Looking for equivalent of ST_ContainsProperly for selecting features in PyQGIS

Question

I'm looking for a solution to select islands (non-black polygons) in polygon A (black).

I think, in PostGIS ST_ContainsProperly() will do this task. But I'm looking for an equivalent solution in PyQGIS.

The function shall only return True when checking the green polygon. Checking the grey MultiPolygon shall return False, because it is partially outside and the red polygon False because it touches the border

I tried the code from this thread

path_1 = r"C:\temp\test_poly.shp"
path_2 = r"C:\temp\test_poly_umr.shp"

layerB = QgsVectorLayer(path_1, "layerB", "ogr")
layerA = QgsVectorLayer(path_2, "layerA", "ogr")

if layerA.isValid():
    QgsProject.instance().addMapLayer(layerA)

if layerB.isValid():
    QgsProject.instance().addMapLayer(layerB)
    
island = False
for bF in layerB.getFeatures():
    for aF in layerA.getFeatures():
         if aF.geometry().intersects(bF.geometry()): 
            if bF.geometry().contains(aF.geometry()): 
                island = True
    print(bF[0], ' ', island)
    island = False

... but it returns island=True for Polzgon #5 instead of polygon #3 (as expected)

Answer 1

You need to check if A's boundary intersects B's boundary. You can use this script.

# ...
# previous lines
# ...

island = False
for aF in layerA.getFeatures():
    
    # get aF's boundary (=line)
    aF_boundary = aF.geometry().convertToType(QgsWkbTypes.LineGeometry)
    
    for bF in layerB.getFeatures():        
        if aF.geometry().contains(bF.geometry()):

            # get bF's boundary (=line)
            bF_boundary = bF.geometry().convertToType(QgsWkbTypes.LineGeometry) 
       
            if not bF_boundary.intersects(aF_boundary):
                island = True
                
        print(bF[0], ' ', island)
        island = False

Answer 2

Use the [6] within predicate with the "Extract By Location".

container = QgsProject.instance().mapLayersByName('container')[0]
islands = QgsProject.instance().mapLayersByName('islands')[0]

selected = processing.run("native:extractbylocation",
                            {'INPUT':islands,
                            'PREDICATE':[6],
                            'INTERSECT':container,
                            'OUTPUT':'TEMPORARY_OUTPUT'}
                        )['OUTPUT']

selected.setName('selected')

QgsProject.instance().addMapLayer(selected)

In the figure, the hatched polygon is the result of Extract by Location.