update enum

Hi for all.
I have an operator that shows an enum(combobox) with default element (‘a’ in this case), but I need change this value when I execute the operator (change ‘a’ by ‘b’ in this case). This is the idea but this operator doesn’t satisfy me.
How can I do??? :confused::confused::confused::confused::confused:
Thanks in advance.

class OBJECT_OT_view_lists(bpy.types.Operator):

subdivide = [('a','a','a')]
subdivides = EnumProperty(name="Groups", description="Generated groups", items=subdivide)

def draw(self, context):
    layout = self.layout
    row = layout.row()
    row.prop(self, 'subdivides')

def execute(self, context):
    subdivide = [('b','b','b')]
    return {'FINISHED'}

When I use Enums I just let Blender manage what the user picks. There is no need to update anything. Just leave it alone and it will work. I believe it is meant to be read, not controlled.