Using SAGA tool in PyQGIS standalone script. Output doesn't return vector layer

Question

In my standalone script I want to use the SAGA tool "merge vector layers". According to the docs I need the following.

Thus, I wrote the following code to implement the tool:

def merge(self):
    Parcel_channel2 = self.Parcel_Channel2()
    intersection = self.intersection()
    param = {'INPUT': [Parcel_channel2, intersection],
             'MERGED': 'TEMPORARY_OUTPUT',
             'SRCINFO': False,
             'MATCH': True,
             }
    merge = processing.run('saga:mergevectorlayers', param)
    vlayer = merge['MERGED']
    QgsProject.instance().addMapLayer(vlayer)
    return vlayer

When I try to run the script I get the following error message:

QgsProject.addMapLayer(): argument 1 has unexpected type 'str'

With native Qgs tools, this code works fine. However, it seems that SAGA doesn't return a VectorLayer but just the string in my 'outDir' variable. How do I get the output when using SAGA?

I'm using QGIS 3.10

Edit: I figured out, that native Qgs tools retun QgsMapLayer objects and saga tools return the path and name of a .shp file

C:/Users/denni/AppData/Local/Temp/processing_c0c1104bbc4447558e00113072ccc4d8/91e580e9c006429597540594cb000407/MERGED.shp

but I need a QgsMapLayer as result... how would I do that?

Answer 1

If you print the vlayer variable, you will see it's a string referencing the QGIS layer path, so the correction would be the following (standalone, tested with only two separated points layers)

layers = QgsProject.instance().mapLayers()
param = {'INPUT': list(layers.values()),
         'MERGED': 'TEMPORARY_OUTPUT',
         'SRCINFO': False,
         'MATCH': True,
         }
merge = processing.run('saga:mergevectorlayers', param)
Here, it's the layer path not a vector layer
layer_path = merge['MERGED']
The main change to fix your issue
vlayer = QgsVectorLayer(layer_path, "My new merged layer", "ogr")
QgsProject.instance().addMapLayer(vlayer)