qgis - Getting the output layer reference returned by Processing tool

After running Processing algorithm ending up with a layer by runalg or runandload in QGIS2, 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.

If it would be a method like processing.run*(...) that returns an instance of output, not path or not 'memory:name', we would create the layer reference by writing just layer = processing.run*(...)

Is there any processing.run*() method that returns layer instance/reference directly, e.g. QgsVectorLayer?

Answer

QGIS 3:

• 
  

  OPTION 1: run() method with memory output

  
  

  result = processing.run("native:buffer",
                          {'INPUT':'D:/foo/bar.shp', 
                           ... # other params
                           'OUTPUT':'memory:buffer' # or 'OUTPUT':'TEMPORARY_OUTPUT'
                          })
  
  # OUTPUT
  
  #`result = {'OUTPUT': }`
  

  
  

  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":"memory:"})["OUTPUT"]

result_layer is now . Since it's a memory layer, if necessary, it has to be added using addMapLayer() method.

Other Processing Options

• 
  

  OPTION 2: run() method with file output

  
  

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

  
  

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

  
  

---

QGIS 2:

• 
  

  OPTION 5: 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 6: 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 7: 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 8: 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 reference for output in QGIS 2.