I'm trying to add some layers created during a process to a project.

There seems nothing to be added (TOC empty) but when I close QGIS it says:

This project includes one or more layers as temporary draft. This layers are not saved at the disc and...

Or something like that (just my own translation)

I defined the layer as:

vl_layerPG = QgsVectorLayer("Polygon", "vl_layerPG", "memory")

Filled with some data and then:

QgsProject.instance().addMapLayer(vl_layerPG, True)

But this is not added to the TOC, almost visible.

I tried without True first as shown at the PyQGIS cookbook, but It was the same result.

def processAlgorithm(self, parameters, context, feedback):
    dxf_file = self.parameterAsString(parameters, self.INPUT_FILE, context)

    current = 0       
    driverIn = ogr.GetDriverByName('DXF')
    dataSource = driverIn.Open(dxf_file, 0)
    dataSource.ExecuteSQL("SELECT * FROM entities WHERE Layer = 'PG-LP'")
    layerIn = dataSource.GetLayer()        
    n_records = layerIn.GetFeatureCount()    
    inFeature = layerIn.GetNextFeature()
    geometryPoly = ogr.BuildPolygonFromEdges(ogr.ForceToMultiLineString(inFeature.GetGeometryRef()), dfTolerance = 10)
    geometryPolyWkt = geometryPoly.ExportToWkt()
    ml = QgsVectorLayer("Polygon", "PG_layer", "memory")
    pr = ml.dataProvider()
    attrib = []
    feat = QgsFeature()
    return {self.MEM_OUTPUT:ml}
  • Can you share more detail from your code?
    – pnz1337
    Sep 12, 2019 at 18:32
  • Maybe try without 'True': QgsProject.instance().addMapLayer(vl_layerPG)
    – pnz1337
    Sep 12, 2019 at 18:32
  • @pnz I edited the question adding the code. I'm a little stuck on that, because if I tried the command QgsProject.instance().addMapLayer(ml) at the python terminal It works as expected.
    – Javier G
    Sep 20, 2019 at 9:27
  • Hi, did you find a way to import your layers in processing?
    – aurel_nc
    Nov 25, 2020 at 5:21
  • No yet, there's the same behaviour.
    – Javier G
    Nov 26, 2020 at 11:40

1 Answer 1


Finally I founded a solution:

I read that is not possible to add a new layer from processAlgorithm anymore.

But you can add then after, at the postProcessAlgorithm

def postProcessAlgorithm(self, context, feedback):
  • I didn't test, but I think that ml in processAlgorithm have to be self.ml (I mean self.ml = QgsVecto....) for this to work in postProcessAlgorithm. Oct 27, 2021 at 8:13
  • Yes, that ml must be self.ml
    – Javier G
    Oct 29, 2021 at 12:12

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