PyQGIS batch input for data attribute table

Question

I have around 50 layers, which are going to have exactly the same column names and their content.

I found a way how to create the new columns in a batch way, which is presented here:

gis.stackexchange.com/questions/344779/applying-field-calculator-for-multiple-layers-at-once

but the Python code doesn't include the content, which would go inside of the newly created column.

I would like to pick the layer name up and put it into the table field, like shown here:

Is there a way to label a layer using its file name or layer name?

so my code was looking like this:

  project_layers = QgsProject.instance().mapLayers().values()


 for layer in project_layers :
  layer.startEditing()
  layer.dataProvider().addAttributes([QgsField("area", QVariant.Double)])
  layer.updateFields()


   for feature in layer.getFeatures():
    area = "layer_property( @layer_id, 'name')"
    feature.setAttributes([area])
    layer.updateFeature(feature)


    layer.commitChanges()

but it throws error.

How can I provide the content to the newly created column?

Answer 1

A couple of things.

• You are trying to update a QVariant.Double field with a string
• The string you use is just that, a string of an expression which is not evaluated and does not get the layer name
• Some confusion with field names. You talk of layer name but the field is called area

changeAttributeValue can be used to update a feature's attributes. It takes the feature id, the field index to change, and the new value:

project_layers = QgsProject.instance().mapLayers().values()

for layer in project_layers :
    layer.startEditing()

    new_field_name = 'layer_name'
    
    # changed QVariant to String to accept the layer name
    layer.dataProvider().addAttributes([QgsField(new_field_name, QVariant.String)])
    layer.updateFields()
    
    # get the field index of the new field
    fields = layer.fields()
    idx = fields.indexFromName(new_field_name)

    for feature in layer.getFeatures():
        # area = "layer_property( @layer_id, 'name')"   this is just a string literal, it is not getting the layer name
        
        # get layer name
        name = layer.name()
        
        # update the feature attribute at the index of the new field
        layer.changeAttributeValue(feature.id(), idx, name)

    layer.commitChanges()