"DeprecationWarning: QgsVectorFileWriter.writeAsVectorFormat() is deprecated" when exporting GeoJSON in QGIS

Question

I am trying to export a layer same to like this question: https://gis.stackexchange.com/a/435753/215676

But the difference is I'd like to do it in PyQGIS with QgsVectorFileWriter. Is there a way to use that 'Use Key/Value' option in codes?

Here is my command which is not working:

QgsVectorFileWriter.writeAsVectorFormat(
    layer,
    outfile,
    'utf-8',
    QgsCoordinateReferenceSystem('EPSG:4326'),
    'GeoJSON',
    layerOptions=['COORDINATE_PRECISION=8'],
    onlySelected=True
    )

And I have this warning:

'DeprecationWarning: QgsVectorFileWriter.writeAsVectorFormat() is deprecated'

As I understand because of having those JSON fields, it works without JSON fields.

Answer 1

'DeprecationWarning: QgsVectorFileWriter.writeAsVectorFormat() is deprecated' is a warning, not an error. If you check the documentation for that method you see that using QgsVectorFileWriter.writeAsVectorFormatV3() is recommended instead.

Once you adjust your code to use that method, the "error" will disappear.