Add geometry Attributes (area and perimeter) using PyQGIS

Question

So this is my code :

    from qgis import processing
    
    inlayDir = iface.activeLayer()
    
    Sp_Offset = 'E:/QGIS/General/20230117_Boundaries R8/PVD Hectares/PH_RPSOffset8-0_AEB.gpkg'
    Sp ='E:/QGIS/General/20230117_Boundaries R8/PVD Hectares/PH_RPS8-0_AEB.gpkg' 
    
    dissolved=processing.runAndLoadResults("native:dissolve",
                                 {'INPUT': inlayDir, 
                                  'FIELD':[],
                                  'OUTPUT': 'TEMPORARY_OUTPUT'})
                                  
    
    
    UN_Sp_Offset = processing.runAndLoadResults( "qgis:union",{
                            'INPUT' : Sp_Offset, 
                            'OUTPUT' : 'TEMPORARY_OUTPUT', 
                            'OVERLAY' : dissolved['OUTPUT'], 
                            'OVERLAY_FIELDS_PREFIX' : '' })
    
                            
    Clip = processing.runAndLoadResults("native:clip",
                                { 'INPUT':UN_Sp_Offset['OUTPUT'],
                                'OVERLAY' : dissolved['OUTPUT'],
                                'OUTPUT' : 'TEMPORARY_OUTPUT',
                                })
    
                                    
    
    # create an instance of QgsDistanceArea
    distance_area = QgsDistanceArea()
    
    # create the fields list
    fields = QgsFields()
    fields.append(QgsField('area', QVariant.Double))
    
    inlayDir = iface.activeLayer()
    
    fields.append(QgsField('perimeter', QVariant.Double))
    
    # start editing the vector layer
    with edit(clip):
        # add the fields
        for field in fields:
            vl.addAttribute(field)
            
        # loop into the features
        for feature in vl.getFeatures():
            geometry = feature.geometry()
            
            # update the feature with teh QgsDistanceArea calculation
            feature['area'] = distance_area.measureArea(geometry)
            feature['perimeter'] = distance_area.measurePerimeter(geometry)
            
            Clip.updateFeature(feature)

# add the layer to the map
project.addMapLayer(Clip)

and I want to add "Area and Perimeter" in another column in my attribute table next to my columns:

Error Message:

# Python Console
# Use iface to access QGIS API interface or type help(iface) for more info
# Security warning: typing commands from an untrusted source can harm your computer
exec(Path('C:/Users/CMCA/AppData/Local/Temp/tmp8h99ttbs.py').read_text())
Traceback (most recent call last):
  File "C:\PROGRA~1\QGIS32~1.0\apps\Python39\lib\code.py", line 90, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
  File "<string>", line 15, in <module>
  File "C:\PROGRA~1/QGIS32~1.0/apps/qgis/./python\qgis\core\additions\edit.py", line 38, in __enter__
    assert self.layer.startEditing()
AssertionError
exec(Path('C:/Users/CMCA/AppData/Local/Temp/tmpd0rbj1lv.py').read_text())
Traceback (most recent call last):
  File "C:\PROGRA~1\QGIS32~1.0\apps\Python39\lib\code.py", line 90, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
  File "<string>", line 42, in <module>
  File "C:\PROGRA~1/QGIS32~1.0/apps/qgis/./python\qgis\core\additions\edit.py", line 38, in __enter__
    assert self.layer.startEditing()
AssertionError
exec(Path('C:/Users/CMCA/AppData/Local/Temp/tmplsle_e88.py').read_text())
Traceback (most recent call last):
  File "C:\PROGRA~1\QGIS32~1.0\apps\Python39\lib\code.py", line 90, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
  File "<string>", line 42, in <module>
  File "C:\PROGRA~1/QGIS32~1.0/apps/qgis/./python\qgis\core\additions\edit.py", line 38, in __enter__
    assert self.layer.startEditing()
AssertionError
exec(Path('C:/Users/CMCA/AppData/Local/Temp/tmp2ubmh9vo.py').read_text())
Traceback (most recent call last):
  File "C:\PROGRA~1\QGIS32~1.0\apps\Python39\lib\code.py", line 90, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
  File "<string>", line 42, in <module>
  File "C:\PROGRA~1/QGIS32~1.0/apps/qgis/./python\qgis\core\additions\edit.py", line 38, in __enter__
    assert self.layer.startEditing()
AssertionError

Can someone help me with the coding?

Answer 1

If you want to use Processing algorithms you can concatenate many algorithms.

In you case:

from qgis import processing

clip = processing.run("native:clip",
                        { 'INPUT':union['OUTPUT'],
                        'OVERLAY' : dissolved['OUTPUT'],
                        'OUTPUT' : 'TEMPORARY_OUTPUT',
                        })['OUTPUT']

processing.runAndLoadResults(
    "qgis:exportaddgeometrycolumns", 
    {
        'INPUT':clip,
        'CALC_METHOD':0,
        'OUTPUT':'TEMPORARY_OUTPUT'
    }
)

Or you can use standard pyqgis code like the following:

clip = processing.run("native:clip",
                            { 'INPUT':union['OUTPUT'],
                            'OVERLAY' : dissolved['OUTPUT'],
                            'OUTPUT' : 'TEMPORARY_OUTPUT',
                            })['OUTPUT']

# get the project instance
project = QgsProject.instance()

# create an instance of QgsDistanceArea
distance_area = QgsDistanceArea()

# create the fields list
fields = QgsFields()
fields.append(QgsField('area', QVariant.Double))
fields.append(QgsField('perimeter', QVariant.Double))

# start editing the vector layer
with edit(clip)
    # add the fields
    for field in fields:
        vl.addAttribute(field)
        
    # loop into the features
    for feature in vl.getFeatures():
        geometry = feature.geometry()
        
        # update the feature with teh QgsDistanceArea calculation
        feature['area'] = distance_area.measureArea(geometry)
        feature['perimeter'] = distance_area.measurePerimeter(geometry)
        
        clip.updateFeature(feature)

# add the layer to the map
project.addMapLayer(clip)