Temporary layer has no attribute '__getitem__' with pyqgis

Question

I used the polygonize tool in order to polygonize a raster layer and it worked well, but now, I would like to save this new vector layer as a temporary layer.

My code from various posts is the following :

grid = QgsVectorLayer("Polygon?crs=epsg:4326", 
                      "Grid_"+str(self.id), 
                      "memory")

#grid = self.pabs+"/1_Grids/Grid_"+str(self.id)+".shp" #non temporary layer initially used

layer = self.pabs+"/0_Inputs/LandUse/lu_{}.tif".format(self.id)
processing.runalg('gdalogr:polygonize',layer,"lu_majorit",grid)

When I execute the code in the python console of Qgis I get:

'QgsVectorLayer' object has no attribute '__ getitem__'

I know that you sometimes have this problem with python class but here I don't see where the problem comes from.

I was wondering if it is linked to the fact I create an empty layer ?

I am using QGIS 2.18.10

Answer 1

Use the None parameter in the processing algorithm to create a temporary output. You can then call and add this output as a vector layer and add your fields etc.

layer = self.pabs+"/0_Inputs/LandUse/lu_{}.tif".format(self.id)
result = processing.runalg('gdalogr:polygonize',layer,"lu_majorit",None)

polygonized_layer = processing.getObject(result['OUTPUT'])
polygon_layer = iface.addVectorLayer(polygonized_layer.source(), "Grid_"+str(self.id), "ogr")
# Add your field
polygon_layer.startEditing()
polygon_layer.dataProvider().addAttributes([QgsField("index", QVariant.Int)])
polygon_layer.commitChanges()

Answer 2

To source this answer comment:

python script in QGis - help using gdal.Polygonize

the third parameter needs to be an instance of the GDAL/OGR vector layer class; you're passing in an instance of the QGIS vector layer class... not the same thing!....

This should produce a temporary vector polygon layer (need to use the None value for the last parameter, this puts the result in memory):

result = processing.runalg('gdalogr:polygonize',layer,"lu_majorit", None)
polygonLayer = processing.getObject(result['OUTPUT'])
QgsMapLayerRegistry.instance().addMapLayer(polygonLayer)