custom normals.py

import bpy
import bmesh
import random

me = bpy.context.object.data

# Get a BMesh representation
bm = bmesh.new()   # create an empty BMesh
bm.from_mesh(me)   # fill it in from a Mesh

# Modify the BMesh, can do anything here...
for v in bm.verts:
    orientations = [[-1,0,0], [1,0,0], [0,1,0], [0,-1,0]]
    v.normal = random.choice(orientations)

# Finish up, write the bmesh back to the mesh
bm.to_mesh(me)
bm.free()  # free and prevent further access