Mostapha, thanks for the reply.
I have copied your 2 lines into a "python script" node and when I run the (dyn) graph, the node turns orange with error message
Warning: IronPythonEvaluator.EvaluateIronPythonScript operation failed.
Traceback (most recent call last):
File "", line 1, in
ImportError: No module named os
Not sure how to proceed further.