blender_bmesh_example.py

import bpy
import bmesh

obj = bpy.context.active_object
mesh = obj.data
bmsh = bmesh.new()
bmsh.from_mesh(mesh)
# Do something with mesh: e.g. add layer
layer = bmsh.verts.layers.int.new('Example')
for id,vert in enumerate(bmsh.verts):
    vert[layer] = id
    print(id, vert, vert[layer])
bmsh.to_mesh(mesh)
mesh.mesh.update_tag()