Make custom function using dynamic variable

Question

In the function editor, I am trying to make this function :

@qgsfunction(args='auto', group='Custom')
def size_reduction( x, feature, parent):
 s = @map_scale/1000/x
 y = if( s<1, s, 1+ln(s) )
 return y

But I get the error shown below. Can anyone help me ?

An error occurred during execution of following code:
from qgis.core import *
from qgis.gui import *
@qgsfunction(args='auto', group='Custom')
def size_reduction( x, feature, parent):
 s = @map_scale/1000/x
 y = if( s return y
 File "", line 6
 s = @map_scale/1000/x
 ^
SyntaxError: invalid syntax

Python version: 3.8.5 (default, Jan 27 2021, 15:41:15) [GCC 9.3.0]

QGIS version: 3.16.4-Hannover 'Hannover', exported

Answer 1

You can fetch the map scale within a function using

from qgis.utils import iface
[...]
iface.mapCanvas().scale()

Answer 2

Here is my working function

import math 
from qgis.utils import iface 
from qgis.core import * 
from qgis.gui import * 
 
@qgsfunction(args='auto', group='Custom') 
def size_reduction( x, feature, parent): 
 s = iface.mapCanvas().scale()/1000/x 
 if s<1 : 
 y = s 
 else : 
 y = 1+math.log(s) 
 return y