Getting vertices of a vertex group from within Python

[PKHG]

@CoDEmanX

maybe like this?

ob =  bpy.context.object
group_lookup = {g.index: g.name for g in ob.vertex_groups}
verts = {name: [] for name in group_lookup.values()}
for v in ob.data.vertices:
    for g in v.groups:
        verts[group_lookup[g.group]].append(v.index)

CoDEmanX , this I should have known yesterday … very nice way accessing vertex_groups

With respect to this I have a question:
I have an object with a group op vertices selected and want to assign these to a vertex_group of the object.
The vertex_group is already set to active
(e.g. object.vertex_groups.active_index = to_list.index(to_groep_name) and in edit mode that group is indeed selected)
BUT I can not assign it via Python : bpy.ops.object.vertex_group_assign() seems not to be possible ???
Forgotten by the implementors?
Select and Deselect commands are schown (bpy.ops.object.vertex_group_select()
bpy.ops.object.vertex_group_deselect() )

but presseing Assign or Remove are done but not shown (in the … window of Blender, opened above the View3D) ???