Setting feature attribute by name via QGIS python api?

Question

I try to set feature attributes by attribute name with this code

pr = layer.dataProvider()
pr.addAttributes([QgsField("test", QVariant.Int)])
layer.updateFields()
for feature in layer.getFeatures():
 attrName = 'test'
 feature[attrName] = 1

but in result I have NULL in all fields. If I use

feature.setAttributes([1])

it works properly.

What I do wrong? Why feature[attrName] = 1 doesn't work?

Update: Find this solution

pr = layer.dataProvider()
pr.addAttributes([QgsField("test", QVariant.Int)])
layer.updateFields()
for feature in layer.getFeatures():
 attrName = 'test'
 feature[attrName] = 1
 pr.changeAttributeValues({feature.id() : {pr.fieldNameMap()[attrName] : 1}})

Answer 1

QGIS can use field names and indexes:

feature['fieldname'] = 10
feature[1] = 10

Make sure you are in edit mode before you do anything on the layer:

layer.startEditing()
feature['fieldname'] = 10
layer.updateFeature(feature)
#Call commit to save the changes
layer.commitChanges()

Answer 2

AFAIK, QGIS uses field indexes, not field names, for assignment. Try using feature[layer.fieldNameIndex(attrName)] = 1.

• dmitry.v.kiselev: I try: feature.setAttribute(attrName, 1) and feature.setAttribute(layer.fieldNameIndex(attrName), 1) with same results (api reference have two setAttribute methods for feature with index and name)
• dmitry.v.kiselev: feature[layer.fieldNameIndex(attrName)] = 1 also the same.
• Jan Šimbera: Have you initialised the attributes (feature.initAttributes(len(attributes)))?
• dmitry.v.kiselev: Add feature.initAttributes(1) with same result. But check feature[attrName] and got 1 which I looking for. But still have NULL if I open Attribute Table in gui.