Remove islands and completely surrounded polygons after polygonization with QGIS

Question

I have a line map of buildings in a city. I was able to correctly polygonize the buildings in QGIS with the Polygonize processing algorithm. All the holes/islands became features as expected. Now I need to select and remove all features that are completely surrounded by other (one or more) features. Examples shown in red on the image:
These are not part of buildings, but inner yards.

Answer 1

You can use for example this python script to highlight or delete the features that are completely within another feature.

layer = iface.activeLayer()  #or the layer you want to use
features_a = layer.getFeatures()
features_b = [feat for feat in layer.getFeatures()]
within_list = list()
for feat_a in features_a:
    for feat_b in features_b:
        if feat_b.id() != feat_a.id():
            if feat_b.geometry().within(feat_a.geometry()):
                within_list.append(feat_b.id())

To highlight the features you can use:

layer.setSelectedFeatures(within_list)

And to delete the highlighted features use:

caps = layer.dataProvider().capabilities()
if caps & QgsVectorDataProvider.DeleteFeatures:
    res = layer.dataProvider().deleteFeatures(within_list)

As a test result I get for example:

But before you execute this script please save your qgis file because if you want to process a large file this script probably crashes your qgis.

Answer 2

an alternative is to use "vector > geoprocessing tools > dissolve" (--Dissolve all-- in the list of dissolve fields), then select the polygons that are "within" the dissolved polygons using the "spatial query" plugin.

Answer 3

You can also duplicate layer and then make spatial query with "touches" between them. Works with prepared data.