import bpy
for mat in bpy.data.materials:
    mat.diffuse_shader = 'OREN_NAYAR'
    mat.diffuse_intensity = 1.0
    mat.roughness = 0
    mat.specular_shader = 'BLINN'
    mat.specular_intensity = 1.0
    mat.specular_hardness = 50
    mat.specular_ior = 10