QGIS attribute data updating by not commiting

Question

I am trying to change the values of an attribute in one of my layers. My code is as follows:

from qgis.utils import iface
from PyQt4.QtCore import *
layer =iface.activeLayer()
pr=layer.dataProvider()
layer.startEditing()
for f in layer.getFeatures():
    geom=f.geometry()
    xval= geom.asPoint().x()
    f['XX']=xval
    layer.commitChanges()
    print f['XX']
for f in layer.getFeatures():
    print f['XX']

For my first print statement, I am getting the values out that I want, but when I run the second print statement, the values are what they were before running the code.

Am I Not committing changes correctly?

Answer 1

You can't directly edit a feature returned by getFeatures() - those are copies of the underlying layer features, so any changes you make won't actually affect the layer itself. What you need to do is use the QgsVectorLayer.changeAttributeValues method here. So something like:

xx_field_index = 0 # or whatever the index is for the field you want to change
layer.startEditing()
for f in layer.getFeatures():
    geom=f.geometry()
    xval=geom.asPoint().x()
    layer.changeAttributeValue(f.id(), xx_field_index, xval)

layer.commitChanges()