[GIS] Using list of values for custom function in QGIS Field Calculator

Question

I'm writing a custom function in the Field Calculator (QGIS 2.18.5).

Creating a new function from Function Editor > New file, I'm able to start using a default function called func:

"""
Define new functions using @qgsfunction. feature and parent must always be the
last args. Use args=-1 to pass a list of values as arguments
"""

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

@qgsfunction(args='auto', group='Custom')
def func(value1, feature, parent):
    return value1

At the beginning of the above code, I noticed this:

Use args=-1 to pass a list of values as arguments

Desiring to pass, for example, this list:

my_list = ['a', 'b', 'c']

as an input parameter, I tried to use args=-1 instead of args='auto':

@qgsfunction(args=-1, group='Custom')
def my_func(my_list, feature, parent):
    return my_list

but I wasn't able to add this list for my custom function because I wrote it using an invalid syntax:

What am I doing wrong? Or maybe I made some confusion about the using of args?

Answer 1

After some trial-and-errors, I found a working solution.

For passing a list as an argument, it would be enough setting the length of the list as a value for the args parameter. Then, since a list can't be returned by the function, it is necessary to specify a value from the list.

In my case, I need to rewrite the function in this way:

@qgsfunction(args=3, group='Custom')
def my_func(my_list, feature, parent):
    val = my_list[1]
    return val

and call the function by directly inserting only a list as a parameter:

As desired, the function returns the second value from the list which is directly specified inside the parentheses (without recurring to the square brackets as it is usually done for lists).