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I have two shapefiles (drenagem and rio_pri). I need to loop through every feature from rio_pri and save all those features from drenagem that intersect rio_pri. So I wrote the following code to be ran in Python Terminal. I'm using QGis 2.18.17 and Linux Mint 19.

import processing
drenagem = QgsVectorLayer('/home/tiago/Documentos/Rios_mata_disol.shp','Rios_mata_disol','ogr')
rio_pri = QgsVectorLayer('/home/tiago/Documentos/rios_linha_costa.shp','rios_linha_costa','ogr')
# Loop
for r in rio_pri.getFeatures():
    processing.runalg("qgis:selectbylocation", drenagem, r,['intersects'],0,0)
    r_cod = r['COCURSODAG']
    path = r_cod+'.shp'
    processing.runalg('qgis:saveselectedfeatures', drenagem, path)

Then for every feature from rio_pri I get this message at console.

Unable to execute algorithm
Wrong parameter value: <qgis._core.QgsFeature object at 0x7ffb6620b770>

I guess I should not use the r as an argument of qgis:selectbylocation, but I could not figure out what should I use instead.

  • Your main problem is that you are trying to run the processing algorithms for every feature in a layer! The algorithms themselves loop over the features. Just call select by location once, pass 'drenagem' as input and 'rio_pri' as intersect, then call save selected features on 'drenagem'. Check the docs: docs.qgis.org/2.18/en/docs/user_manual/processing_algs/qgis/… docs.qgis.org/2.8/en/docs/user_manual/processing_algs/qgis/… – Ben W Jun 6 at 22:51
  • @BenW But I don't want to select features from 'drenagem' that intersect 'rio_pri' just ONCE. I need to do the select by location feature by feature from 'rio_pri'. – tiago Jun 7 at 11:57
  • @smiler I check that before, but I could not adapt to my code. – tiago Jun 7 at 11:59
  • OK, I misunderstood your goal. Do you mean that you want to save a new shapefile for every feature in rio_pri containing just the intersecting features from drenagem? What are the geometry types of the layers? – Ben W Jun 7 at 12:55
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Edit: After reading your subsequent comments above about what you are trying to achieve, I now doubt this actually what you want...

I have written the code below which you should be able to adapt. I have tested this in QGIS 3.4 and have included some commented lines which should allow for adaptation to QGIS 2.18. No guarantees though as I have not used the QGIS 2 API for a while now. If it's possible for you, I would recommend upgrading to at least QGIS 3.4. The script below assumes the 2 layers are loaded in the current project. My output files are in geopackage format, named 'test_file_' appended with the id of the intersecting feature to which they belong- you can change as required. Also, double backslashes in the output file path string works on Windows- I'm not sure about Linux.

I hope this helps.

import processing

rio_pri = QgsProject().instance().mapLayersByName('rios_linha_costa')[0]
#QGIS 2# rio_pri = QgsMapLayerRegistry().instance().mapLayersByName('rios_linha_costa')[0]
drenagem = QgsProject().instance().mapLayersByName('Rios_mata_disol')[0]
#QGIS 2# drenagem = QgsMapLayerRegistry().instance().mapLayersByName('Rios_mata_disol')[0]
index = QgsSpatialIndex(drenagem.getFeatures())
for feat in rio_pri.getFeatures():
    select_ids = []
    geoms = feat.geometry()
    engine = QgsGeometry.createGeometryEngine(geoms.constGet())
    engine.prepareGeometry()
    intersection = index.intersects(geoms.boundingBox())
    candidates = drenagem.getFeatures(QgsFeatureRequest().setFilterFids(intersection))
    for f in candidates:
        if engine.intersects(f.geometry().constGet()):
            select_ids.append(f.id())
    drenagem.selectByIds(select_ids)
    out_layer = 'Path\\To\\Output\\Location\\test_file_{}.gpkg'.format(str(feat.id()))
    # change extension to .shp if preferred
    processing.run('native:saveselectedfeatures', {'INPUT': drenagem,
    'OUTPUT': out_layer})
  • Thank you @Benw, I'll try it. – tiago Jun 9 at 20:21

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