Updating attribute table with area

Question

I want to update a certain field with the current area. What I have tried:

updated_area = iface.addVectorLayer(xx.shp, "", "ogr")
selected_feature = updated_area.selectedFeatures()
updated_area.startEditing()
for feature in selected_feature:
 feature["area"] = "$area"
 updated_area.updateFeature(feature)
updated_area.commitChanges()

I'm not getting any errors, but nor is it updating any fields.

• Kasper: Do you need the python approach ? If not, you can create a virtual field from the field calculator, defined as $area. Otherwise if you want pyqgis to do the evaluation, your feature["area"] field should be something like 'feature.geometry().area()"
• Tobias Bowley: Yes, unfortunately I do. It's a part of a bigger script.
• Kasper: does your shapefile have the area attribute already ? or does it need to be created by the script ?
• Tobias Bowley: There is a field called area already, but because of some clipping in the script, it needs an update. I wouldn't mind deleting it and adding a new instead, any work-around would do.

Answer 1

I've found a way to update the ares, but for what ever reason the numbers are whacked. Like in an unidentified unit.

updated_area = iface.addVectorLayer(xx.shp, "", "ogr")
 
area_idx = updated_area.fields().lookupField('area')
for f in updated_area.getFeatures():
 area = f.geometry().area()*626336.7007
 updated_area.dataProvider().changeAttributeValues({f.id(): {area_idx: area}})

By multiplying it with 626336.7007 I'm getting the correct numbers, however, but I would like to skip this work around.

• etrimaille: You haven't told us the CRS of your layer and what is the unit you are expecting.
• Tobias Bowley: I'm not sure how to do that. =/
• etrimaille: Go in your vector layer properties, the first tab, you have a section "Coordinate Reference System (CRS)". Please edit your question with these info.
• etrimaille: If you don't know how to retrieve the CRS of the current layer using QGIS desktop, it seems you have a few more things to know in GIS and QGIS in general before diving into PyQGIS
• Tobias Bowley: I meant, I don't know how to do that in code. Using the GUI is not an option.