qgis - Troubles Updating Values of Selected Features with PyQGIS

I'm trying to modify the values of some selected features. I have written this small script, but nothing happens when I run it. Is there something that I've missed? Here is my code.

lyr = qgis.utils.iface.activeLayer()
features = lyr.selectedFeatures()

lyr.startEditing()
lyr.updateFields()
for f in features:

    fid= f.id()
    lyr.changeAttributeValue(fid,12,"0")
    lyr.updateFeature(f)

print "elemento alterado"

lyr.commitChanges()

Answer

Just remove the lyr.updateFeature(f) line that is inside your for loop. You don't need it and, according to the docs, it's also inefficient.

If that doesn't work, make sure the 13th field is of type String.