Adding today's date to attribute table using PyQGIS

Question

I have the following script to merge a group of layers, and I want to edit the script to add a field containing today's date in the attribute table and also delete some fields by their names?

root = QgsProject.instance().layerTreeRoot()
group = root.findGroup("RL")
layers = [layer.layer() for layer in group.children()] 

merged = processing.run("native:mergevectorlayers",
            {'LAYERS': layers, 'OUTPUT':"TEMPORARY_OUTPUT"})["OUTPUT"]

QgsProject.instance().addMapLayer(merged, False)
root.insertLayer(0, merged)

Answer 1

You can use this script:

from datetime import date


#
# previous lines
#

provider = merged.dataProvider()

## add date field
i = merged.fields().indexFromName("date")
if i == -1:
    provider.addAttributes([QgsField('date', QVariant.Date)])
    merged.updateFields()
##

## populate date field
i = merged.fields().indexFromName("date")
attr_map = {f.id(): {i: QDate(date.today())} for f in merged.getFeatures()}
provider.changeAttributeValues(attr_map)
##

## delete some fields
field_names = ['Shape'] # field names to be deleted
field_indices = [merged.fields().indexFromName(f) for f in field_names]
provider.deleteAttributes(field_indices)
merged.updateFields()
##

QgsProject.instance().addMapLayer(merged, False)
root.insertLayer(0, merged)
``

Answer 2

One easy way is to execute Refactor field manually. Add the field and calculate with the expression now(), and delete whatever fields you like:

Then press Ctrl+Alt+H and copy paste the code:

processing.run("native:refactorfields", {'INPUT':'C:/GIS/data/tempdata/grid.shp','FIELDS_MAPPING':[{'expression': '\"id\"','length': 20,'name': 'id','precision': 0,'type': 6},{'expression': '\"left\"','length': 23,'name': 'left','precision': 15,'type': 6},{'expression': '\"top\"','length': 23,'name': 'top','precision': 15,'type': 6},{'expression': '\"right\"','length': 23,'name': 'right','precision': 15,'type': 6},{'expression': '\"bottom\"','length': 23,'name': 'bottom','precision': 15,'type': 6},{'expression': 'now()','length': 0,'name': 'now','precision': 0,'type': 14}],'OUTPUT':'TEMPORARY_OUTPUT'})