How to colorize each face of the mesh in blender

How can I use the bpy module in Blender to load a human body mesh from a .obj file and colorize each face of the mesh based on the angle between the face's normal vector and the direction of the sun?

Specifically, I want to color each face of the mesh using the following formula: (vn•e + 1) / 2, where vn is the normal vector of the face, e is the direction of the sun, and • represents the dot product between vn and e.

import bpy 
import random

filepath = "body_vertor.obj"



# Import the mesh from the .obj file
bpy.ops.import_scene.obj(filepath=filepath)



#bpy.ops.mesh.primitive_plane_add()
body = bpy.context.selected_objects[0]
#body = bpy.context.active_object

#-****************************

r = random.random()
y = random.random()
g = random.random()
alfa = 1.0

color = (r, g, y, alfa)

mat = bpy.data.materials.new("body")
mat.diffuse_color = color 

body.data.materials.append(mat)

bpy.ops.paint.vertex_paint_toggle(color)