Can't call GRASS commands with Python on QGIS 2.18

Question

I'm trying to run a grass command (v.split.length) using the processing library in pygis. The script is as follows:

import processing

layer = r'path\to\shp.shp'
extent = QgsVectorLayer( layer, '', 'ogr' ).extent()
xmin = extent.xMinimum()
xmax = extent.xMaximum()
ymin = extent.yMinimum()
ymax = extent.yMaximum()

res = processing.runalg("grass:v.split.length",layer,1000, "%f , %f, %f, %f "% (xmin , xmax , ymin , ymax), -1,0, 0, r'path\to\out_shp\out_shp.shp')

When I execute the script I get the following message box:

GRASS folder is not configured. Please configure it before running GRASS algorithms.

I am running QGIS 2.18.22 with GRASS 7. What I have noticed in the processing options is as follows:

The Grass7 folder appears to be defined but Grass folder is not. Further more inside my QGIS installation this is what I can see

C:\Users\aboufira>cd C:\Program Files\QGIS 2.18\apps\grass

C:\Program Files\QGIS 2.18\apps\grass>dir
 Volume in drive C is OS
 Volume Serial Number is 944A-AABE

 Directory of C:\Program Files\QGIS 2.18\apps\grass

07-Aug-18  15:49    <DIR>          .
07-Aug-18  15:49    <DIR>          ..
07-Aug-18  15:49    <DIR>          grass-7.4.1
               0 File(s)              0 bytes
               3 Dir(s)  31,910,600,704 bytes free

There are two possible solutions:

• install the regular version of GRASS and add the folder to apps path such that when I open the providers menu I can see the path appear under regular GRASS
• Find a way to call the grass 7 algorithms from the Python console. It seems that processing.runalg by default attempts to run the regular GRASS algorithms

Which of these options is the more straightforward one and how do I implement it? If there is an alternate option I am also open to that.

Answer 1

Call the GRASS 7 scripts by simply using grass7 in the command instead of grass.

E.g.:

res = processing.runalg("grass7:v.split.length",layer,1000, "%f , %f, %f, %f "% (xmin , xmax , ymin , ymax), -1,0, 0, r'path\to\out_shp\out_shp.shp')