Sets the vertex normals of a face selection, averaging normals shared by multiple selected faces.

vertex_normals_from_sel_faces.py

# ---------------------------------------------------------------------------------------
# - pompVertNormFaceAvg
#
# - Sets the vertex normals of a face selection,
#   averaging normals shared by multiple selected faces.
#
#
# - by Pontus Karlsson 2013 (www.pomperi.com)
# ---------------------------------------------------------------------------------------

import maya.cmds as cmds

faces = cmds.filterExpand(sm=34, ex=True)
verts = cmds.ls((cmds.polyListComponentConversion(faces, ff=True, tv=True)), flatten=True)
normals = [[], [], []]

for v in verts:
    conFaces = cmds.ls(cmds.polyListComponentConversion(v, fv=True, tf=True), flatten=True)
    shaFaces = list(set(conFaces).intersection(set(faces)))
    faceNorm = cmds.polyInfo(shaFaces, fn=True)
    
    for normal in faceNorm:
	    label, vertex, x, y, z = normal.split()
	    normals[0].append(float(x))
	    normals[1].append(float(y))
	    normals[2].append(float(z))
	    
    x_avg = (sum(normals[0]) / len(shaFaces))
    y_avg = (sum(normals[1]) / len(shaFaces))
    z_avg = (sum(normals[2]) / len(shaFaces))
    
    cmds.select(v)
    cmds.polyNormalPerVertex(xyz = (x_avg, y_avg, z_avg))
    
    normals[:] = [[], [], []]

cmds.select(cl=True)