PyQGIS update field in line dataset when it is within a polygon

I am trying to update a field in a (Road centerline) table based on a value from another table when it is within a Polygon (Locality boundaries).

Based on posts like QGIS layer.geometry.intersection() not finding intersections between layers I have the following code. Both datasets are in EPSG:28355. It never prints ntext=b['Locality_N'] - so obviously something is wrong with the if within statement.

    import re

    def Update_Field(fieldName, ntext): # Function to simplify updating fields

        print (ntext)
        fieldIndex = layer.fields().indexFromName(fieldName)
        dpr.changeAttributeValues({f.id(): {fieldIndex: ntext}})
        layer.commitChanges()

    RoadType_dict = {'Rd':'Road', 'Avenue': 'Ave'}
    layer = iface.activeLayer()
    selection = layer.getFeatures() #all features
    #selection = layer.selectedFeatures() #only selected features


    #Locality Data
    Locality_layer=QgsVectorLayer(r'Z:\My Drive\Mangoesmapping\Spatial Projects\Gen_Library\DSC\vector\DSC_Localities_areas.shp', "Locality", "ogr")

    dpr = layer.dataProvider()

    for f in selection:
        print (f.id())
        text=f['RoadMntnc']
        print (text)
        stext=re.split('_+', text)


    ##Update Road Type
    #    ntext=RoadType_dict.get(stext[-1:][0])
    #    fieldName='Type'
    #    Update_Field(fieldName,ntext)

    ##Update LocalityName
        for b in Locality_layer.getFeatures():
            if f.geometry().within(b.geometry()):
                print (b.geometry())

                ntext=b['Locality_N']
                fieldName="Locality"
                Update_Field(fieldName,ntext)

    layer.commitChanges()
    layer.updateFields()

I have commented out the code that works but is not required for these tests.

--- Based on another answer I also tried

e = QgsExpression( "geomwithin('Locality_layer','Locality_N') ")

f[fieldName] = e.evaluate()
layer.updateFeature(f)

But the values aren't updated in the table.

Answer

After ensuring you loaded your vector layer, you could use the following:

##Update LocalityName
e = QgsExpression( "geomwithin('Locality','Locality_N') ")
f[fieldName] = e.evaluate()
layer.updateFeature(f)