Deleting parts of MultiPolygon based on area using PyQGIS

Question

I have a layer with MultiPolygons. Some of them have parts less than five m² which I want to delete.

The QgsGeometry class has methods: parts() and deletePart(self, partNum: int). On parts() I can apply area() to get the area. https://qgis.org/pyqgis/master/core/QgsGeometry.html

What I am missing, is a method to get the partNum.

Can someone point me in the right direction?

Here is the code I came up so far. But I doubt that the index of the list asGeometryCollection is the same as the numPart index. Alternatively, I could create a new geometry, and loop over the parts, and if a part is > 5, add the part to the new geometry.

# Minimum area threshold
minimum_area = 5

# Loop through all the features in the layer
for feature in layer.getFeatures():
    # Retrieve the multipolygon geometry
    geometry = feature.geometry()
    geom = geometry.asGeometryCollection()
    numParts = len(geom)
    # loop over parts and numParts ( same index ??)
    for part, numPart in zip(geometry.parts(), range(numParts)):
        if part.area() < minimum_area:
            geometry.deletePart(numPart)

Answer 1

You can make a new part list from the MultiPolygon instead of deleting the parts.

layer = iface.activeLayer()

# Minimum area threshold
minimum_area = 2000

layer.startEditing()

# Loop through all the features in the layer
for feature in layer.getFeatures():

    # get polygons collection from geometry
    geoms = feature.geometry().asGeometryCollection()

    # filter the parts      
    filtered_geoms= [g.asPolygon() for g in geoms if g.area() > minimum_area]

    # build a new multipolygon
    new_geom = QgsGeometry.fromMultiPolygonXY(filtered_geoms)
    
    # change the geometry
    layer.changeGeometry(feature.id(), new_geom)

# layer.commitChanges()

Test the script. If it meets your need, uncomment the last line. It saves the changes to the data source.