[GIS] How to add new columns with attributes in existing Qgis layer

Question

How to add new columns with attributes in existing Qgis layer?
Doing the "same thing" here described

Answer 1

To add info to an existing layer instead of in memory layer you need to point to a layer on disk with QgsVectorLayer:

vl = QgsVectorLayer("C:\Temp\pt.shp", "pt", "ogr") 

Then you may continue to reference the code snippet from this Q/A:

How to create a new empty vector layer programmatically?