Groups by materials #Blender

blender-groups-by-materials.py

import bpy

data = {}
for obj in bpy.data.objects: # bpy.context.selected_objects
    mat_slots = obj.material_slots.keys()
    if mat_slots:
        data_key = "-".join(mat_slots)
    else:
        data_key = "None"
    data.setdefault(data_key, []).append(obj)

for key, value in data.items():
    group_name = "{}".format(key)
    
    if group_name not in bpy.data.groups:
        bpy.ops.group.create(name=group_name)
        
    grp = bpy.data.groups.get(group_name)
    for obj in value:
        if len(obj.users_group) == 0:
            grp.objects.link(obj)