I’d like to write a QGIS Expression function in Python that takes a list of strings and joins them with a separator that can be given as an optional named argument sep which also has a default value. For example, I would use it in an expression with the default separator concat_sep('a', 'b', 'c') or with a custom separator concat_sep('a', 'b', 'c', sep := ' - ').
How can I pass Qgis expression named parameters to Python? My Python code currently is:
@qgsfunction(args='auto', group='custom', handlesnull=True, usesGeometry=False)
def concat_sep(*strings, sep=', ', feature, parent):
non_empty_strings = filter(lambda s: s is not None and s != '', strings)
if not non_empty_strings:
return ''
result = sep.join(non_empty_strings)
return result
It works fine when using the default separator, i.e. writing an expression without sep := …. But when I try to use sep := …, there’s an error popup in the expression editor that says “concat_sep does not support named QgsExpressionFunction::Parameters”.
How can I define a named parameter in the expression that is passed through to the Python function? Is it at all possible? I guess something needs to happen in the @qgsfunction decorator, but I couldn’t find anything in the documentation.
