PYQGIS 3 Using the ouptut from GRASS v.distance algorithm

Question

I have a script which is trying to run v.distance on two existing shape files,
buildings.shp which containing some polygons and singleasn.shp which contains a single point. This will be run from the console using QGIS 3.4

Using the algorithm via the Processing toolbox works fine, but the code has a problem.

I'm using the following python code:

e = buildings_layer.extent()
extents_str = str(e.xMinimum()) +"," + str(e.yMinimum()) +"," + str(e.xMaximum()) +"," + str( e.yMaximum())

parameters = {'from':'C:QGISProjectsQGIS3 Devarea 1buildings.shp',
             'from_type':[0,1,3],
             'to':'C:QGISProjectsQGIS3 Devarea 1singleasn.shp',
             'to_type':[0,1,3],
             'dmax':-1,
             'dmin':-1,
             'upload':[0],
             'column':['calculated'],
             'to_column':None,
             'from_output':'memory:nearest',
             'output':'memory:distance',
             'GRASS_REGION_PARAMETER':extents_str,
             'GRASS_SNAP_TOLERANCE_PARAMETER':-1,
             'GRASS_MIN_AREA_PARAMETER':0.0001,
             'GRASS_OUTPUT_TYPE_PARAMETER':0,
             'GRASS_VECTOR_DSCO':'',
             'GRASS_VECTOR_LCO':'',
             'GRASS_VECTOR_EXPORT_NOCAT':False}

algout = processing.run('grass7:v.distance', parameters)
for ft in algout['output'].getFeatures():
    #do something here#

The algorithm appears to run ok, but when it reaches the for loop i get the following error:

AttributeError: 'str' object has no attribute 'getFeatures'

Similar code to this worked for a native algorithm but not for a grass one - is there a difference in the output and if so how do I address it?

Comrade Che · Answer

You are trying to iterate over a string variable. Check out the output. Execute print(processing.algorithmHelp('grass7:v.distance')) to get help information for this algorithm:

----------------
Outputs
----------------

from_output:  <QgsProcessingOutputVectorLayer>
    Nearest

output:  <QgsProcessingOutputVectorLayer>
    Distance
None

Answered by Comrade Che on October 22, 2020

jfoguet · Answer

I'm not completely sure but i think the string of your output is the path for the vector output. Print it and check it. If so, you can 'cast' that output into a QgsVectorLayer.

The run method returns a dictionary with one or more output names (the
  ones shown in the algorithm description) as keys and the file paths of
  those outputs as values

qgis.doc

algout = processing.run('grass7:v.distance', parameters)['output']
vector = QgsVectorLayer(algout, '', 'ogr')
for ft in vector.getFeatures():
    #do something here#

hope it helps

PYQGIS 3 Using the ouptut from GRASS v.distance algorithm

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