PyQGIS Processing - wrong number of parameters

Question

I'm facing a very strange issue from the module Processing using it as a standalone (not in the Qgis python console). I'm trying to create voronoi polygons from a points csv file. I can load my file without issue but the Processing.runAlgorithm always fail with " wrong number of arguments ". There are only 3 arguments described in the help.

#create uri
uri = "file://{}?delimiter={}&xField={}&yField={}".format(os.path.abspath(fichier.name),",","x","y")
#open file
layer = QgsVectorLayer(uri, "tmp", "delimitedtext")
# check
if not layer.isValid():
 print "Layer didn't load properly."
 print "Check your filename and the file integrity."
# UTF-8
layer.setProviderEncoding('UTF-8')
#feedback
print("File successfully loaded as " + layer.name() + " layer.")
Processing.runAlgorithm("qgis:voronoipolygons",layer,0.0, "test2.shp")

Traceback :

File successfully loaded as tmp layer.
Error: Wrong number of parameters
ALGORITHM: Voronoi polygons
 INPUT <ParameterVector>
 BUFFER <ParameterNumber>
 OUTPUT <OutputVector>

but it works from the Qgis python console :

mapcanvas = iface.mapCanvas()
layers = mapcanvas.layers()
processing.runalg("qgis:voronoipolygons",layers[0],0.0,"test.shp")

Output :

{'OUTPUT': 'test.shp'}

Even stranger, when i'm trying to add an unknown argument like :

Processing.runAlgorithm("qgis:voronoipolygons",layer,0.0, "test2.shp",0.0)

it returns :

File successfully loaded as tmp layer.
Error: Wrong parameter value: test2.shp

With adding 2 unknown arguments :

Processing.runAlgorithm("qgis:voronoipolygons",layer,0.0, "test2.shp",0.0,0.0)

TB :

File successfully loaded as tmp layer.
Error: Wrong number of parameters
ALGORITHM: Voronoi polygons
 INPUT <ParameterVector>
 BUFFER <ParameterNumber>
 OUTPUT <OutputVector>

Looks like there is a forth argument ... Any idea ?

Answer 1

Syntax is:

runAlgorithm(algOrName, onFinish, *args, **kwargs). 

Just set None to onFinish argument.