create mirror of active vertex group as a new vertex group

add_vertex_group_mirror.py

import bpy

bpy.ops.object.vertex_group_copy()
bpy.ops.object.vertex_group_mirror(flip_group_names=False,use_topology=True)

# only select one object at a time
selected_obj = bpy.context.selected_objects[0]

active_vertex_group = selected_obj.vertex_groups.active

if active_vertex_group.name.find('_L_') != -1:
  print(1)
  active_vertex_group.name = active_vertex_group.name.replace('_L_', '_R_').replace('_copy', '')
elif active_vertex_group.name.find('_R_') != -1:
  print(2)
  active_vertex_group.name = active_vertex_group.name.replace('_R_', '_L_').replace('_copy', '')
  
  
  
# change all
import bpy

selected_obj = bpy.context.selected_objects[0]
vertex_groups = selected_obj.vertex_groups

ops = bpy.ops.object

for group in vertex_groups:
    if group.name.find('_L_') != -1:
        bpy.ops.object.vertex_group_set_active(group = group.name)
        ops.vertex_group_copy()
        ops.vertex_group_mirror(flip_group_names=False,use_topology=True)
        vertex_groups.active.name = vertex_groups.active.name.replace('_L_', '_R_').replace('_copy', '')