C4DtoA ships with a python API (since C4DtoA 4.2.0) which allows you to work with Arnold specific components, such as the Arnold Material, custom gui elements, etc.

The API is located in the C4D_HOME/library/scripts/arnold folder, in specific python modules. In your python script you have to make sure the scripts folder is added to the python path:

import c4d, os, sys

# add the script folder to the system path
scripts_folder = os.path.join(c4d.storage.GeGetC4DPath(c4d.C4D_PATH_LIBRARY), "scripts")
if scripts_folder not in sys.path:
    sys.path.append(scripts_folder)

Then you can import the specific arnold modules, for instance:

import arnold.util as arnold_util
from arnold.material import *

The following pages contain examples of how to use the API and how to query and modify Arnold related nodes in the scene.