Why is feature.geometry() None in my user-defined expression function?

Question

I'm trying the user-defined expression functions for QGIS, as listed on Nathans blog: http://nathanw.net/2012/11/10/user-defined-expression-functions-for-qgis/

I always get the feature.geometry() as None, when running the user expression in the expression builder updating a text column? This happens for all geometries like point, lines and polygons. The data format is ESRI Shape. The code is in an userfunctions.py referenced from startup.py.

Simplified example:

from qgis.utils import qgsfunction
from qgis.core import QGis

@qgsfunction(0, "Python")
def test(values, feature, parent):
    geom = feature.geometry()  
    if geom is None: 
        return 'geom is None'

    return 'geom is NOT None'

My text column is updated with the text: 'geom is None'. Why can I not reference the geometry of a feature?

Answer 1

You need to specify that this function requires the geometry with the kwarg usesgeometry=True. If you don't, you may get a geometry but there is no guarantee (as you realized).

This works starting from QGIS 2.4

from qgis.utils import qgsfunction
from qgis.core import QGis

@qgsfunction(0, "Python", usesgeometry=True)
def test(values, feature, parent):
    geom = feature.geometry()  
    if geom is None: 
        return 'geom is None'

    return 'geom is NOT None'

This requirement was introduced for performance reasons, so a (possibly large) geometry does not need to be fetched from the database in many cases and therefore relieves the database server and network server where possible.