonize_2
    Running Polygonize 2 (named output) [3/4]
    Input Parameters:
{ BAND: 1, EIGHT_CONNECTEDNESS: False, FIELD: 'DN', INPUT: 'C:/Users/rudi_000/Dropbox/GIS/shared/named_output_used_as_input_test/dem.tif', OUTPUT: 'C:/Users/rudi_000/AppData/Local/Temp/processing_37290c9f65694832953690d6bd83266d/bfa5cb1aca8a4c8b8a9b7da1b85da366/gdal_polygonize_2_DEM_vector.shp' }
    GDAL command:
    cmd.exe /C gdal_polygonize.bat C:/Users/rudi_000/Dropbox/GIS/shared/named_output_used_as_input_test/dem.tif C:/Users/rudi_000/AppData/Local/Temp/processing_37290c9f65694832953690d6bd83266d/bfa5cb1aca8a4c8b8a9b7da1b85da366/gdal_polygonize_2_DEM_vector.shp -b 1 -f "ESRI Shapefile" None DN
    GDAL command output:
    0...10...20...30...40...50...60...70...80...90...Creating output C:/Users/rudi_000/AppData/Local/Temp/processing_37290c9f65694832953690d6bd83266d/bfa5cb1aca8a4c8b8a9b7da1b85da366/gdal_polygonize_2_DEM_vector.shp of format ESRI Shapefile.

100 - done.

OK. Execution took 29.425 s (1 outputs).
    Prepare algorithm: native:extractbyattribute_2
    Running Extract by attribute [4/4]
    Input Parameters:
{ FIELD: 'DN', INPUT: '', OPERATOR: 0, OUTPUT: 'memory:', VALUE: '1' }
    Could not load source layer for INPUT: invalid value
    Error encountered while running Extract by attribute
    Error encountered while running Extract by attribute
    Execution failed after 59.70 seconds

Loading resulting layers
    Algorithm 'named_output_used_as_input' finished