How to write a vector layer with defined WKB type

Question

I would like to save a selection of a polygon vector layer to a newly created line vector layer. Manually, this can be done by: Right-Click on vector layer > Save As > Geometry Type > Line String.

How would I get the same result with PyQGIS?

Until now this works, but creates a polygon vector layer (same as input):

QgsVectorFileWriter.writeAsVectorFormat(vlayer, lines, "utf-8", None, "ESRI Shapefile", 1)

From https://qgis.org/api/classQgsVectorFileWriter.html, I don't understand how or if it is possible to overwrite the output WKB type to MultiLineString. I tried several versions of:

QgsVectorFileWriter.writeAsVectorFormat(vlayer, lines, "utf-8", None, "ESRI Shapefile", 1, Qgis.WKBMultiLineString)

Does anyone know how to do this?

Answer 1

You're close. Try:

QgsVectorFileWriter.writeAsVectorFormat( vl, "/tmp/lines.shp", "utf-8", None, "ESRI Shapefile", True, overrideGeometryType=QgsWKBTypes.MultiLineString )

As you're not providing all parameters between onlySelected and overrideGeometryType, you need to pass the geometry type as named parameter (parameter name and value).

overrideGeometryType=QgsWKBTypes.MultiLineString

From the URL you mentioned, you can see that the expected geometry type should be taken from the QgsWKBTypes.Type enum.