Updating layer attribute in QGIS processing script

Question

In a QGIS processing script I try to update an attribute value in a feature with changeAttributeValue but I am not able to.

I'm following: https://courses.spatialthoughts.com/pyqgis-in-a-day.html#processing-scripts

from PyQt5.QtCore import QCoreApplication
from qgis.core import (QgsProcessing,
 QgsProcessingAlgorithm,
 QgsProcessingParameterFeatureSource,
 QgsProcessingParameterField,
 QgsProcessingParameterFileDestination,
 )
#from qgis import processing
class SaveAttributesAlgorithm(QgsProcessingAlgorithm):
 """Saves the attributes of a vector layer to a CSV file."""
 OUTPUT = 'OUTPUT'
 INPUT = 'INPUT'
 X_FIELD = 'x_field' # champ centroid X
 Y_FIELD = 'y_field' # champ centroid Y
 SURFACE_FIELD = 'surface_field' # champ surface
def initAlgorithm(self, config=None):
 self.addParameter(
 QgsProcessingParameterFeatureSource(
 self.INPUT,
 self.tr('Input layer'),
 [QgsProcessing.TypeVectorAnyGeometry]
 )
 )
 self.addParameter(
 QgsProcessingParameterField(
 self.X_FIELD,
 'Champ centroide X',
 '',
 self.INPUT))
 self.addParameter(
 QgsProcessingParameterField(
 self.Y_FIELD,
 'Champ centroide Y',
 '',
 self.INPUT))
 self.addParameter(
 QgsProcessingParameterField(
 self.SURFACE_FIELD,
 'Champ Surface',
 '',
 self.INPUT))
def processAlgorithm(self, parameters, context, feedback):
 source = self.parameterAsSource(parameters, self.INPUT, context)
 csv = self.parameterAsFileOutput(parameters, self.OUTPUT, context)
 
 x_field = self.parameterAsString(
 parameters,
 self.X_FIELD,
 context)
 y_field = self.parameterAsString(
 parameters,
 self.Y_FIELD,
 context)
 surface_field = self.parameterAsString(
 parameters,
 self.SURFACE_FIELD,
 context)
 fieldnames = [field.name() for field in source.fields()]
 # Compute the number of steps to display within the progress bar and
 # get features from source
 total = 100.0 / source.featureCount() if source.featureCount() else 0
 
 features = source.getFeatures()
 
 fieldIdx = source.fields().indexFromName(surface_field)
 feedback.pushInfo(str(fieldIdx))
 feedback.pushInfo(surface_field)
 feedback.pushInfo(x_field)
 feedback.pushInfo(y_field)
 
 for current, f in enumerate(features):
 # Stop the algorithm if cancel button has been clicked
 if feedback.isCanceled():
 break
 feedback.pushInfo(str(f.geometry().centroid()))
 feedback.pushInfo(str(f.geometry().area()))
 feedback.pushInfo(str(f.id()))
 #changeAttributeValue(f.id(),source.fields().indexFromName(surface_field), f.geometry().area()) 
 
 
 # Update the progress bar
 feedback.setProgress(int(current * total))
 return {self.OUTPUT: csv}
def name(self):
 return 'save_attributes'
def displayName(self):
 return self.tr('Update X,Y,area')
def group(self):
 return self.tr(self.groupId())
def groupId(self):
 return ''
def tr(self, string):
 return QCoreApplication.translate('Processing', string)
def createInstance(self):
 return SaveAttributesAlgorithm()

• Bera: What do you mean by not able? Do you get errors?
• Alex: In tried features.changeAttributeValue(f.id(),source.fields().indexFromName(surface_field), f.geometry().area()) but it doesnt work
• Alex: the error is Traceback (most recent call last): File "<string>", line 90, in processAlgorithm AttributeError: 'QgsFeatureIterator' object has no attribute 'changeAttributeValue'
• Alex: Thanks for the ansewer but f.changeAttributeValue gives AttributeError: 'QgsFeature' object has no attribute 'changeAttributeValue'
• Germán Carrillo: You cannot call changeAttributeValue from features (which is a QgsFeatureIterator). You need to call it from a QgsVectorLayer (source object in your case) if you open an edit session. That's well documented in the PyQGIS Cookbook. Have a look at: docs.qgis.org/testing/en/docs/pyqgis_developer_cookbook/…

Answer 1

I found the way by replacing source = self.parameterAsSource(parameters, self.INPUT, context) with source = self.parameterAsVectorLayer(parameters, self.INPUT, context)