How to read the error when writing vector files in QGIS 3.0

Question

Upgraded to QGIS 3.0 and now I cant read writer errors. The following (post 2to3) code doesnt print the error to the console:

CRS=QgsCoordinateReferenceSystem().fromEpsgId(4326)
error = QgsVectorFileWriter.writeAsVectorFormat(vl,fp, "system", CRS, "ESRI Shapefile")
if QgsVectorFileWriter.hasError(error):
    print("uh oh an error")
    print(QgsVectorFileWriter.errorMessage(error))

rather than reporting the error I get:

TypeError: QgsVectorFileWriter.hasError(): first argument of unbound method must have type 'QgsVectorFileWriter'

How can I modify this so that the write error is reported?

Answer 1

Have a look at the API Documentation which mentions the following signature for this method:

writeAsVectorFormat(layer: QgsVectorLayer, fileName: str, fileEncoding: str, destCRS: QgsCoordinateReferenceSystem = QgsCoordinateReferenceSystem(), driverName: str = '', onlySelected: bool = False, datasourceOptions: Iterable[str] = [], layerOptions: Iterable[str] = [], skipAttributeCreation: bool = False, newFilename: str = '', symbologyExport: QgsVectorFileWriter.SymbologyExport = QgsVectorFileWriter.NoSymbology, symbologyScale: float = 1, filterExtent: QgsRectangle = None, overrideGeometryType: QgsWkbTypes.Type = QgsWkbTypes.Unknown, forceMulti: bool = False, includeZ: bool = False, attributes: Iterable[int] = [], fieldValueConverter: QgsVectorFileWriter.FieldValueConverter = None) → Tuple[QgsVectorFileWriter.WriterError, str]

The last part is interesting here, it's the return value: → Tuple[QgsVectorFileWriter.WriterError, str]

Tuples can be read like this:

error, error_string = QgsVectorFileWriter.writeAsVectorFormat(....)

so you probably want to use something like this

error, error_string = QgsVectorFileWriter.writeAsVectorFormat(....)
if error != QgsVectorFileWriter.NoError:
    print('Oh an error has happened: {details}'.format(details=error_string))