PyQGIS Expression – Resolving Unrecognized Geometry Methods in QGIS Expression Field Calculator

pyqgisqgis-3qgis-expression

It appears as though the function editor on the Field Calculator is not recognizing the Geometry() method on the feature.

Example code below, always returns -1 into the field I populate from the Field Calculator. The features have valid geometry. The source table is standalone and not a join.
When I also do a print(geometry()) call, it returns <QgsGeometry: null>
The weird thing is that on the Field Calculator preview, it shows the correct value.

The issue is similar to this issue:

Expression not saving in QGIS Field Calculator?

However the tables I am working with have no joins. I have tested on both 3.18 and 3.22 versions with a blank project. No joins/relationships.

from qgis.utils import qgsfunction
from qgis.core import *
from qgis.gui import *
    
@qgsfunction(args='auto', group='Custom')
    
def test_geom(feature, parent):
    
    area_calc = feature.geometry().area()
        
    return area_calc

Best Answer

Seriously, I've had this problem all day, scouring google, 10 minutes after posting the question - of course, I stumble across the answer on the git forums.

The reason why this doesn't work is explained in detail here https://github.com/qgis/QGIS/issues/41695

In short, the Preview doesn't use the same code as the actual function method, and in order to get it to work, you need to declare usesgeometry=True in the function declaration.

So this works.

from qgis.utils import qgsfunction
from qgis.core import *
from qgis.gui import *
    
@qgsfunction(args='auto', group='Custom', usesgeometry=True)
    
def test_geom(feature, parent):
    
    area_calc = feature.geometry().area()
        
    return area_calc

Related Solutions

QGIS – How to Reproject Geometry with a QGIS Expression

Make sure that the assigned CRS is set to EPSG:3005 and correct the value of your target EPSG.

You should put this as a default values for your lon/lat fields, respectively :

format_number(x( transform( $geometry, layer_property(@layer,'crs'), 'EPSG:4326')),6)
format_number(y( transform( $geometry, layer_property(@layer,'crs'), 'EPSG:4326')),6)

PyQGIS – Returning Multiple Values in Custom SQL Query with Python Expression Function

The best solution I could find at the moment is to use the new SQLite JSON functions and operators (available since SQLite 3.38.0): https://www.sqlite.org/json1.html

So I changed my Python expression function to return a JSON string:

from qgis.core import qgsfunction,QgsProject,QgsExpressionContextUtils

@qgsfunction(args='auto', group='Custom')
def getLayoutMapGridParam(layoutname, feature, parent):
    layout = QgsProject.instance().layoutManager().layoutByName(layoutname)
    space = QgsExpressionContextUtils.projectScope(QgsProject.instance()).variable('grid_space')
    print(space)
    if not space:
        space = 1000
        QgsExpressionContextUtils.setProjectVariable(QgsProject.instance(), 'grid_space','1000')
    else:
        space = int(space)

    if layout:
        map = layout.referenceMap()
        if map:
            extent = map.extent()
            xmin = int(extent.xMinimum()/space)*space
            ymin = int(extent.yMinimum()/space)*space
            xcount = round(extent.width()/space,0)+1
            ycount = round(extent.height()/space,0)+1
            return '{"xmin":%s,"ymin":%s,"xcount":%s,"ycount":%s,"space":%s}' % (xmin,ymin,xcount,ycount,space)
    if not extent:
        return None

and modified the SQL to use JSON operators:

WITH RECURSIVE   
    r(x) AS (
        SELECT getLayoutMapGridParam('MyLayout') -> 'xmin'
        UNION ALL      
        SELECT x+getLayoutMapGridParam('MyLayout')->'space' FROM r
        LIMIT getLayoutMapGridParam('MyLayout') -> 'xcount'
    ),
    c(y) AS (
        SELECT getLayoutMapGridParam('MyLayout') -> 'ymin'
        UNION ALL
        SELECT y+getLayoutMapGridParam('MyLayout')->'space' FROM c
        LIMIT getLayoutMapGridParam('MyLayout') -> 'ycount'
    ) 
SELECT row_number() over() as id,geomfromtext('POINT('||r.x||' '||c.y||')',31255) as geom /*:point:31255*/ FROM r,c

The default grid spacing (1000) can be changed through setting the QGIS project variable grid_space.

BTW: I'm using QGIS 3.26.3 on Windows (SQLite 3.38.1)

* UPDATE *

with a small change in our SQL code we can reduce the number of calls to the PYTHON function getLayoutMapGridParam

WITH RECURSIVE 
   data(xmin,ymin,xcount,ycount,space) AS MATERIALIZED    
       (SELECT
          json->'xmin' AS xmin,
          json->'ymin' AS ymin,
          json->'xcount' AS xcount,
          json->'ycount' AS ycount,
          json->'space' AS space
      FROM (SELECT getLayoutMapGridParam('MyLayout') AS json)),
  r(x) AS (
      SELECT xmin FROM data
      UNION ALL
      SELECT x+space FROM r,data
      LIMIT (SELECT xcount FROM data)
  ),  
  c(y) AS (
      SELECT ymin FROM data
      UNION ALL
      SELECT y+space FROM c,data
      LIMIT (SELECT ycount FROM data)
  )
SELECT row_number() over() as id,geomfromtext('POINT('||r.x||' '||c.y||')',31255) as geom /*:point:31255*/ FROM r,c

Best Answer

Related Solutions

QGIS – How to Reproject Geometry with a QGIS Expression

PyQGIS – Returning Multiple Values in Custom SQL Query with Python Expression Function

*** UPDATE ***

Related Question

* UPDATE *