Example illustrating how a cli could be executed directly using subprocess.Popen. This could be particularly useful when investigating issue associated with CLI. Source: http://stackoverflow.com/questions/706989/how-to-call-an-external-program-in-python-and-retrieve-the-output-and-return-cod

slicer_raw_execute_cli.py

import os
from subprocess import Popen, PIPE

#cli_path = "/home/jchris/Projects/Slicer-AHM-Superbuild-Debug/Slicer-build/lib/Slicer-4.2/cli-modules"
#cli_name = "AddScalarVolumes"
#cli_filepath = os.path.join(cli_path, cli_name)
cli_filepath = slicer.modules.addscalarvolumes.path

process = Popen([cli_filepath, "--xml"], stdout=PIPE)
output = process.communicate()[0]
print(output)

Since my module was not loading in Slicer, I had to use the commented piece of code and comment out "slicer.modules.addscalarvolumes.path". Very useful though!