Getting error while adding few layers to GeoPackage using PyQGIS?

Question

I get error message when trying enter few layers to geopackage using PyQGIS 3.6.2.

This is my code:

from qgis.core import   QgsVectorFileWriter 
options = QgsVectorFileWriter.SaveVectorOptions()
options.actionOnExistingFile = QgsVectorFileWriter.CreateOrOverwriteLayer  
options.EditionCapability = QgsVectorFileWriter.CanAddNewLayer  
options.onlySelectedFeatures = True 
GpName = 'c:\\qgis_dat\\proj\\proj1' 

for layer in  iface.mapCanvas().layers() :  
    namee = layer.name()  # it can be 'roads' or 'buildings' ...
    options.layerName =  namee    
    _writer = QgsVectorFileWriter.writeAsVectorFormat(layer, GpName, options )
    print (_writer) 
    # print the error's message : 
    #    (2, 'Opening of data source in update mode failed (OGR error: )')  .
    # the geoPackage and layers are not created

Answer 1

If the file does not exists the create file option has to be set:

    if not os.path.exists(geo_package): # if the ouput file doesn't already exist
        options.actionOnExistingFile = QgsVectorFileWriter.CreateOrOverwriteFile
    else:
        options.actionOnExistingFile = QgsVectorFileWriter.CreateOrOverwriteLayer

Answer 2

If you're using QGIS 3.x-3.16, use QgsVectorFileWriter.writeAsVectorFormatV2(). If you're using QGIS 3.20 or beyond, QgsVectorFileWriter.writeAsVectorFormatV3()

Here's how your code might look ( I cleaned it up a little) using .writeAsVectorFormatV2(). It's important that GpName be a complete path, not just a directory. One big difference is that the V2() function needs a context argument:

from qgis.core import QgsVectorFileWriter 
options = QgsVectorFileWriter.SaveVectorOptions()
options.actionOnExistingFile = QgsVectorFileWriter.CreateOrOverwriteLayer  
options.onlySelectedFeatures = True 
GpName = 'c:\\qgis_dat\\proj\\proj1\\test_01.gpkg' # full path here 

for layer in  iface.mapCanvas().layers() :  
    context = QgsProject.instance().transformContext()
    options.layerName = layer.name()  # it can be 'roads' or 'buildings' ...   
    _writer = QgsVectorFileWriter.writeAsVectorFormatV2(layer, GpName, context, options)
    print (_writer)
    if _writer[0] == QgsVectorFileWriter.NoError:
        print("Success!\n")
    else:
        print(_writer)

Good luck, hope that helps.