Getting the output layer reference returned by Processing tool

Question

After running Processing algorithm ending up with a layer by run or runAndLoadResults in QGIS 3, these methods return a result which is mostly a dictionary like {'OUTPUT': 'file_path'} etc. So, after result = processing.run*(...), I can get the output by result['OUTPUT']. But its value is mostly just a string.

Is there a way method that returns layer instance/reference directly, e.g. QgsVectorLayer, so that we can get the layer reference by writing just layer = processing.run*(...)["OUTPUT"]?

Answer 1

Please note that all solutions explained here are related to vector layer used as the input and QGIS native algorithms. The answer doesn't explain the results of other providers' tools. (GRASS, GDAL, SAGA, ...). They may be different from native ones.

QGIS 3:

• OPTION 1: run() method with memory output:

    result = processing.run("native:buffer",
                            {'INPUT':'D:/foo/bar.shp', 
                             ... 
                             'OUTPUT':'TEMPORARY_OUTPUT'
                            })
  
    # OUTPUT
    #`result = {'OUTPUT': <qgis._core.QgsVectorLayer object at 0x00...>}`
  

Result is a dictionary. result['OUTPUT'] gives an instance of a layer (QgsVectorLayer). No layer is added. Option 1 is one and only solution that returns a reference for QgsVectorLayer in QGIS 3.

It can be used in the following way:

result_layer = processing.run("alg_name", {...,  "OUTPUT":'TEMPORARY_OUTPUT'})["OUTPUT"]

result_layer is now qgis._core.QgsVectorLayer. Since it's a memory layer, it should be added to the project using addMapLayer() method.

Other Processing Options

• OPTION 2: run() method with file output:

    result = processing.run("native:buffer",
                            {'INPUT':'D:/foo/bar.shp', 
                             ... 
                             'OUTPUT':'c:/foo/baz.shp'})
  
    # OUTPUT
    # `result = {'OUTPUT': 'c:/foo/baz.shp'}`
  

Result is a dictionary, value is a string. No layer is added.

• OPTION 3: runAndLoadResults() method with file output

    result = processing.runAndLoadResults("native:buffer",
                                          {'INPUT':'D:/foo/bar.shp', 
                                           ... 
                                           'OUTPUT':'c:/foo/baz.shp'})
  
    # OUTPUT
    # `result = {'OUTPUT': 'c:/foo/baz.shp'}`
  

Result is a dictionary, value is a string. A layer is added.

• OPTION 4: runAndLoadResults() method with memory output

    result = processing.runAndLoadResults("native:buffer",
                                          {'INPUT':'D:/foo/bar.shp', 
                                           ... 
                                           'OUTPUT':'TEMPORARY_OUTPUT'
                                          })
  
    # OUTPUT
    # `result = {'OUTPUT': 'buffer_0ae....'}`
  

Result is a dictionary, value is a string. A layer is added.

---

QGIS 2: (Old version)

• OPTION 1: runandload() method with file output

    result = processing.runandload("qgis:fixeddistancebuffer",
                                   "c:/foo/bar.shp", 10, 5, False,
                                   "c:/foo/baz.shp")
    # OUTPUT
    # `result = <*****.FixedDistanceBuffer instance at 0x00...>`
  

Result is an instance of related algorithm class. A layer is added.

• OPTION 2: runandload() method with memory output

    result = processing.runandload("qgis:fixeddistancebuffer",
                                   "c:/foo/bar.shp", 10, 5, False,
                                   "memory:mem_layer")
    # OUTPUT
    # `result = <*****.FixedDistanceBuffer instance at 0x00...>`
  

Result is an instance of related algorithm class. A layer is added.

• OPTION 3: runalg() method with file output

    result = processing.runalg("qgis:fixeddistancebuffer",
                               "c:/foo/bar.shp", 10, 5, False,
                               "c:/foo/baz.shp")
    # OUTPUT
    # `result = {'OUTPUT': "c:/foo/baz.shp"}`
  

Result is a dictionary, value is a string. No layer is added.

• OPTION 4: runalg() method with memory output

    result = processing.runalg("qgis:fixeddistancebuffer",
                               "c:/foo/bar.shp", 10, 5, False,
                               "memory:mem_layer") 
  
    # OUTPUT
    # `result = {'OUTPUT': "memory:mem_layer"}`
  

Result is a dictionary, value is a string. No layer is added.

Neither runalg nor runandload returns a layer reference/instance for output in QGIS 2.

Answer 2

If you want to save the output file on the processing output folder, please see this example:

1 - where is the processing output folder (QGis3)? Settings -> User Profiles -> Open Active Profile Folder -> "processing" folder -> "outputs" folder

2 - save the output of this processing algorithm on the above "output" folder and put it on the TOC (or legend):

    #.......................
self.addParameter(QgsProcessingParameterVectorLayer('Areadeestudo', 'Area de estudo', types=[QgsProcessing.TypeVectorPolygon], defaultValue=None))
self.addParameter(QgsProcessingParameterVectorLayer('COS', 'COS', types=[QgsProcessing.TypeVectorPolygon], defaultValue=None))
self.addParameter(QgsProcessingParameterVectorDestination('CosAreaEstudo', 'COS Area Estudo', type=QgsProcessing.TypeVectorAnyGeometry, createByDefault=True, defaultValue='COS Area Estudo.shp'))
    
    #......................... other algorithms, etc
    
    # Polygon clipping2
            alg_params = {
                'CLIP': parameters['Areadeestudo'],
                'S_INPUT': parameters['COS'],
                'S_OUTPUT': parameters['CosAreaEstudo']
            }
            outputs['PolygonClipping2'] = processing.run('saga:polygonclipping', alg_params, context=context, feedback=feedback, is_child_algorithm=True)
            results['CosAreaEstudo'] = outputs['PolygonClipping2']['S_OUTPUT']
    
    #..............................

The important factor here is the [defaultValue='COS Area Estudo.shp']

If the output is a raster, and if the processing algorithm is a SAGA one, the output file must be written [name of the raster file].sdat ; if the processing algorithm is GDAL or QGis native, you may use *.tif