How to write a custom function for a label?

Question

One of my projects has a layer label that is calculated by an expression from seven field values as follows:

 if(coalesce("caption" ,'') = '', '', "caption" || '\n') || trim(concat("Distance",' ', "Unit", ' ', "Label", ' ',"Distance 2",' ', "Unit 2", ' ',"Label 2", ' '))

I want to replace this with a custom function in order to do some additional processing without writing a very complex expression.

Using the function editor I have put this custom function in.

from qgis.core import *

from qgis.gui import *

@qgsfunction(args='auto', group='Custom')

def loclabels(capt, dist1,unit1,label1,dist2,unit2,label2, feature, parent):

    cap=coalesce(capt,'')

    cap2 = trim(dist1 + ' ' + unit1 + ' ' + label1 + ' ' + dist2 + ' ' + unit2 + ' ' + label2)

    if cap == '':
            fullcapt = cap2
    else:
            if cap2 != '':
                fullcapt = cap + '\n' + cap2
            else:
                fullcapt = cap

    return fullcapt

Calling this and passing the same fields as the original expression always returns a null value. In fact even trying to return a constant returns a null. The function is syntactically correct (no errors in the code) but simply fails to return any useful data.

What is missing?

Answer 1

You can try :

cap = capt if capt is not None else ''

instead of :

cap=coalesce(capt,'')

and :

cap2 = (dist1 + ' ' + unit1 + ' ' + label1 + ' ' + dist2 + ' ' + unit2 + ' ' + label2).strip()

instead of :

cap2 = trim(dist1 + ' ' + unit1 + ' ' + label1 + ' ' + dist2 + ' ' + unit2 + ' ' + label2)

Also you might test your function in the python console (removing the expression extra stuff) before using it in an expression.