Getting polygon's geometry from layer

Question

I would like to retrieve a polygon's geometry from a layer to use it in a cql_filter. Right now, I'm using the code below. It is working, however, it's creating a rectangular geometry out of the polygon.

    # Get the polygon layer
    polygon_layer = QgsProject.instance().mapLayersByName(layer_name)[0]

    # Get the extent of the polygon layer
    extent = polygon_layer.extent()

    # Create a polygon geometry from the extent
    polygon = QgsGeometry.fromRect(extent)

I've tried searching online and reading the QGIS API documentation but I couldn't find what I'm looking for...

Answer 1

A given layer will have multiple features (could be one, but could be many).

So once you have the layer, you need to iterate over the features, and then get the geometry for each feature eg

# Get the polygon layer
for feature in polygon_layer.getFeatures():
    if feature.geometry().isMultipart(): # multipolygon
        for geom in feature.geometry().asGeometryCollection():
            polygon = geom
            <insert further code>
    else:
        polygon = feature.geometry()
        <insert further code>

A cleaner approach to avoid dealing with the difficulties of single/multi part features is to use the parts() or constParts() iterators. constParts() is probably sufficient for your requirements eg:

for feature in polygon_layer.getFeatures():
    for part in feature.geometry().constParts():
        polygon = part
        <insert further code>