Promoting to multipart using PyQGIS?

Question

I am creating a code that aggregates neighbouring polygons if they have similar value on the field 'DN' and if they are closer than 50 m.Then I want to iterate this 3 times.

I could already get the smale ID to the polygons with this restrictions.

for i in range (3):
for feature in GFW_catch.getFeatures():
    geom = feature.geometry()
    ll = []
    for feature_n in GFW_catch.getFeatures():
        geom_n = feature_n.geometry()
        euclidean = QgsGeometry.distance(geom, geom_n)
        if abs(feature ["DN"] - feature_n ["DN"]) <= 2:
            if euclidean < 50 and feature.geometry().area() < feature_n.geometry().area():
                ll.append((feature_n.geometry().area(),feature_n ["ID"]))
    if len (ll):
        feature ["ID"] = max(ll)[1]
    GFW_catch.updateFeature(feature)
processing.runalg('saga:polygondissolve', GFW_catch,"ID", False, '2018-11-15-GFW_AR5_Forest.shp')

Since I did not find a code for promoting to multipars in PyQGIS, I thought about dissolving by ID with SAGA, that creates multiparts by default in the GUI. However, it is not working with the message: Error: Algorithm not found.

in https://qgis-docs.readthedocs.io/en/latest/docs/user_manual/processing_algs/saga/shapes_polygons.html the doccumetation shows this code:

processing.runalg('saga:polygondissolve', polygons, field_1, field_2, field_3, dissolve, dissolved)

any tip?