Geometry generator and calling an expression from a custom function

Question

I am using geometry generator symbol layer type and some expressions that makes the result a line that is shorter then the original line.

The expression works and looks like this:

make_line(line_interpolate_point($geometry,10),line_interpolate_point($geometry,35))

However I would like to make my own function that makes it easier to do this operation. Inside the function I am trying to use the make_line expression but this results in an error in the expressions panel when using the function.

My function looks like this:

from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom')
def smallerline(geometry,value1, feature, parent):
tmpline = make_line(line_interpolate_point(geometry,value1),line_interpolate_point(geometry,value1))

return tmpline

I'm using the function in the expression tab but there's an eval error that says "Eval Error: global name 'make_line' is not defined"

Answer 1

You are mixing up expression syntax and python syntax.

The function you write is in python. Python does not understand the expression syntax.

You need to find a matching function in the pyqgis API to make the line shorter.

Looking at the sourcecode of the interpolate expression function it uses lineGeom.interpolate(distance)

make_line creates a new line (lineString = QgsLineString()) and then loops over the vertices and calls lineString.addVertex( vertex ).

Puttin the pieces together is left as an exercise for the reader.