PyQGIS: using changeAttributeValues on new feature

Question

I try to write a Python script with PyQGIS to update automatically a field when a feature is added or when a geometry is changed. It's work well with the geometryChanged listener but not with the featureAdded listener : For a new feature the QgsFeatureId is negative before a commit (-2 for example) and it seems that changeAttribute does not like that :

Layer point: Feature -2 for attribute update not found.

How can I update a field automatically before a commit?

Here is my code :

#### variables ####
layerName = "point"
fieldName = "test"
## create layer object
layer = (QgsMapLayerRegistry.instance().mapLayersByName(layerName))[0]
## function updateField
def updateField(QgsFeatureId):
 field = layer.fieldNameIndex(fieldName)
 layer.dataProvider().changeAttributeValues({QgsFeatureId:{field:"updateTest"}})
 
## listeners
layer.geometryChanged.connect(updateField)
layer.featureAdded.connect(updateField)

I work with QGIS 2.14.4

• Germán Carrillo: By the way, unless you definitely need to implement this by yourself in a script, you could use the AutoFields plugin, which handles automatic vector field updates for you.
• Maxime Bescu: Thanks, I tested your plugin, it works well but the attributes are not updated until you commit your edit change.
• Germán Carrillo: Right, they're not saved until you save. That something we could adjust for a next version. :)

Answer 1

Have a look at this post about listening to featureAdded in a safe way.

In your case, this should work:

layerName = "point1"
fieldName = "test"
addedFeatures = []
layer = (QgsMapLayerRegistry.instance().mapLayersByName(layerName))[0]
field = layer.fieldNameIndex(fieldName) 
def storeFeaturesIds(featId):
 addedFeatures.append(featId)
def updateAttributesAfterAdding():
 while addedFeatures:
 featureId = addedFeatures.pop()
 layer.beginEditCommand('Updating test field')
 layer.changeAttributeValue(featureId, field, "updateTest")
 layer.endEditCommand()
layer.featureAdded.connect(storeFeaturesIds)
layer.editCommandEnded.connect(updateAttributesAfterAdding)