Calculating information for selected parts of the polygon with PyQGIS

Question

I have a polygon that is formed using Quick OSM.

After that, I add an area calculation field to it, automatically using the QGIS tools.

How can I use PyQGIS to get the sum of the areas of the selected parts of the polygon multiplied by 5 and display the answer in the Python Console?

Answer 1

To sum up all features' areas can be done either via

the "FIELD_NAME" field, that contains area values:

# imports
from qgis.core import QgsProject, QgsAggregateCalculator

# getting polygon layer by its name
layer = QgsProject.instance().mapLayersByName("YOUR_LAYER_NAME")[0]

# summing area of all features
area_total_sum = layer.aggregate(QgsAggregateCalculator.Sum, "FIELD_NAME")

print(area_total_sum) # (19594.140492000002, True)

or the $area variable directly:

# imports
from qgis.core import QgsProject, QgsAggregateCalculator

# getting polygon layer by its name
layer = QgsProject.instance().mapLayersByName("YOUR_LAYER_NAME")[0]

# summing area of all features
area_total_sum = layer.aggregate(QgsAggregateCalculator.Sum, "$area")

print(area_total_sum) # (19594.140491519036, True)

Then if necessary proceed with area_total_sum[0] * 5.

If one applies this expression area($geometry) the result will be slightly different (19603.3203125, True). Therefore, pay attention to this thread: Calculating polygon areas in shapefile using QGIS.

To sum up only selected features' areas can be done either via:

the aggregate() method of the QgsVectorLayer class:

# imports
from qgis.core import QgsProject, QgsAggregateCalculator

# getting polygon layer by its name
layer = QgsProject.instance().mapLayersByName("YOUR_LAYER_NAME")[0]

# summing area of selected features
area_selected_sum = layer.aggregate(
                            aggregate=QgsAggregateCalculator.Sum,
                            fieldOrExpression="FIELD_NAME",
                            fids=layer.selectedFeatureIds()
                            )

print(area_selected_sum) # (1958.09388, True)

or the calculate() method of the QgsAggregateCalculator class:

# imports
from qgis.core import QgsProject, QgsAggregateCalculator

# getting polygon layer by its name
layer = QgsProject.instance().mapLayersByName("YOUR_LAYER_NAME")[0]

# constructing the aggregate calculator
agg_calc = QgsAggregateCalculator(layer)
# setting a filter for selected features only
agg_calc.setFidsFilter(layer.selectedFeatureIds())

# summing area of selected features
area_selected_sum = agg_calc.calculate(QgsAggregateCalculator.Sum, "FIELD_NAME")

print(area_selected_sum) # (1958.09388, True)

As was already previously shown, instead of the "FIELD_NAME" one can also use either $area or area($geometry).

Then if necessary proceed with area_selected_sum[0] * 5.

---

References:

• Summing values in one field using PyQGIS

Answer 2

here is the python script based on the data in the column Area from the attribute table

# Access the active layer (which should be your polygon layer)
layer = iface.activeLayer()
# Check if the layer is indeed a vector layer
if not layer or not isinstance(layer, QgsVectorLayer):
    print("Active layer is not a vector layer.")
else:
    # Initialize a variable to hold the sum of the areas
    sum_of_areas = 0
    
    # Loop through each selected feature in the layer
    for feature in layer.selectedFeatures():
        # Assuming the field that stores the area is named 'Area'
        # Make sure to replace 'Area' with the actual name of your area field
        area = feature['Area']
        
        # Multiply the area by 5 and add to the total sum
        sum_of_areas += area * 5
    
    # Print the total sum in the Python Console
    print(f"The total sum of the selected areas multiplied by 5 is: {sum_of_areas}")