basic example how to write getfeatureinfo response to selected features of a vectorlayer in QGIS / pyqgis

grab_featureinfo_pyqgis.py

# coding: utf-8
from qgis.PyQt.QtCore import Qt
from qgis.gui import QgsMapTool
from qgis.utils import iface


class SendPointToolCoordinates(QgsMapTool):
    """ Enable to return coordinates from clic in a layer.
    """
    def __init__(self, canvas, layer):
        """ Constructor.
        """
        QgsMapTool.__init__(self, canvas)
        self.canvas = canvas
        self.layer = layer
        self.setCursor(Qt.CrossCursor)

    def canvasReleaseEvent(self, event):
        point = self.toLayerCoordinates(self.layer, event.pos())
        
        print(point.x(), point.y())
        
        wmsLayer = iface.activeLayer()
        # Change with your coordinates
        ident = self.layer.dataProvider().identify(QgsPointXY(point.x(),point.y()), QgsRaster.IdentifyFormatFeature)

        # You can look at ident.results() and the features
        #print(ident.results()[0][0].features())

        # To see one feature from the getfeatureinfo behind the scene
        fields = [f.name() for f in ident.results()[0][0].features()[0].fields()]
        attributes = ident.results()[0][0].features()[0].attributes()
        featureinfo_dict=dict(zip(fields, attributes))
        print(featureinfo_dict)
        print(featureinfo_dict['Art'])
        layers = QgsProject.instance().mapLayersByName('testlayer') 
        layer = layers[0]  # We assume there is at least 1

        field_idx = layer.fields().indexOf('testfeld')
        new_value = featureinfo_dict['Art']
        
        #version1: vectorlayer has to be in editing mode and you have to save the changes yourself
        for feature in layer.selectedFeatures():
            feature["testfeld"] = new_value
            layer.updateFeature(feature)
            
        #version2: vectorlayer will be set to edit-mode and changes commited without asking you
        #with edit(layer):
        #    for feat_id in layer.selectedFeatureIds():
        #       print(feat_id)
        #        layer.changeAttributeValue(feat_id, field_idx, new_value)
        

layer, canvas = iface.activeLayer(), iface.mapCanvas()

send_point_tool_coordinates = SendPointToolCoordinates(
    canvas,
    layer
)
canvas.setMapTool(send_point_tool_coordinates)

#use: create a vectorlayer named testlayer with a field called 'testfeld' (type string).
#active Layer before running the code: WMS-Layer with enabled getfeatureinfo. example https://geodienste.ch/db/av_0 , layers=Aggregierte_Amtliche_Vermessung

#based on
#https://github.com/webgeodatavore/pyqgis-samples/blob/master/gui/qgis-sample-QgsMapTool.py
#https://gis.stackexchange.com/questions/102119/changing-the-value-of-attribute-using-qgsfeature-in-pyqgis