AttributeError when calculating area in hectares in PyQGIS

Question

I want to calculate the area of the polygon and add it to the attribute table as another column. I used this script:

from qgis import processing
    
inlayDir = iface.activeLayer()
P3_boundary = r'C:\Users\CMCA\OneDrive - Boskalis\Documents\Trial PyQGIS\outTemp.gpkg'
    
dissolved = processing.runAndLoadResults("native:dissolve",
                                 {'INPUT': inlayDir, 
                                  'FIELD':[],
                                  'OUTPUT': 'TEMPORARY_OUTPUT'})
                                  
union = processing.runAndLoadResults( "qgis:union",{
                            'INPUT' : P3_boundary, 
                            'OUTPUT' : 'TEMPORARY_OUTPUT', 
                            'OVERLAY' : dissolved['OUTPUT'], 
                            'OVERLAY_FIELDS_PREFIX' : '' })
                            
clip = processing.runAndLoadResults("native:clip",
                                { 'INPUT':union['OUTPUT'],
                                'OVERLAY' : dissolved['OUTPUT'],
                                'OUTPUT' : 'TEMPORARY_OUTPUT',
                                })
                             
provider = clip.dataProvider()

field = QgsField("area3", QVariant.Double)

provider.addAttributes([field])

clip.updateFields()
    
index = provider.fieldNameIndex('area3')
    
for feat in clip.getFeatures():
    provider.changeAttributeValues({feat.id(): {index: feat.geometry().area()}})

result :

The error I get:

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 27, in <module>
AttributeError: 'dict' object has no attribute 'dataProvider'

Answer 1

Since processing.runAndLoadResults gives you a dictionary, you get the error. You have to use processing.run to get a layer and add it to QGIS manually using QgsProject.instance().addMapLayer(clip).

....

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

....

QgsProject.instance().addMapLayer(clip)

Answer 2

You get that error because layer has no method fieldNameIndex, but provider has. Use the following script:

layer = iface.activeLayer()
provider = layer.dataProvider()

field = QgsField("area3", QVariant.Double)
provider.addAttributes([field])
layer.updateFields()

index = provider.fieldNameIndex('area3')

for feat in layer.getFeatures():
    provider.changeAttributeValues({feat.id(): {index: feat.geometry().area()}})

Note: This is the answer to the question before it was changed.