Creating Vector layer from selected features with PyQGIS

Question

In my QGIS plugin I select dynamically features from a Vector layer. And currently I create a new layer (shapefile) from selection on combining all feature into a new feature:

theField = QgsField
self.theString = (os.path.expanduser("~")+'\.qgis\\statsRectangle.shp')
feat = QgsFeature()
shapeLayer2 = QgsVectorLayer(self.theString, "Upstream Area Of Interest", 'ogr')
geomtotSubwatershed = QgsGeometry.fromWkt('GEOMETRYCOLLECTION EMPTY')
nodLayer.setSelectedFeatures(selectFeatureIDlist)
UpstreamGeometry = QgsGeometry.fromWkt('GEOMETRYCOLLECTION EMPTY')

for elem in nodLayer.selectedFeatures():
   UpstreamGeometry = UpstreamGeometry.combine(elem.geometry())

feat.setGeometry(UpstreamGeometry)
QgsMapLayerRegistry.instance().addMapLayer(shapeLayer2)

but combining selected feature take a long time and froze QGIS UI.

Answer 1

In QGIS 3, there is new helper for that:

layer = iface.activeLayer()
new_layer = layer.materialize(QgsFeatureRequest().setFilterFids(layer.selectedFeatureIds()))
QgsProject.instance().addMapLayer(new_layer)

This will give you a memory layer. Then you can still write it to disk if necessary with the QgsVectorFileWriter.

PS, IMHO, you should try to avoid shapefile. People might come with some layers if longer field names and they will be truncated. Especially in QGIS3, GeoPackage is the default format.

Answer 2

Old question but I was searching for this today - here is what I did:

layer = iface.activeLayer()
QgsVectorFileWriter.writeAsVectorFormat( layer, 'H:/temp/' + layer.name() + ".shp", "utf-8", layer.crs(), "ESRI Shapefile", 1)

The boolean option '1' at the end of the command results in saving just the selected features of the layer.