Blender Python API: Example of BMesh layers

layers_example.py

import bpy
import bmesh
 
def element_id_layer(bm, elems, id_name):
    """
    This function will create/print verts/edges/faces layer
    """
 
    # Try to get layer of elem IDs
    elems_iter = getattr(bm, elems)
    lay = elems_iter.layers.int.get(id_name)
    if lay is not None:
        print('Layer ', id_name, ' exist')
        # Print values in layer
        for elem in elems_iter:
            print(elem[lay], elem.tag)
    else:
        print('Creating layer ', id_name)
        # Set values in layer
        lay = elems_iter.layers.int.new(id_name)
        # NEW: values will not be copied
        lay.use_force_default = True
        # NEW: it is possible to set default value
        lay.default_value = -1
        for elem in elems_iter:
            elem[lay] = elem.index
            elem.tag = True
            print(elem[lay], elem.tag)

def update_layers():
    """
    This function update (add) layers to active mesh object
    """
    obj = bpy.context.active_object
    mesh = obj.data
    edit_obj = bpy.context.edit_object
    if edit_obj is None:
        bm = bmesh.new()
        bm.from_mesh(mesh)
    elif obj == edit_obj and edit_obj.type == 'MESH':
        bm = bmesh.from_edit_mesh(mesh)
    else:
        pass
     
    element_id_layer(bm, 'verts', 'VertIDs')
    element_id_layer(bm, 'edges', 'EdgeIDs')
    element_id_layer(bm, 'faces', 'FaceIDs')

    if edit_obj is None: 
        bm.to_mesh(mesh)
        bm.free()

if __name__ == '__main__':
    update_layers()

This example requires this patch: https://developer.blender.org/T42865